Meiosis for General Biology

welcome to the lecture on meiosis for general biology for the laramie county community college in a previous lecture we talked about the type of cell division called mitosis whose goal was to create two identical daughter cells today we'll be talking about meiosis with slightly different goals meiosis is the process of the cell that leads to the production of something called gametes these gametes in in animals and in plants we would know as egg and sperm cells these types of cells contain half the number of chromosomes of a somatic or of a cell dividing via mitosis so meiosis is really concerned with reducing the number of chromosomes in each cell this is because these gametes or these sex cells will go on to combine to form a new organism and when they combine they will add their genetic material together so what we are interested in with the process of meiosis is in creating what are called haploid cells these are cells that only have one set of chromosomes and we usually indicate that with the lower case letter in and we're going to differentiate the gametes those eggs and sperm from the somatic cells which in humans would be all of the other cells in our bodies all of these cells are what are called diploid meaning they have two sets of chromosomes we indicate that by putting the number two in front of the lower case n okay so gametes are typically the result of meiosis somatic cells are typically the result of mitosis so as a reminder mitosis also called binary fission in prokaryotes and in some organelles is an asexual cellular division the goal is simply to create an identical copy meiosis on the other hand is to create cells that are not identical copies now this is a meiosis is a process entirely linked with sexual reproduction for that reason we find it only in the eukaryotes not all eukaryotes but no prokaryotes so we now know that the primary goal of meiosis is reduction reduction of genetic material specifically the chromosomes but why do we want to reduce the genetic information here we're looking at the karyotype of homo sapiens or us notice that in our genetic material we have 46 total chromosomes okay now if to create gametes we used mitosis that would mean that we would have two copies of chromosome one in the gamete from mom and two copies of chromosome one from dad this would leave us with four copies of chromosome one or twice the normal amount so instead of doing that our cells are cells to produce gametes will undergo meiosis to cut this in half so that the gamete for mom is only contributing one chromosome one and the gamete from dad is only contributing one chromosome one that way together we get back to that 2 in situation now contrary to mitosis meiosis was actually named for the process so the greek my meaning to lessen or to make small mitosis remember was taking one parent cell and getting two identical daughter cells okay and through the entirety of mitosis we are staying diploid okay or 2n in meiosis we're taking that same diploid parent cell but this time we're going to create four haploid cells so each of these cells will have half the genetic material as the parent cell so as i said previously the goal the entire goal of meiosis is to create these haploid cells because these are the cells that will come back together during fertilization to start a new organism typically what this means is that sexually reproducing organisms will have this alternation of generations between a haploid stage and a diploid stage so let's look at that in a couple of organisms so when we take the example of animals it's interesting to note how little time they spend in this haploid generation okay so during sexual reproduction eggs are produced sperm is produced those two cells come together to create a diploid zygote at that point the zygote will undergo mitosis several billion times to create every cell and every organ in your body okay at the appropriate stage some of those cells will undergo meiosis to again produce the haploid gametes so animals which would include humans do not spend a lot of time in this haploid stage we really spend a brief moment there the majority of an animal's life is spent in this diploid stage where the cells are undergoing mitosis but animals are not the only organisms on the planet what if we compare and contrast plants and fungi well plants are interesting in that when the gametes come together the egg and the sperm we get fertilization this creates a zygote very similar in animals the zygote cells will undergo mitosis to create a larger plant at the appropriate time some of those cells will undergo meiosis where they will produce haploid cells now one thing to contrast with animals is that these haploid cells at this stage are not gametes they do not have the appropriate proteins the appropriate morphologies but these haploid cells will start undergoing mitosis okay so the animal haploid cells the gametes do not undergo mitosis the plant haploid cells do giving us a new organism a haploid organism that at the appropriate stage will modify its cells into gametes thus alternating to the diploid stage okay fungi on the other hand are sort of the complete opposite of the animals and two gametes come together they form a zygote and that zygote immediately forms haploid cells those haploid cells undergo meios mitosis and become the larger organism like the mushrooms that you might be familiar with okay so animals spend most of their lives in this diploid stage spend most of their lives in the haploid stage and plants seem to have more of a balance between the diploid and the haploid stages so what is going on in the actual process of meiosis and how does this compare to mitosis well we are going to use the same terms prophase anaphase metaphase but because meiosis goes through two cellular divisions we're going to refer to the individual stages as prophase one or prophase 2. so just based on this you should