Transcript for:
Overview of Cell Cycle and Reproduction

so now we need to learn about the cell cycle basically cellular reproduction there's a couple ways that that's that happens so we're going to go over the similarities and differences of both of those cycles and this would be just for cells like our own not for prokaryotes or something like that you'll learn that elsewhere so when cells divide it's more about dna and conserving that dna from one generation to the next more than it is about anything else so we have to consider chromosomes for a minute even though that's going to be more of a vocabulary word and topic for genetics but because that is the form of the active dna in your cells we need to get into this realm chromosomes is actually the form of the protein and dna in your cells only during mitosis and meiosis the two forms of cell division that we're going to look at here cell rep reproduction so chromosomes means condensed dna and proteins the proteins specifically being the ones that organize that dna and not any of the other proteins that you find in the nucleus so chromosomes are that i think you already had the term chromatin chromatin is the same thing as chromosomes it's the same dna it's the same organizational proteins but it's not densely packed when chromatin is densely packed that's when you call it a chromosome now we use the term chromosome much more loosely in general terms but that's the actual definition i mean when it comes to saying chromosome we're not just talking about all the dna in your cells and such it's very specific the form in which that dna comes in and when we call the chromosome it's specifically when it's condensed like this picture you see here this is the form it takes and you can actually see a shadows of these or really the you can see them under a microscope so that means that dna is very much condensed if you can see it under a microscope and so when it's in its more loosely packed form it's filling up the nucleus that's when we call it chromatin that you learned before so chromatin is active dna that dna is doing something in that form and when it's densely packed like this it can't do anything so chromosomes are actually only in existence for a few brief moments while mitosis and meiosis are going on at no other time now usually we just use that in more of a familiar term for the 46 fragments of dna that has our information on it you know it's used more loosely like that but again since this is a biology class you should know the actual definition and then use it loosely as you so please so again chromatin and chromosomes are composed of dna and proteins there's a ton of different proteins in the nucleus and the cell in general we're only talking about the organizational proteins so that's the material that we're most interested in when it comes to the topic of cell division or cell replication so let's go ahead and see how the cell accomplishes this so again it's better to call this the cell cycle and not you know cell division because cell division is a little too one-dimensional and you'll see what i mean by that in a minute so we call this the cell cycle and what that really refers to is not just reproduction but the entire life of a cell until it reproduces and so what you get where you actually see mitosis going on is just this little corner of the pie here and notice that cytokinesis is a part of that so this little point this little period of time right here that we call m phase or mitotic phase in meiosis it would be called meiotic phase but we're not there yet this is where we have chromosomes and these slivers of the pie would actually be almost non-existent you almost can't see them considering all the rest of the time in the life of that cell that's about to do them um we just had to squeeze in the letters and font to to get in there in reality mitosis and cytokinesis you know they happen extremely rapidly and interphase which is the rest of the life of a cell can last a very very long time so here's this little m phase and the rest of the life is interphase and that's 100 of the life of one cell and as soon as that cell divides it becomes the next generation so it's not like organismal reproduction like you and me where you know parents have their kids that represents two different generations one of those generations is gonna die the next one's gonna live on and do the same thing that's not the way cellular reproduction works cells don't die yes you can kill them they can age and die and such like that but the ones that go through this process are starting the next generation they are the next generation and that's where you get this perpetuity of life that we talked about in the in the cell theory where we could trace life all the way back to that very first cell 4 billion years ago because we know that this is what happens all cells if they're going to reproduce they don't die they divide into two and that becomes the next generation so a quite different way to look at reproduction than what we're used to with organismal reproduction so m phase and interphase are the periods in the life that make up the whole life of that cell where different things are going on eventually and ultimately leading up to division so interphase the textbooks always say that takes up about 90 percent of the cell cycle meaning 90 percent of the life of a cell in reality it's a lot more than that because as i mentioned m phase is extremely short um so interphase is really most of the life of a cell and intervase is divided up into three sub phases the g1 s and g2 phases we'll get into that in a minute but this is again a period of time where different things happen leading up to that event of making the next generation so in the life of the cell most of which is interphase the cell is taking in nutrients getting rid of wastes metabolizing carrying out the functions of life it's what it does at some point it has to copy its chromosomes or copy all of its dna because again that's the most important type of molecule that has all the information for life and carrying out all of life's processes you might think that dna is just some set of instructions that made us what we are today but it's just a set of instructions it sits sequestered inside the nucleus and doesn't do anything when in fact all day long every day for the life of a cell that dna is extremely busy because those instructions are active instructions to tell that cell what to do to carry out the processes of life at any given moment so again dna is extremely important even to the day-to-day cycles of the cell and other processes and then again it it will eventually divide so see there's cell division that's at the very last thing i mean the cell dividing into two that cell division not everything that leads up to it and that's again why we need to call this uh the cell cycle now the type of cell cycle we're going to look at first is mitosis you may have heard of that before but mitosis is not the whole cell cycle it is just that little wedge in the pi where nuclear division is happening mitosis actually means nuclear division not cell division so nuclear division actually happens early on can't happen until all the chromosomes have been copied etc so that's what we're getting into here that's kind of a prelude to the whole story but let's first look at the three sub phases of interphase how a cell divides up its day and lifetime so that g1 phase that you saw in that pie chart it was named that g1 as first gap phase because under the microscope the people that discovered all these things it looked like nothing was going on under the microscope so it seemed to be a period of time where nothing was happening so they called it a gap phase then they knew something was happening happening in s phase because they could quantify the dna before and after it and they could see that there's exactly twice as much dna after this and they could see some stuff going on in the nucleus during this time as well so they called it s phase s for synthesis specifically synthesis of dna then once all that synthesis of dna is done once you have two copies of dna now it enters the g2 phase which again was called a gap phase because it looked like nothing was going on before cell division finally started so i don't want you to think of g1 and g2 as gap phases there's certainly plenty going on there in the first g1 phase if you imagine one cell dividing into two well those two baby cells so to speak are going to be half as big as the parent cell by definition right so there's some growing to be done so the first gap phase they're busy metabolizing taking in nutrients growing until they reach their adult size and then at some point not up to them they will get a message that hey we're going to need you to divide soon and so when they get that message s phase starts they copy their dna and then they wait for the final instruction okay divide now and then the rest of it after that could happen pretty quickly um so g2 phase is just kind of waiting around for that message that it's time to divide and in the meantime it's very active metabolically to carry out life's processes and like right before it finally is allowed to divide it there's quite a few things it still has to kind of take care of before that gets done we're not going to get into all those details necessarily but that's what's going on certainly not gap phases then after mitosis in this case if you'll look back at that chart it says cytokinesis those are the two parts of m phase mitosis and cytokinesis cytokinesis literally is just the cell dividing into if it's an animal cell or a cell like yours it's a little bit different in