be able to tell the difference between when we're talking about meiosis versus mitosis okay so if i were to write prophase prophase one and prophase two this would indicate to you that prophase without a number is talking about mitosis prophase one would be talking about the first round of meiosis and prophase two would be talking about the second round of meiosis okay now just because we use the same terms does not mean that things will look the same especially when it comes to meiosis one meiosis ii does bear much more similarity to mitosis and we can see that just briefly in these and these two cells here we have metaphase and here we have anaphase so notice let's look down here at meiosis ii because we're more familiar with mitosis in meiosis in metaphase during mitosis the chromosomes line up all in a row they do that as well in meiosis ii in meiosis one though all of our chromosomes kind of bunch together and we get pairs of chromosomes that line up those pairs are then separated in anaphase one the cell splits during telophase then we go back through once more through prophase prometaphase metaphase ii now those chromosomes are lining up the same as they would look during mitosis and anaphase are sister chromatids are pulled apart so we'll want to keep track of chromosomes versus chromatids okay now this allows these differences allow for two major goals and one that we talked about already was reduction okay and when we talk about ploidy we're talking about chromosome number okay so we want to reduce the number of chromosomes so that when these gametes come back together the the resulting zygote will have the appropriate number of chromosomes but another thing that happens is crossing over okay so homologous chromosomes or in other words chromosomes are the same number so in humans you know chromosome 1 and chromosome 1 would be homologous chromosomes chromosome 2 and chromosome 2 would be homologous chromosomes this is because they have the same genes on each one of the chromosomes one of them just happened to come from your mom one of them happened to come from your dad okay what crossing over allows is for increased variation which remember from our lectures on natural selection variation is what allows species to adapt so when we look at crossing over and during prophase one what happens is that these homologous chromosomes line up together so here we have chromosome 1 from dad and here we have chromosome 1 from mom notice that these chromosomes have already gone through the first stage of division so they have duplicated the dna remember from our lecture on mitosis this means that each one of these chromosomes currently has two sister chromatids okay when these chromosomes are lined up next to each other it allows for this process of crossing over where genetic information from one chromatid is able to be exchanged with the genetic information from a non-sister chromatid meaning a chromatid from the other chromosome okay so now we can see in the simple example how much variation can occur because now through meiosis we are going to produce four different cells one of those cells will have the chromosome entirely from dead one of those gametes will have a chromosome that is mostly dad but a little bit of mom one of those chromosomes will have a chrome will have genetic information that is mostly mom but a little bit dad and then our fourth cell our fourth gamete will have a chromosome that is entirely from mom in this way we increase that genetic variation usually we think of this as adaptive because variation is great it allows us to adapt now sometimes this can be a problem because what if you know there are some truly beneficial genes here that increase the fitness of the organism and we just took them out of this chiami's genetic information well that's just biology that's one of the downsides that that will happen so to look at this this process just kind of to give you an overview okay so the cell is still going to go through a step of interface where organelles will divide the genetic information will be duplicated in and the chromosomes will form with their respective chromatids okay one and two one and two then during the first step of meiosis when the homologous chromosomes are lined up and crossing over is allowed the result of that will be two haploid cells and now i want to make sure to emphasize this because this is a point that's a little bit confusing okay so we take our initial diploid cell a one chromosome two chromosomes the genetic information is duplicated remember we're making the x's with the protein in the middle to keep them together all right so one chromosome one two chromatids okay and then those x's are separated into two daughter cells at the end of meiosis one so at the end of meiosis one each daughter cell is now haploid we have already reduced the genetic information okay and now we will go through a second round of meiosis meiosis ii to separate those sister chromatids the same as we would during mitosis so we won't spend too much time on the specifics because they they are very much the same features as mitosis they just look somewhat differently so for example prophase one the chromosomes are still condensing the nucleus is still dissolving but in this case we have homologous chromosomes pairing up and crossing over occurring during meiosis one and during metaphase all right instead of what we would see with mitosis where all of our chromosomes are lined up okay all in a single file line we see those homologous chromosome pairs lined up so we see the duplicate chromosomes here okay the microtubules the spindle fibers are still attaching they're still pulling apart and we can really see that during anaphase is one of our major differences okay so here is meiosis one okay metaphase those homologous chromosomes are paired up anaphase the entire chromosome is separated okay then in meiosis ii and mitosis the chromosomes themselves line