plants and some other organisms and you'll see that later but for now that's what we need to look at then those two daughter cells we call them daughter cells as if there's a parent and daughter those two daughter cells that are the two cells that parent cell divided into may then go on and repeat that cycle now it depends on what cell type it is as to whether that cell cycle is going to repeat or not because if you think about our species we can use us for an example you have probably heard that some cells never grow and divide again they're just not allowed to whereas other cells have to grow and divide all day long to replace damaged cells like the ones of our integumentary system our skin on the outside uh what lines our digestive tract things like that are those cells are constantly being killed and need to be replaced so if it's a epidermal cell then there's certainly going to be a lot more repeating the cycle if it's a neuron or a muscle cell or a fat cell then the cell cycle is not going to repeat at some point at certain point in our development those three types of cells are no longer allowed to grow and divide again we're born with as many of most of those cells as we're ever going to have our nervous system continues to develop again fat may as well but muscle cells you're born with as many of those as you're going to have so there are cell types that will not do this and just have to be long lived in order to carry out their lifelong function so what we're going to do is i mean everything's going on in interphase you're going to learn about various things that happen there in other chapters what we need to do here is focus on m phase specifically mitosis and cytokinesis and then we get to the next one meiosis and then the cytokinesis is the same after that so we are diving in to mitosis mitosis is subdivided into five categories of activities all a sequence of events of course this is very fluent and happens very quickly we're just kind of stopping you know taking little snapshots along the way when different things are happening and so that's how we've characterized each one of these sub phases based on what's going on at that period of time now each one of these subphases also has its own internal mechanism for telling the cell when it can enter prophase when it's done when it can enter the next phase those are all regulated highly but we're not going to get too much into that either just a couple quick things about regulation after this is all over so the because m phase is after interface we already have duplicate chromosomes because you know that s phase dna synthesis happens in interphase so that means when interphase is over and it's time for m phase to start we already have twice as much dna as we're supposed to because all of our chromosomes were copied all of our dna was replicated you'll learn how that happened later on that's a genetics chapter so for now just realize that there are two copies two sets of dna in this cell in preparation for cell division because if one cell divides into two you need twice as much dna right and exactly the same copies of dna otherwise nothing's gonna work so when you have two copies of that dna so that the two copies of each chromosome don't get kind of lost during this time each set of copies is held together by something called a centromere that centromere holds two copies together and when they're held together like that we have a new vocabulary term chromatid so if you jump back to the first slide and see that picture of a condensed chromosome you could see it look like two things paired together that's actually what it is there are two copies of dna that are held together at the centromere and when they're in that condition we can still call the chromosome but that chromosome is composed of two sister chromatids so when that when those two copies break apart now you just call them chromosomes this word goes away but when they're held together when you have two copies held together after replication after s phase we call them chromatids so chromatids are the exact same thing as chromosomes it just implies that there's two copies of that chromosome held together by a centromere so make sure you get all that down for that definition has to be qualified by being held together by a centromere it's just a gluey mechanism that holds these two things together so sister chromatids sister implying that they came through this s phase and are related to each other they are they should be perfect uh copies of each other so when we start prophase and for most of m phase or at least of mitosis you're gonna have two copies of all that dna in the form of chromatids so that is to say you have two copies of chromosome one two copies of chromosome two all the way through 46 right we have 46 chromosomes and so if we went through s phase now we have 92 chromatids we still have 46 chromosomes but there's 92 chromatids because we have a copy of an extra copy of each one of them but again they have to be tied together those copies before you use this particular term otherwise chromatid is the same thing as a chromosome which is the same stuff as chromatin right so write all those words down and play with them a bit spend some time with them so chromosome chromatids i put sister in parentheses there and chromatin they're related words that can cause some confusion if you don't spend some time with it so spend some time with those all right so in prophase which is the first subphase of mitosis there's a couple things going on there's something called a mitotic spindle that's forming you can't quite see the spindle shape that gives it the word spin look a spindle is kind of a football shape thing but we will see that later on it's getting started they call it mitotic because this spindle is going to conduct all of the nuclear division so this mitotic spindle is made up of two centrosomes go back to your organelle chapter and make sure you remember what that those centrosomes do they generally make and break and rearrange microtubules for the cytoskeleton for flagella for cilia and for the cellular reproductive process specifically nuclear division nuclear division couldn't happen if you didn't have two centrosomes doing their job there's nothing else that's directly involved in organizing those two copies of dna to make sure that two daughter cells get all the copies that there should that they should so the other thing about this is not just making a little list of what's going on here that but let's make another little list so right down here these are going to be points that you need to memorize so you can recognize a picture and say oh dot dot dot that's a picture of prophase you need to be able to recognize these from a picture this is a very fairly complicated and fancy picture this is a real cell that's been stained with fluorescent dyes i may or may not use these pictures the more black and white type may be a little more straightforward but you can very much see what's going on by the way all these yellow thread-like stuff those are all microtubules being made by those centrosomes that will pretty soon be used for the process of mitosis and meiosis for that matter okay so for your list to recognize this as prophase i mean you have to know that this is going on but that's not how you recognize what's going on this list is just for recognizing the pictures so notice that the nucleus has a nice tight membrane around it you know that because it's a nice perfect circle right or maybe a little oblong whatever you don't have chromosomes like falling out and unorganized they're highly organized inside of what you can obviously see must be a nuclear membrane containing all that dna so that's one thing you have an intact nuclear membrane the other is that the appearance of the chromosomes inside looks wormy i call it wormy looks like a bunch of worms in there doesn't it those are actually the condensed sister chromatids you can see them they're so dense as i told you before so if this was back in interface you do have to recognize interphase 2 it would just be a nice homogeneous one color with one dark dot for your nucleolus if you forget what a nucleon nucleolus is go back to your organelle assignment so the nucleolus would be evident in interphase that's all you have to look for to recognize a picture of a cell that's in interphase all you have to do is look at the nucleus is it round like this and does it have a dark dot inside that's interphase and can't be anything else because notice there is no dark dot in this so right away you lose the nucleolus when mitosis starts marking the beginning of prophase it's because all the chromosomes are condensed i mean that dark dot that is the nucleolus they're very specific genes very hard at work and they're denser because there's a lot of their products those parts of the rna sorry ribosome the rna rna from the ribosome but when the dna rearranges itself like this that goes away so you don't see a nucleolus so intact nuclear membrane a wormy appearance and no nucleolus that's how you know you're in prophase and nothing else so what's next take a look at this nucleus see not a nice tight circle around it we know that that nuclear membrane is gone so that's on your little list of what's going on in prometaphase pro metaphase is depending on what book i can't remember which one they use in ours but sometimes prometaphase is not separated as a step it should be and we're going to do that if if your book doesn't so characterizing prometaphase so you can recognize a picture of it no nuclear membrane and notice that the chromosomes all look very unorganized now you don't have anything to compare that