up not in pairs this time and this allows for the chromatids to be separated during anaphase or anaphase ii if we're talking about meiosis and so anaphase very similar same with telophase okay important to recognize that we are still going through telophase and cytokinesis even at the end of meiosis one where we will now have two daughter cells okay and remember each of these daughter cells is now haploid meaning they each only have one set of chromosomes the chromosomes still have duplicated dna so we still write them as x's but they are still one chromosome so haploid okay meiosis ii very very similar to mitosis the nuclear envelope dissolves again the spindle fibers form the chromosomes align along that metaphase plate during anaphase ii the chromatids are separated from each other and then telophase ii that nucleus reforms following followed by cytokinesis okay so let's take a look at this visually if we review meiosis or excuse me if we review mitosis we start with a diploid cell the genetic information is duplicated so we draw our chromosomes as x's because each chromosome now has two sister chromatids those chromosomes line up during metaphase and then during anaphase the chromatids are pulled apart from each other resulting in two diploid daughter cells which are genetically identical to the diploid parent cell if we look at meiosis during prophase one we get this we still have the duplic we have still duplicated the dna so we still have our chromosomes in x's but at this stage we're putting our homologous chromosomes together what that means is in a human with 46 chromosomes during mitosis that metaphase plate will basically have 46 lines and during meiosis 1 it will have 23 lines because our chromosomes stay together in homologous pairs okay crossing over has already occurred that occurs in prophase one so then in anaphase one we now separate chromosomes and notice that some of these chromosomes have new genetic material because of crossing over but each one now has half the number of chromosomes they started with and so we have now created two haploid cells each of those haploid cells will now undergo another round of meiosis where we will end with four haploid cells because these chromosomes with their two sister chromatids split apart giving one chromatid and each of the four daughter cells so if we again compare mitosis whose goal is growth and repair with meiosis whose goal is production of gametes for sexual reproduction mitosis has one cell division whereas meiosis goes through two mitosis starts with one diploid cell and ends with two diploid cells meiosis starts with one diploid cell and ends with four haploid cells mitosis the daughter cells are genetically identical and this makes sense okay this is the process that say we use for when we have a scrape or when we have a cut on our knee right we don't want the skin the new skin cells to somehow be genetically different they are already doing their job we just need more of them okay but in meiosis where the goal is to make a new organism we do want some variation so that that new organism can adapt to a changing environment so now let's look at a quick bioflex video kind of summarizing the process of meiosis and if after this lecture you're still a little uh confused i would encourage you to watch both the mitosis and the meiosis bioflix videos back to back to help you compare and contrast the two different processes humans produce gametes eggs and sperm through the process of meiosis here we'll follow the production of male gametes by focusing on this cell as it goes through meiosis let's begin in the nucleus where genetic information is stored in chromosomes most of a person's cells are diploid with two sets of chromosomes one set is from their mother shown here in red and the other set is from their father shown in blue each maternal chromosome has a corresponding paternal chromosome these matched pairs are called homologous chromosomes during interphase chromosomes are duplicated each chromosome now consists of two identical copies called sister chromatids zooming in we see that each sister chromatid is made up of dna wound around histone proteins each strand coils up into a tight helical fiber as meiosis begins a spindle forms and duplicated centrosomes start to migrate toward opposite poles of the cell back in the nucleus the chromosomes are condensing in meiosis homologous chromosomes stick together in pairs the close association of homologous chromosomes allows segments of non-sister chromatids to trade places this recombination of maternal and paternal genetic material is a key feature of meiosis after the spindle forms and the nuclear envelope breaks down microtubules from opposite poles attach to each chromosome of the homologous pair resulting in a tug of war at metaphase one the chromosome pairs are positioned in the middle of the cell the next stage begins when homologous chromosomes separate from each other and move toward opposite poles each chromosome still consists of two sister chromatids this cell began meiosis with 46 chromosomes but each daughter cell now has only 23 chromosomes in meiosis ii microtubules from opposite poles attach to the chromosomes which then move to the center of the cell next the sister chromatids separate becoming full-fledged chromosomes that move to opposite poles nuclear envelopes reform and each daughter cell divides into two cells we started with a single diploid cell and now that meiosis is complete we have four haploid cells cells with a single set of chromosomes these haploid cells mature into gametes that can contribute to the next generation so now that we have a good feeling for the process of meiosis how it's similar to and different from the process of mitosis let's look at what happens when things could go wrong so we've talked in previous lectures about mutations and how changes in a particular gene depending on where that gene is can lead to some