yet to to say i don't know if it's organized or not wait a minute look at the next phase or two and you'll see what i mean by organization it's very obvious so right now you have no nuclear membrane they're just kind of spilled out unorganized that's the best way to recognize prometaphase now the mechanisms that are going on here so you're going to learn you know the mechanisms behind mitosis is that those two centrosomes have separated and still are cranking out all those microtubules some of those microtubules are going to grab on to the copies of dna the copies of chromosomes and others are going to go over and attach to microtubules across the way which will effectively push these two centrosomes far apart so nuclear envelope fragments and the microtubules now have access to the chromosomes and will start the organization process those microtubules literally grab onto them and are involved in their organization so that one copy goes into each of the daughter cells eventually daughter nuclei i should say the nuclei are produced first all right so there that's some organized dna right there yeah the legs are kind of unorganized out the sides there because they're not attached to anything and they are just kind of hanging there but look at how densely packed those microtubules are all smashing the dna together right there that's literally what they're doing they're both sides are pushing with their microtubules to make sure all the dna is right in the middle of the cell and in that process they're also organizing it so that the microtubules on this side are attached to a copy that will end up over there and the microtubules on this side are attached to the other copy of those sister chromatids so that that copy ends up over here so you get all 46 copies going that direction the other 46 copies go in the other direction when this cell is ready to divide so the way you recognize metaphase from a picture is by what's called a metaphase plate so because all those chromosomes are already very densely packed and now they're being squished together in the middle it looks like a plate like a disk in the middle really if you're thinking three dimensionally where all the chromosomes have been squished together so the plate itself is actually made up of all those micro all those chromosomes that are smashed together there then the next is anaphase notice that it's still highly organized but there's two clusters now there's distinctly two clusters of chromosomes so the copies have been separated from each other if this is a human cell we have 46 chromosomes over here that are moving that way and 46 chromosomes here that are moving this way so that when the curtain comes down and the cell divides right in the middle there there are no chromosomes left behind so anaphase is characterized by the separation of sister chromatids and that's what you see happening in this picture here very easy to recognize in a picture you can't confuse that with anything else i guess the only thing you might confuse that with is let's jump back is maybe that but that's highly disorganized and there's not two distinct batches of them when you look at that there's clearly two parts and you can see the mitotic spindle has broken in half really and you see the dense thing on either side there's your centrosome and the other center so all right so sister chromatids separate during anaphase of mitosis that's the mechanism that's what you're describing that's what you're seeing here to help you identify the picture as well you then have telophase telophase is the last phase of mitosis so you see what's happened are those chromosomes have gotten bunched up at the end where that centrosome is and in reality this is all the way out at the edge of the cell to make sure that all these copies are as far away from these copies as possible so that when division happens nothing gets left behind if even one little piece of one chromosome got left behind that's enough to kill both cells that'll cause problems notice that the cell is taking on an hourglass shape so cytokinesis which is not a part of mitosis but the very next stage that actually causes the division starts during telophase so it's not a part of mitosis but it does start a little early and so you get this indentation so you that you know that the cell hasn't divided yet but notice you got these two clusters and look it's starting to look pretty round right there the nuclear membrane is starting to reform see it getting nice and round on this side too so those are going to seal up into nice new daughter nuclei and then the cell is going to divide cytokinesis is finally going to do that by the way cytokinesis means division of the cytoplasm because that's all that's happening here we've already gone through all the steps to replicate the dna and make sure all the copies ended up in two daughter nuclei before that cell divides so the nuclei the nuclei are starting to reform you might even start to see the nucleolus again the dna is becoming less tightly packed because when dna is densely packed it is inactive it cannot do anything if dna can't do its thing the cell will die quickly that's why this part all of mitosis has to happen very very quickly because during the time that you have an actual dense chromosome the dna is inactive it's tight it's too tightly packed to do its job and if it doesn't do its job pretty quick that cell's going to die it's kind of like holding your breath so this whole cell is holding its breath holding off all life's processes until this gets done so it has to get done quick then again it's followed by cytokinesis where the cell will divide so let me just go through those steps in a more in a better more clear way a drawn picture again pretty far from reality because how nice and neat these pictures are drawings are but it'll show you the important parts so this can help you if you missed anything over the last few slides only thing i want to note is they made this look like a wormy appearance just to show you that there is dna in there i would have drawn it that way i would just have one nice solid color with the dark circle there that is the nucleolus because during interphase the dna doesn't look like this um so go ahead and review this match it up with your notes from what we just went over so that you know exactly what's going on i might point out here that when in that metaphase plate because it's hard to see what's going on in a real cell when that's happening but all the copies are lining up on that metaphase plate so that notice the two sister chromatid one is ready to go that way and one's ready to go that way it's ready to go that way and ready to go that way as soon as the centromere breaks remember the centromere holds together sister chromatids and the centromere gives way and they separate during anaphase etc so please review that and again that is punctuated by cytokinesis this is a real cell this is a zygote meaning the first cell that you ever were you know when your dad's sperm and your mom egg united into one cell you were one cell until you started doing this a lot so that you eventually became a hundred million cells through this process of mitosis that i just showed you so here this shows the first one's already been done so this single celled zygote has divided once so this one going across here is complete so we have one whole cell here and the other cell matching on the other side and then each of those has already started the next round of mitosis but let me ask you this notice here is the what they call the cleavage furrow so the cleavage furrow is going but it ends right there and here's the rest of the cell it's even puckered because during like this type of animal cell during cytokinesis it literally pinches itself in half there is there are fibers just inside the plasma membrane that are like a belt on the cell but on the inside around the equator that literally cinches up like a belt to pinch it in half that's why it looks like it's puckering there it's forcefully being pinched apart but it's only right here so you tell me what phase are these two cells in right now based on what you see here cytokinesis has already happened right here and you see it's starting here but what are they in what is the subphase must be telophase right because cytokinesis has begun but has not finished that's the only time cytokinesis can be seen happening is in telophase until it's complete so that would be the four cell stage of an embryo now and that's going to continue on until there's a hundred million cells in a human [Music] that's a lot of mitosis and it's entirely mitosis let me tell you now mitosis is a cloning process also that dna is perfectly copied if everything goes well through a process we'll see later on to carry this out so what is the definition of cytokinesis again the division of the cytoplasm all right so that's the whole process of mitosis and as i mentioned there's a lot of things that regulate when certain things start and other things stop through the entire process we're not going to get into all those details that's much higher level cell biology class but we will get into a few of the easy ones now there's things called growth factors they come in two forms one is chemical growth factors which mainly are steroids and hormones and communication molecules that tell you know so cells can communicate with other cells to tell them when to do this we're not going to get into those either you can wait for another class physiology or something for your endocrinology that's involved here we're just going to look at the physical growth factors and there's two physical growth factors that must be met before a cell is allowed to divide to grow and divide for that matter because there may not be any room for growth either so the first one is density dependent inhibition that's what you see here i mean this is like one of your skin cells thrown into a petri dish and then grown over time but this could be the cells on your arm or something so density dependent inhibition specifically says you know based on this physical requirement that there must be room for cells to replicate and divide before they're allowed to if there's no room then you know if they just kept growing and dividing what's that that's a tumor right and we'll talk about that later on but normal growth would be that those cells if we let's look at this one here so long as there's no elbow room so to speak since they bump into a neighbor and have a neighbor they don't grow or divide because there's no room for it now if there's an injury like to remove some cells from the petri dish or if you actually cut yourself or something then notice how one two three four five six seven of the cells don't have neighbors here there's room to grow so those cells right there and those alone are allowed to grow and divide because they're the ones that have room to do it although they're going to actively grow and divide these cells over here won't because there's not room and as soon as they fill in that gap and start bumping into each other again they stop so that it's very tightly controlled because all the different tissues of your body there's not room for more of most of them and so if all goes well you should just have that one layer of that cell type based on that kind of regulation so again you could you could make this happen and watch it in a lab or just watch it by injuring yourself so density dependent inhibition when the density is high growth and division is inhibited that's what we're getting at there if density is low then it's not inhibited they're allowed to grow and divide the next one is called anchorage dependence so anchorage dependence says that oops i won't talk about that just yet anchorage dependent says that if a cell is not physically anchored to something that something is usually another cell that's neighbor or the extracellular matrix or something like that but if it's not attached if it becomes dislodged then it's no longer allowed to grow and divide that's anchorage dependence it depends on being anchored if it's anchored and all those other growth factors are in place then it will be allowed to grow and divide so even if all the growth factors are there physic all the other physical ones well density dependent inhibition there's room if all the steroids and hormones that promote growth and division and stuff all that is in place but that cell becomes dislodged it's not allowed to grow and divide why because if a cell gets from a tissue in your body if it gets dislodged and starts moving around in the fluids of your body if it was allowed to grow and divide then it would start to plug up veins and arteries and stuff right wherever it ends up so we don't want a cell that gets dislodged to be able to do that our cell's going to our body is going to get rid of it because it could cause problems but cancer as you know has escaped both of these physical growth factors you know that it doesn't listen to density dependent inhibition because cancer cells just grow and divide at will and create these big tumors of overgrowth uh too many cells that's a cell that has a violated density dependent inhibition and only cancer cells or pre-cancer cells do that and then once a cell from a tumor that already has a problem gets dislodged they stop listening to this anchorage dependence as well and even though they're dislodged and floating around in your lymphatics or your blood they will start growing and dividing and that's why tumors often end up in lymph nodes because lymph nodes filter the lymph where these types of things might end up in travel and there's kind of a little mesh work in there that captures them like a net they get stuck in there they start growing and you have lymphatic tumors so cancer cells do not listen to anchorage dependence or density dependent inhibition they have undergone various genetic changes that allow them to ignore that and just live a selfish life so when cells have undergone this change that allows them to ignore density dependent inhibition what do you have you have this right here a tumor and all tumors are benign but all tumors have the potential to become cancers like full blown cancer now i say that all tumors are benign with an asterisk you know a disclaimer because there are certain places that a single seemingly harmless tumor can grow where they might interfere with organ function and cause major problems like brain tumor or something like that but generally speaking these things pop up everywhere and are benign until they undergo the last change that allow them to dislodge themselves they stop listening to this anchorage dependence now we have full blown cancer because those cells can end up anywhere and they're going to cause problems and there's no stopping it so cancer cells have undergone genetic changes that make that happen so just to relate to this a little bit better this idea of cancer cells this is this by the way is the cellular understanding of cancer we're going to get into the genetics of it in the genetics chapters as well so we're only looking at this from a cellular perspective or scale so this is a horrible thing i hate this but it's in the textbooks and i guess it's a place to start but it says that most mammalian cells divide 20 to 50 times before they stop age and die so that range first of all is ridiculous because all cells go through tons of mitosis to get us to the 100 million cell stage so from that point on i guess you could say once the individual has reached an adult scale like that then those cells may divide another 20 or 50 times before they stop age and die but not all cells remember some cells have already stopped growing and dividing and need to be long lived cells like our nerve cells and stuff i should say neurons um but others are going to do this to the day you die now there's going to be a lot of problems with the aging process that are going to first start getting in the way here and i think that's what they're getting at just the fact that there is an aging process and that cells will die and will eventually lead to our our death so what we know about cancer cells is they seem to be immortal well immortal means to live forever and we haven't seen forever so how do we know that they live forever of course that's a relative thing it could certainly kill cancer cells and a lot of times they kill themselves on accident but what we mean here is all the evidence we have shows that cancer cells stay young and healthy indefinitely they don't go through this aging process prior to death they are able to fix all their dna their you know repair dna which is one thing that we start to lose more and more as we age that's why our cells you know stop producing elastin and we start getting wrinkles and things like that well cancer cells just keep on metabolizing and and making sure that everybody everything's working right and they even change their dna in ways so that they could live forever because we basically have cells that live a certain period of time and start breaking down and dying and cancer cells don't do that they stay young so to speak how do we know this the best evidence so far is one of the first tumors that we ever harvested cells from to study cancer were harvested from a woman henrietta lacks back in 1951 and these cells have been growing in culture in countless labs around the world cancer research labs you can order these through a biological supply company if you are a scientific lab still to this day they've been one of the single most important things in cancer research so very influential cells here and what we can see in these cells that were harvested in 1951 they still show no signs of slowing down or aging or anything very healthy cells cancer cells that are that old so where do cancer cells come from then ask yourself that where do cancer cells come from are they cells that you catch like a communicable disease or a virus or something where do they come from literally any cell in your body that undergoes the right genetic changes that we'll learn about later can become a cancer cell and that starts you know if you have one cancer cell then it's going to create more and more as it goes through the mitosis process that you just learned about so cancer all sends back to a single cell that underwent those changes and again it could be anything normally i mean we get cancer cells pretty much every day of our lives but a healthy immune system finds and destroys them before they become a problem now if you have an immune system that's that's compromised with age or with pathology or something like that then you're much more likely to have tumor pop up at any given time but a healthy immune system usually destroys them early on so again things start off with as a benign tumor and once they stop listening to anchorage dependence they become malignant and to become malignant we're talking about metastasis the cells metastasize that means they dislodge themselves and start traveling through the body mainly through our lymphatics and our blood vessels etc and what they do is riddle our body with little tumors because they get clogged up in places they can't move on their own they're moving around in our fluids and if they end up inside of critical organs they're just going to keep on growing tumors and although