pretty serious differences and complications and development if it's a mutation in a control gene that can stop or speed up the production of protein now we want to look at larger scale chromosomal art alterations and so remember when we're talking about the chromosome we're talking about hundreds if not thousands of genes usually when this happens we get severe impacts most often these result in miscarriages some chromosomal alterations the zygote can survive but with a variety of developmental disorders so the first type of chromosomal issue that we'll talk about is a fairly common one called non-disjunction during non-disjunction the problem occurs during meiosis ii during the process of anaphase where the sister chromatids do not separate this results in one of the gametes having too many chromosomes some of you might be familiar with trisomy 21 where you have three copies of the chromosome and in the other gamete you have one too few usually this is this always results in a miscarriage the only exception is if the the issue arises in the sex cells and one of the x's is lost but one x still remains additions of chromosomes are slightly better odds so interestingly the odds of survival vary based on which chromosome has had an extra so for example chromosome number 16 is quite bad almost 30 percent of spontaneous abortions or of miscarriages are accounted for with issues in chromosome number 16 okay notice that we know that based on trisomy 21 down syndrome that having alterations in this chromosome are survivable but we still have a significant chunk of those which do not survive maybe this has to do with when it when the aberration presents or other developmental issues and talking about trisomy 21 and so this is when we have one extra chromosome number 21 okay rather interestingly the odds of non-disjunction go increase with the age of the mother okay and so the the biological reason for this is as that zygote okay so going all the way back to development as the as the zygote is developing the embryo is undergoing mitosis in in human females the egg cells are already starting to undergo meiosis meaning that when born all of the egg cells that you will ever have are already there waiting to finish meiosis now we see this the strong uptick in non-disjunction events after the age of 35 which is when most doctors would consider a pregnancy to be more high risk of non-disjunction and this is because those cells those gametes those the sister chromatids they have been together for 35 years and so it's just more likely that during anaphase ii they stay together resulting in non-disjunction events okay now moving away from anaploidy which is just having one extra chromosome we also have organisms that are polyploids meaning they have an extra set of chromosomes now for whatever reason extra genetic material seems to be very bad excuse me for animals plants are much more able to handle having duplications of the entire genome okay even having triploids tetraploids if you have ever eaten strawberries from the grocery store and those are octopoids meaning they have eight full sets of chromosomes okay now moving beyond uh non-disjunction events and looking at other alterations in the structure of chromosomes okay one of those could be a deletion where during the course of meiosis a an entire section of the gene of the chromosome gets deleted okay this can be quite a problem if there are control genes there or if there are important developmental genes there not always a good thing you can also have duplication where let's say an entire section of the chromosome suddenly gets duplicated okay we will look at this a little bit further later in the in the lecture because sometimes this can be beneficial because having this duplication now means that you can conserve the original copy and allow the extra copy to accumulate mutations and this can result in new functions or different functions or slightly better functions which we'll see in a little bit okay other ways that chromosomes can be altered you can have an inversion where this the stretch of dna is swapped on the same chromosome okay this can be pretty detrimental to an organism you can also have translocations translocations are essentially crossing over but for non-homologous chromosomes for example if you took this stretch of dna from chromosome 2 and put it onto chromosome 15 and then you take this stretch of dna from chromosome 15 and put it on to chromosome 2. okay it does not always result in miscarriage some effects of translocation can be minor but we'll talk about in a minute some can be pretty major okay so going back to those duplications and how they can be beneficial is that we basically have now an extra copy okay so if you have gene a and that's duplicated into two a's now evolution mutations natural selection can start acting on that second copy without losing the function of the original copy okay so if we take for example this deep sea fish we know from genetic research that they had a digestive enzyme which became duplicated hey so the first duplicate continued doing that function continued producing the protein for this enzyme and then the second one became slightly modified into an antifreeze protein this would allow these fishes to move into a new environment a new colder habitat where they could where they were better adapted perhaps had more fitness had a new food source or fewer predators but in some way this new function of the duplicate provided a beneficial outcome we know that in yeast the entire genome has been duplicated and even if in our own hemoglobin okay specifically the beta hemoglobin having the duplicate has allowed for sub functionalization of these of the beta chains to allow for different sections of hemoglobin to carry oxygen more efficiently okay so this would be an example of sub functionalism where it's still serving the same function [Music] just better if we go back to the translocations we talked about how these can be have minor consequences some of them can have major consequences and there you'll notice with a lot of these diseases there's a lot of cancers here and so for example uh larcel lymphoma if you take a chunk of chromosome 5 move it over to chromosome 2. leukemia taking chromosome 12 moving some of that over to chromosome 1. [Music] and these cancers are much different than the cancers that we've talked about previously because these are not the result of mitosis these are the result usually of meiosis meaning it's not just a single cell it's every cell in your body has the genetic information translocated and so it's not as simple as identifying the one spot where there's an issue it's every cell okay and so treatment of these cancers is much different than cancers that we've previously talked about okay so this is our lecture on meiosis so hopefully you learned that meiosis is a type of cell division that only occurs in eukaryotes that produce sexually that should say sexually okay hopefully you can now compare and contrast meiosis with mitosis and you can discuss somewhat the causes and effects of different chromosomal alterations

welcome to the lecture on meiosis for general biology for the laramie county community college in a previous lecture we talked about the type of cell division called mitosis whose goal was to create two identical daughter cells today we&#39;ll be talking about meiosis with slightly different goals meiosis is the process of the cell that leads to the production of something called gametes these gametes in in animals and in plants we would know as egg and sperm cells these types of cells contain half the number of chromosomes of a somatic or of a cell dividing via mitosis so meiosis is really concerned with reducing the number of chromosomes in each cell this is because these gametes or these sex cells will go on to combine to form a new organism and when they combine they will add their genetic material together so what we are interested in with the process of meiosis is in creating what are called haploid cells these are cells that only have one set of chromosomes and we usually indicate that with the lower case letter in and we&#39;re going to differentiate the gametes those eggs and sperm from the somatic cells which in humans would be all of the other cells in our bodies all of these cells are what are called diploid meaning they have two sets of chromosomes we indicate that by putting the number two in front of the lower case n okay so gametes are typically the result of meiosis somatic cells are typically the result of mitosis so as a reminder mitosis also called binary fission in prokaryotes and in some organelles is an asexual cellular division the goal is simply to create an identical copy meiosis on the other hand is to create cells that are not identical copies now this is a meiosis is a process entirely linked with sexual reproduction for that reason we find it only in the eukaryotes not all eukaryotes but no prokaryotes so we now know that the primary goal of meiosis is reduction reduction of genetic material specifically the chromosomes but why do we want to reduce the genetic information here we&#39;re looking at the karyotype of homo sapiens or us notice that in our genetic material we have 46 total chromosomes okay now if to create gametes we used mitosis that would mean that we would have two copies of chromosome one in the gamete from mom and two copies of chromosome one from dad this would leave us with four copies of chromosome one or twice the normal amount so instead of doing that our cells are cells to produce gametes will undergo meiosis to cut this in half so that the gamete for mom is only contributing one chromosome one and the gamete from dad is only contributing one chromosome one that way together we get back to that 2 in situation now contrary to mitosis meiosis was actually named for the process so the greek my meaning to lessen or to make small mitosis remember was taking one parent cell and getting two identical daughter cells okay and through the entirety of mitosis we are staying diploid okay or 2n in meiosis we&#39;re taking that same diploid parent cell but this time we&#39;re going to create four haploid cells so each of these cells will have half the genetic material as the parent cell so as i said previously the goal the entire goal of meiosis is to create these haploid cells because these are the cells that will come back together during fertilization to start a new organism typically what this means is that sexually reproducing organisms will have this alternation of generations between a haploid stage and a diploid stage so let&#39;s look at that in a couple of organisms so when we take the example of animals it&#39;s interesting to note how little time they spend in this haploid generation okay so during sexual reproduction eggs are produced sperm is produced those two cells come together to create a diploid zygote at that point the zygote will undergo mitosis several billion times to create every cell and every organ in your body okay at the appropriate stage some of those cells will undergo meiosis to again produce the haploid gametes so animals which would include humans do not spend a lot of time in this haploid stage we really spend a brief moment there the majority of an animal&#39;s life is spent in this diploid stage where the cells are undergoing mitosis but animals are not the only organisms on the planet what if we compare and contrast plants and fungi well plants are interesting in that when the gametes come together the egg and the sperm we get fertilization this creates a zygote very similar in animals the zygote cells will undergo mitosis to create a larger plant at the appropriate time some of those cells will undergo meiosis where they will produce haploid cells now one thing to contrast with animals is that these haploid cells at this stage are not gametes