each individual tumor is just a mass of cells it disrupts organ function where they are and that's what you ultimately die of when you die of cancer this picture is just to demonstrate that it could pop up anywhere this is a very typical type of cancer breast cancer that tumor started again from a single cell turned into this tumor spread eventually metastasized started riding the lymph and blood vessels around other parts of the body because you know that breast cancer doesn't really matter that the cancer is in your breast that's not what you diet if you die when it makes it around your body gets stuck in other more critical organs and disrupts their function so for meiosis we'll talk mainly about the differences and i'll point out some similarities but mainly that's kind of getting at a compare and contrast idea something you really need to get used to in the sciences are compare and contrast how are things similar and how are things different that's what it means so we don't need to get into a lot of the background stuff the mechanisms of meiosis you know the cellular machinery that are controlling things in the background for this cell replication process are virtually identical so we don't really have to talk much about them i might remind you a little bit that it's going on there but we're mainly going to look at what's going on in the nucleus and the big differences that are so important so first of all remember mitosis is a cloning process so long as s phase goes correctly then every generation of cells are going to be identical to each other and to the parent cell that that divided into them when it comes to meiosis meiosis is only about making gametes making eggs and sperm that's it so there's a very special place in your body that that happens it's not happening everywhere like mitosis is and where it does happen in your body the gonads specifically gonads is not gender specific for testes and ovaries so gonads in general make gametes and if they're testes then they're gonna make sperm if they're ovaries they're going to make eggs if everything's working right so those cells of the gonads do go through mitosis also and if a cell was in the gonads was starting one of those cell cycles we wouldn't know if it was mitosis or meiosis until we saw m phase and what was going on otherwise even meiosis starts with interphase that's exactly the same interface we saw before now if gonads are for making egg and sperm why would mitosis have to happen why would some of those cells before they ever become egg and sperm have to clone themselves well if you think about it you know at least with our species you probably heard that women are born with as many eggs as they're ever going to have and it's a few million that's correct they do all that before they're born and have enough eggs to last them a lifetime and never have to make any more but for men of our species they don't even do meiosis until they hit puberty right so 12 13 somewhere in there is when they'll start meiosis and when they start meiosis they're making 400 to 800 million sperm with every ejaculate so men are making lots of sperm all the time and they're shedding them well how many cells do you think a testicle is made of i mean if you're turning those cells into 400 to 800 million sperm every time you're going to run out of testicle really quick if you don't copy and more cells and make sure that the gonads still there to produce these so you can kind of think of half the cells and you're going in male gonads and testes going through mitosis to replace the cells being lost and the other half being lost once you make sperm they are ejected from the body uh females don't so much have to do that uh interesting though because when meiosis occurs and the two genders is so different one before birth the other one not until puberty so there's some very interesting per um differences and similarities for that matter on lots of different scales all right so we're going to go through this like we did before but again we're kind of accelerating this because we really just have to look at the big differences because everything else going on in the background is the same so what do you have i mean you start off with interphase and so you're going to end up with two full copies of dna 92 chromos chromatids 46 chromosomes if it's our cells so it it starts the same the other big difference i'll tell you right off before we get into it all is that mitosis there's just one round of mitosis one parent cell becomes two identical daughter cells that are identical to that parent cell well there are two rounds of meiosis meiosis 1 and meiosis 2 designated by the number 1 or 2 after this and each subphase so we're going to talk about meiosis 1 and look at the sub phases notice you'll see another difference looking at you there we are missing pro metaphase it's not that the events of prometaphase don't happen but they happen at a different time they happen during the events of prophase so it's a little bit accelerated but otherwise all the same stuff has to happen from beginning a prophase to the end of telophase when it comes to the background machinery and stuff that's going on all right the other thing when it comes to meiosis one and two why are there two rounds of this that means that one cell is going to go all through all this and divide into two and then each one of those are going to divide into two so you start off with one cell one parent cell and end up with four daughter cells twice as many so that's a big difference why did that happen well as i told you meiosis starts off with interphase interphase that has dna replication now think about eggs and sperm how many chromosomes do they have do you know that how many chromosomes well step back a sec how many chromosomes do your cells have the rest of them in your body we already mentioned that 46 chromosomes right how many are in your eggs or sperm depending on your gender half of that there's 23 chromosomes only one set because all the other cells in our body that 46 that's actually two sets of chromosomes one whole set that we got from mom from her egg one whole set that we got from dad from his sperm united into one egg bringing that total up to 46. so we have to get the cell that starts this process in the gonad which has 46 chromosomes down to just 23 because if a sperm or egg had 46 chromosomes then the number of chromosomes is going to double every time first generation 92 euchromous chromosomes the next twice that the next one you're going to have way too much dna real quick and when it comes to animal cells we're extremely sensitive to the number of genes and chromosomes that we have just a little bit extra or not enough of dna is enough to kill the cells and perhaps the whole organism so we need to reduce the number of chromosomes to make egg and sperm that's something that makes meiosis special and it does it with a second round of division but the second round is not preceded by an interface so the dna is not copied again so if the dna is copied at first you go from 46 to 92 then the first round of meiosis will get you down to 46 again chromatids though and then the next round if you don't double it again what's half of 46 23 we're at the number we need so meiosis is also known as reduction division all right so let's get down to it and in this case it's mainly knowing the differences because if you looked at meiosis and mitosis under the microscope it'd be hard to tell them apart for anybody so recognizing the pictures based on those criteria we did last time is just about you recognizing the stages of mitosis not meiosis so let's get back to this so again prometaphase is missing because it's happening at the same time as prophase so that means you don't recognize prophase by having an intact membrane that membrane breaks apart real quick so that the microtubules can start grabbing those chromosomes and rearranging them or organizing them alright so as you know again interphase even though it's reduction division does duplicate all its chromosomes first there is an interphase before meiosis one but not before meiosis ii if we had a interphase before meiosis ii we would double the dna again and be right back to where we started with gametes of 46 chromosomes and we can't have that all right so whether it's the interphase prior to mitosis or meiosis those sister chromatids are identical and attached by a centromere we're calling them chromatids again chromatids are two copies of chromosomes held together by a centromere so that's all the same nothing different there so let's look at prophase then and notice we call it prophase one if there was no one there you wouldn't know what we're talking about because there's there's a prophase in mitosis there's a prophase one and a prophase two and meiosis so if there is no number you know we're talking about prophase of mitosis if there's a one or two you know that we're talking about either meiosis 1 or meiosis 2 respectively so again everything in the background add to the list and we have our centrosomes making microtubules you have the events of prometaphase you see the membrane has broken apart so if this was a real picture like i showed you before you just see disorganized dna kind of spilled out into the cytoplasm but what is crazy what is really really different about meiosis compared to mitosis is that the daughter cells are going to be genetically distinct they're going to be different from each other and the parent cell that made them why is that well keep in mind the cells that are undergoing this process like all the other cells in your body uh they have one full set of chromosomes from your dad and one whole set of chromosomes from your mom hopefully your mom and dad are not too closely related so those are probably different uh types different versions of genes blue eyes versus brown eyes etc and so in mitosis all of moms and dad's chromosomes are just independent floating around inanimate objects that are moved around by those microtubules to organize them well what's crazy almost sci-fi about meiosis one specifically starting in prophase is that the chromosomes wake up and start doing some crazy things they become animated uh in some pretty extreme ways again prior to this they're all just inanimate objects being moved around by other mechanisms in the cell in particular those microtubules we talked about but when prophase starts in meiosis one prophase one all of the chromosomes wake up and go find their counterpart your mom's chromosome one wakes up and goes and finds dad's chromosome number one and chromosome number two finds chromosome number two etc such that all homologous chromosomes find each other so dads and mom's versions of the same chromosome that's chromosome one mom's chromosome one those are homologous chromosomes they're not sister chromatids they're not identical because they're from different individuals so we call them homologous chromosomes so homologous chromosomes pair up all of mom's dna goes and finds dad's dna chromosome per chromosome and then they start trading parts this process right here that you see is called crossing over it looks like crossing over under the microscope looks like the legs are crossed over that's where it got it's got its name and because dad's two copies two sister chromatids after interphase moms if those are their colors uh if you're under the microscope it looked like four chromosomes and so they called these tetrats the tetrads represent mom and dad's chromosomes homologous chromosomes that have paired up and again as soon as they're in contact with each other they start trading parts there's some chemistry going on here that you don't see there's enzymes that come in here and break both chromosomes and switch the legs so that now we have hybrid chromosomes we have one chromosome that has both your dads and mom's chromosome uh genes on the same chromosome that's not the way it was before you add a set of dad's chromosomes and a set of mom's chromosomes now they're all going to be hybrid chromosomes and that's what makes the daughter cells genetically unique because those daughter cells have combinations of genes from two different people that you didn't have when you had them they were on different chromosomes so when they find each other that process is called synapsis and again when they start trading parts that's called crossing over alright so again in the background you still have your spindle starting to form notice it is taking on a football shape to give it that spindle name because the microtubules are growing notice the ends of the microtubules they're attached to each other and as they elongate from each centrosome that results in the centrosomes pushing each other apart and it i think i mentioned last time that they will actually push up against the plasma membrane to be as far apart as possible because they're going to make sure that the copies of the chromosome end up on opposite ends of the cell so they don't get left behind when cytokinesis occurs so that's prophase one synapsis and crossing over pretty crazy stuff and that's going to continue all the way on up until they are no longer physically in contact with each other some books say just prophase or something but from the time they pair up to the time they're physically removed in anaphase from each other they're crossing over and you can see more of it going on here during metaphase on the metaphase plate another difference that you might notice under the microscope is that tetrads are lined up on the plate not sister chromatids like we saw before put that on the back burner we'll talk about it later on but notice how the chromosomes have red and blue that represents the hybrid chromosomes now that didn't exist before brand new chromosome combinations here because of that otherwise metaphase is defined by the metaphase plate the organization of that chromosome from the microtubules because again notice some of the microtubules are attached to the centromeres of the opposite sides of the tetrad to make sure that these end up over there and these end up over there you get just the right number of chromosomes in both daughter cells there all right and then anaphase again is the separation of chromosomes but here's a huge difference for you to note and again you should be making all these notes about the major differences here what do you see different about this picture it's not sister chromatids detaching and separating like we defined anaphase from mitosis right go back and look at that it's homologous chromosomes that got separated so anaphase one homologous chromosomes separate not sister chromatids in anaphase of mitosis sister chromatids are separating huge difference there because we still have two copies they're not identical anymore because they traded parts but these are still essentially those sister chromatids they need to be separated and end up in daughter cells also that doesn't happen in anaphase one or meiosis one all right then telophase nothing real different here again you see cytokinesis getting started the nuclear membrane is starting to reform you're gonna have your daughter nuclei and your daughter cells so that cleavage furrow is recognizable again if you were to see it under a microscope the dna is going to start becoming less packed perhaps depending on what kind of gamete this is so this may or may not be well should say this is going to end with cytokinesis of course this is your first two daughter cells they are not sperm or egg yet we'll call them pre-gametes for now so depending on whether this is a male or female things can go quite differently if we're specifically looking at human gamete production so either way we'll just look at it generically at first these are the two daughter cells that are going to start meiosis ii but remember we don't want another round of interphase we don't want to copy the that dna the one thing that we do need though is notice that when this cell divides there's going to be one centrosome in each cell it takes two of them to conduct nuclear division so before meiosis ii does begin the cell at least needs to replicate that organelle it doesn't need to do anything else in interphase it just needs to do that because it requires two of those in each cell to carry out nuclear division so that's going to happen before meiosis ii they call that interkinesis just simply replicating the centrosomes so that there's two of them to carry out the next round of meiosis all right so prophase two now we do not have um any animation this from this point on meiosis ii is pretty much identical to mitosis there's a very important differences obviously but under a microscope and based on basic behavior you wouldn't know the difference because the mechanisms in the background you know that spindle apparatus the nuclear membrane breaking down etc all that's the same and if the cells are not pairing up i'm sorry if the chromosomes are not pairing up and trading parts then hey we're talking about something very basic and inanimate like mitosis so in that generic respect it's meiosis ii is much more like mitosis than meiosis one meiosis one is the crazy one where the chromosomes get animated and pair up and trade parts and all that kind of stuff all right so prophase two again because it's meiosis we don't have prometaphase that's happening at the same time as prophase so you see the nuclear membrane breaking apart which is going to give access to these microtubules so they can grab the chromosomes and start organizing so same basic stuff in the background i'm going to go through this pretty quickly because it's all the same notice here though very important thing going on it's not tetrads lining up on the metaphase plate those are sister chromatids the ones that didn't separate in the last round now they're getting separated so another thing that resembles mitosis because in mitosis chromatids were separated and that's what's happening here again under a microscope you wouldn't be able to tell the difference unless you're able to count the number of chromosomes and see that there's half a half as many or you can somehow quantify the amount of dna and see that there's half as much dna or if you could actually sequence it and see that you have hybrid chromatids instead of identical chromatids those are some obvious differences but not ones that are going to say that this process is all that much different than mitosis because all this is trying to do is reduce the number of chromosomes so that the final gametes the final daughter cells will all have 23 chromosomes each and again one cell divides into two those two each divide into two giving you four daughter cells so getting there again characterized all exactly the same way looks the same and cytokinesis is going to give us our four daughter cells now if this was a female something different happens although females are born with as many eggs as they're going to have in their whole life they're not mature eggs they're called primary oocytes which have not finished meiosis ii yet so it and more interestingly if you as a woman do not ever introduce your primary eggs to a sperm you don't have a reproductive event your eggs will never finish meiosis your eggs only finish meiosis if uh if a sperm starts knocking at the door so to speak and they are kind of in suspended animation in that point of meiosis ii until that happens so very different events there you have that pause for females for males they're just going to produce as many um they're going to produce as many gametes as possible they're just going to fire through this as quickly as possible no stalls for the rest of their life from the time that you hit puberty till the time you die as a male human you're going to be producing hundreds of millions of sperm so again you know that's not true for our females too females enter menopause where they stop this process uh stop releasing eggs and so forth so interesting things going on on higher scales but for now just worry about what's going on with these cells and the production of daughter cells and the comparisons with mitosis so those four daughter cells are said to be haploid haploid means there's half as many chromosomes in the cell as the normal cell count for that species the normal uh sorry chromosome count the normal chromosome count for our species is 46 so our gametes have 23. so here are four haploid daughter cells by the way diploid is the other thing that's going to become more important in later chapters but it's a term that shares the same root ha here literally means half but di means two so two what does ployd mean sets of chromosomes so here's half the sets of chromosomes dye is two full sets of chromosomes so that is to say all the cells in your body except your gametes once they've been produced have two sets of chromosomes one for mom and one from dad that gives you the diploid character that brings us to the variation that we introduce into this process as i mentioned the daughter cells of meiosis are not identical to each other and not identical to the parent cells there are there is so much genetic variation produced very rapidly through that crossing over and through other processes we'll look at here that pretty much all the gametes we make are unlike any others so let's look at how that happens as far as the chromosomes becoming animated and finding each other this is where most of the variation arises because those chromosomes that are finding each other are from two different people mom's chromosomes are going to find dad's chromosomes and then they're trading parts let's forget about crossing over and trading parts just yet i'll introduce it on a list but we're going to go over something before that that's very important so there's three things that happen here to increase genetic variability in our gametes so independent sort assortment is the first this is when mom and dad's chromosomes find each other synapsis happens they don't care what's going on in the cell they're just busy trading parts with each other but when those microtubules start grabbing onto them and organizing them that introduces some variation and you'll understand that when we go through that here in just a minute obviously crossing over is going to cause diversity because with crossing over that's called genetic recombination uh like a gmo i mean we're literally chopping up dna and rearranging other dna and putting it together just like we would in a lab or something like that so this is literally a genetic recombination that's going to result in brand new chromosomes that never existed before by mixing together two unrelated people's dna and then random fertilization obviously we have no idea you know if all the sperm are different from each other and all the eggs are different from each other we don't know which sperm is going to is going to fertilize which egg every one of those is going to give you a different outcome so that's going to just you know which sperm fertilizer which egg is going to bring about a lot of that variation as well so let's look at each one of these individually i mean we don't have to look at crossing over that's pretty straightforward we talked about that a lot but we need to talk about independent assortment there's a little bit of math in it and then random fertilization a little bit more math to really appreciate all the genetic variation that's introduced into our gametes by the processes of meiosis so independent assortment here's a picture of it pretend this is one of our cells we have 46 chromosomes which would be 23 tetrads if they all found each other right so here's dad's chromosome 1 mom's chromosome 1 that have paired up dad's chromosome 2 mom's chromosome 2 and then 21 other pairs that are going to find each other picture would be too complicated so we're just showing you two sets instead of all 23 sets but here are your homologous chromosomes to practice using the terminology each one of these is a sister chromatid attached by a centromere for dad and mom's two sister chromatids there so forget about crossing over we are not going to consider that yet just this process alone this random process introduces a lot of variability because how many of dad's chromosomes and how many of mom's chromosomes end up in the same cell is going to give you quite different outcomes right and the way that that's worked out is when these two chromatid well when these two chromosome sets pair up they're just kind of randomly floating around uh you can imagine them kind of rotating on their axis until a microtubule from a centrosome from that mitotic spindle or meiotic spindle in this case attaches to one of them and then that gives a predestination for that chromosome to go to that side of the cell and once it's positioned like that now the microtubule from the other side is going from the other centrosome is going to attach to the other one so that say dad's is going to go to the cell on the left and moms are going to go to the cellulite so here are the microtubules here that have attached to both sides already this red one can't go over here because it's attached to a microtubule here that will take it over here so once the microtubules attach that tells you where these chromosomes are going to end up and in this case all of dad's chromosomes ended up in one daughter cell and all the moms in another so they're quite different from each other but that's quite different than if well maybe this tetrad rotated another 180 degrees before those microtubules attached and now the chromos one of dads and one of moms is going to end up in the same very different than two of dads and none of moms so that could happen for all 23. i mean there could again ignoring crossing over all 23 of dad's chromosomes could end up in one cell and all moms in the other and that's going to be very different than if there were 20 of dads and one of moms and 10 of dads and 13 of moms etc all of those different possible combinations just of dad and mom's whole chromosomes creates different outcomes because you have dna from two different individuals and different amounts of them in each cells they're going to interact differently there's going to be different gene sequences to give you different outcomes so that's genetic variability right there in a very random way now the math behind it is pretty simple it's just a coin flip because it's either dad's chromosome that's going to end up over here or mom's chromosome so there's two possible outcomes like the flip of a coin there are two possible outcomes heads or tails so you can relate to it that way and each tetrad is a flip of a coin because this one ended up being with dads on the left and moms on the right but this one isn't doing the same thing as that it's random so it could have been dads or moms on this side just depending on when those microtubules attached and that's 23 coin flips for each one to determine whether moms or dads are going to end up in this daughter cell or the other so the math behind this is exponential math so this is math that you're expected to know but it's simple don't worry uh i'll go over it so this 2 and this raised to the power of n is the equation that equation is all you need to know what is n n is the haploid number remember what haploid is half the number of chromosomes that the species has so our species has 46 chromosomes our haploid number is 23. so you would plug 23 into that if you're working this out for our species so there's going to be a different end for all the different species out there right humans squirrels chimpanzees whatever and so you can see how many different chromosome combinations you can make meaning how many different eggs and sperm can a species make based on just this mechanism here so let's 23 is a big number let's work with a smaller one first let's plug in 3 for n 3 goes there how do you do this math is it 2 times 3 is it six six different possible outcomes based on this if a species only has three chromosomes for its haploid number no it's two raised to the three that means you multiply the root to itself as many times as n says so that means you multiply two by itself three times if n equals three so it's not six it's two times two times two which equals eight the other side of this to make sure you understand and really appreciate the math here why is that a 2 why isn't it a 3 or a 4 or 5 or any other number why 2 because there's 2 possible outcomes like a coin flip this would be the exact same equation if we were trying to look at random events of flipping coins it's the same math like everything else you need a huge sample size you need to do this a lot of times to have any confidence in it we you know that's the math by the sorry the science side of that but there's the math so how many different a or sperm can that species make that has a haploid number of three you know this could be an ant or something um eight there's eight different possible combinations of whole mom and dad chromosome combinations so going from you know one of dads and two of moms etc through that whole possible list of combinations per gamete produced there's eight different ones before you start getting random repeats all right so let's do this math for our species our haploid number is 23. so put 23 in for n 2 to the 23 well 2 times itself 23 times doesn't sound like a whole lot right wrong it's over 8 million 2 times itself 23 times is over 8 million so as a species we make a over 8 million different unique gametes before we start getting random repeats from this process because it is a completely random process just like the flip of a coin um so that's a you know over eight million different sperm or egg that's a lot of diversity already that means eight million daughter cells none of which are the same as each other or the parent cells that made them very different from the cloning process of mitosis where they're always identical from one generation to the next if replication doesn't have any mistakes but we'll talk about that later on we'll return to s phase that's basically our first genetics chapter so crossing over again very obvious how this creates new combinations and diversity in dna because you have brand new sequences of dna and chromosomes that didn't exist before so this is something we don't even introduce into that calculation because we know that during crossing over each tetrad trades parts about four times on average before they can't do it anymore um it's some more some less just depends and so we don't even know how many times it happened in the cell you know it's hard to watch it it's hard to quantify it all that so it's just really hard to quantify and so this isn't even put into these numbers these numbers are from an independent assortment that we looked at last that is simply just how many moms and dads chromosomes are in each gamete so again crossing over happens starts in prophase one and continues on up until anaphase when they're physically separated from each other it's the homologous chromosomes again just to play with our terminology here homologous chromosomes in the form of tetrads that synapsed are now trading parts the parts that they're trading are parts of their sister chromatids all right that brings us to random fertilization random fertilization is just a continuation of our calculation of independent assortment because any sperm confused with any egg we know that our species has over 8 million different sperm over 8 million different eggs and we don't know which one is going to fertilize which so how do you do the math there well it's over 8 million doing the 2 to the 23 for the egg and the sperm so do we add those together 8 million plus 8 million and there's that many more possible combinations that are unique no you multiply them together this is probabilities here and exponential math so that means that a resulting zygote where one random sperm fertilized a random egg there are uh 70 trillion over 70 trillion different possible combinations you have to multiply 8 million by 8 million and so over 70 trillion different possible combinations once an egg fertilizes is fertilized by sperm so that means 70 over 70 trillion different zygotes that means none of us are going to be the same genetically at least not by random that's what we're saying there that it would take over 70 trillion zygotes before we might start seeing random repeats and then if you consider that crossing over genetic recombination is happening to every one of those chromosomes along the way well now it's infinite now we virtually now virtually every single zygote that's produced is going to be unique when you really consider that all together but again we can't quite calculate that in but if you really think about that 23 tetrads trading parts every time those are those are very unique uh results that are going to mean that every zygote is going to be different from the next so we learned about mitosis and meiosis as if there's no mistakes at all but there are definitely mistakes that can happen the most common and obvious one the one that's easy to relate to is something called non-disjunction we'll talk about that in a minute that's what's in this picture here but we can't underestimate mutation mutation drives everything including evolution without random mutations happening to our dna you wouldn't have evolution evolution wouldn't work because everything would be identical no change over time means everything's identical and evolution can't happen evolution can only happen if there's lots of different forms of something to choose from so to speak in that natural selection idea all right so non-disjunction is something we can see happening where look what happened in here or whether it's here it could happen in meiosis 1 or meiosis 2. it could happen in mitosis but if it happens in mitosis the consequences are almost none when it happens in meiosis it's more profound because it's going to screw up the dna in a gamete and that means if that gamete fertilizes something when that gam when that zygote undergoes mitosis every generation of cells is going to have that mistake in it so that's it so 100 trillion cells with a major mistake that's pretty profound whereas if this happened in mitosis it could just be a few cells that die and don't affect you so what happened here in meiosis 1 or meiosis 2 for that matter all of the chromatids got pulled to one side and being sep instead of being separate so here the homologous chromosomes did not separate here the sister chromatids did not separate so you got double the copies in one and none in the other in both cases see double the copies over there none to go into this cell over here so that's called nondisjunction and what does it look like happen here now there's a lot of different things that could happen but in general it's a microtubule issue either it didn't attach there or it broke over time or something like that it's a microtubule failure and that's going to result in all the chromosomes attached to the other one ending up in that cell and as i mentioned already at least in the animal kingdom we're very sensitive to chromosome and gene dose so to speak if you get a little bit too much it's enough to cause major problems if not the death of death cell usually the death of the cell especially if you consider the one that doesn't have that chromosome at all it's missing all of those genes chromosomes have hundreds to thousands of genes on each one missing any one of those could kill the cell let alone missing all of them so these are called monosomies because it only has it because it's missing something once a fertilization event occurs but what i want you to realize is these right here are gametes gametes don't know what dna they're carrying or how screwed up it is doesn't matter to them they're simply vehicles to get that dna to the next generation in a fertilization event and then once they get to that fertilization event the dna takes over starts trying to work and live and it can't because there's these kind of major problems too much dna or too little dna and that's going to cause the zygote to just die basically this non-disjunction event that affects gametes is what causes some very specific uh conditions one in particular you might be familiar with and that's down syndrome or trisomy 21. um you've probably even heard the statistics that you know at certain ages women are much more likely to have a trisomy child than a normal one you know the odds go up to one and four once you hit your 40s or something like that something crazy so it's extremely common uh and it always you know that's the odds for trisomy 21. but what about all the other chromosomes what about trisomy 20 trisomy 15 trisomy 1. those happen just as often as trisomy 21 but those are not survivable so if a fertilization results from gametes that have undergone non-disjunction at any of the other chromosomes other than 21 then that's going to be lethal and it's just not possible there's a couple exceptions to that but that's pretty much the rule trisomy 21s they obviously have some major issues as for chromosomal issues go so that there's way too many genes for that 21st chromosome they have you know two of everything else like they're supposed to but they just happen to have three because it would happen here and that one extra chromosome is enough to cause all those problems associated with that disorder but it is survivable so that's why it seems like it's so common common and the other ones aren't when in fact there's just this is happening for all the other ones too it's just not survivable unless it's the 21st chromosome and the sex chromosomes but we won't get into that otherwise very few exceptions the other thing that could happen that's a mistake or problem in this is that chromosomes can break during this process and if a chromosome breaks one or both of the broken parts could be lost and then the cell would be missing those genes or they might pick them up from another cell or something in this process again causing the wrong amount of dna and problems so that's pretty obvious to you but this is mostly about non-disjunction and things that could happen in meiosis so again if this happened i mean if you look at a freckle on your arm and the cells in that little spot one of those cells could have trisomy 21 but is it going to affect you if it's just one cell are you going to be considered down syndrome no that's just one cell that is involved in something else and is not every cell in your body if it happens to the gamete that means every cell in the body is going to be trisomic like that and that's the end of this chapter so uh get it down as usual