they do not have the appropriate proteins the appropriate morphologies but these haploid cells will start undergoing mitosis okay so the animal haploid cells the gametes do not undergo mitosis the plant haploid cells do giving us a new organism a haploid organism that at the appropriate stage will modify its cells into gametes thus alternating to the diploid stage okay fungi on the other hand are sort of the complete opposite of the animals and two gametes come together they form a zygote and that zygote immediately forms haploid cells those haploid cells undergo meios mitosis and become the larger organism like the mushrooms that you might be familiar with okay so animals spend most of their lives in this diploid stage spend most of their lives in the haploid stage and plants seem to have more of a balance between the diploid and the haploid stages so what is going on in the actual process of meiosis and how does this compare to mitosis well we are going to use the same terms prophase anaphase metaphase but because meiosis goes through two cellular divisions we&#39;re going to refer to the individual stages as prophase one or prophase 2. so just based on this you should be able to tell the difference between when we&#39;re talking about meiosis versus mitosis okay so if i were to write prophase prophase one and prophase two this would indicate to you that prophase without a number is talking about mitosis prophase one would be talking about the first round of meiosis and prophase two would be talking about the second round of meiosis okay now just because we use the same terms does not mean that things will look the same especially when it comes to meiosis one meiosis ii does bear much more similarity to mitosis and we can see that just briefly in these and these two cells here we have metaphase and here we have anaphase so notice let&#39;s look down here at meiosis ii because we&#39;re more familiar with mitosis in meiosis in metaphase during mitosis the chromosomes line up all in a row they do that as well in meiosis ii in meiosis one though all of our chromosomes kind of bunch together and we get pairs of chromosomes that line up those pairs are then separated in anaphase one the cell splits during telophase then we go back through once more through prophase prometaphase metaphase ii now those chromosomes are lining up the same as they would look during mitosis and anaphase are sister chromatids are pulled apart so we&#39;ll want to keep track of chromosomes versus chromatids okay now this allows these differences allow for two major goals and one that we talked about already was reduction okay and when we talk about ploidy we&#39;re talking about chromosome number okay so we want to reduce the number of chromosomes so that when these gametes come back together the the resulting zygote will have the appropriate number of chromosomes but another thing that happens is crossing over okay so homologous chromosomes or in other words chromosomes are the same number so in humans you know chromosome 1 and chromosome 1 would be homologous chromosomes chromosome 2 and chromosome 2 would be homologous chromosomes this is because they have the same genes on each one of the chromosomes one of them just happened to come from your mom one of them happened to come from your dad okay what crossing over allows is for increased variation which remember from our lectures on natural selection variation is what allows species to adapt so when we look at crossing over and during prophase one what happens is that these homologous chromosomes line up together so here we have chromosome 1 from dad and here we have chromosome 1 from mom notice that these chromosomes have already gone through the first stage of division so they have duplicated the dna remember from our lecture on mitosis this means that each one of these chromosomes currently has two sister chromatids okay when these chromosomes are lined up next to each other it allows for this process of crossing over where genetic information from one chromatid is able to be exchanged with the genetic information from a non-sister chromatid meaning a chromatid from the other chromosome okay so now we can see in the simple example how much variation can occur because now through meiosis we are going to produce four different cells one of those cells will have the chromosome entirely from dead one of those gametes will have a chromosome that is mostly dad but a little bit of mom one of those chromosomes will have a chrome will have genetic information that is mostly mom but a little bit dad and then our fourth cell our fourth gamete will have a chromosome that is entirely from mom in this way we increase that genetic variation usually we think of this as adaptive because variation is great it allows us to adapt now sometimes this can be a problem because what if you know there are some truly beneficial genes here that increase the fitness of the organism and we just took them out of this chiami&#39;s genetic information well that&#39;s just biology that&#39;s one of the downsides that that will happen so to look at this this process just kind of to give you an overview okay so the cell is still going to go through a step of interface where organelles will divide the genetic information will be duplicated in and the chromosomes will form with their respective chromatids okay one and two one and two then during the first step of meiosis when the homologous chromosomes are lined up and crossing over is allowed the result of that will be two haploid cells and now i want to make sure to emphasize this because this is a point that&#39;s a little bit confusing okay so we take our initial diploid cell a one chromosome two chromosomes the genetic information is duplicated remember we&#39;re making the x&#39;s with the protein in the middle to keep them together all right so one chromosome one two chromatids okay and then those x&#39;s are separated into two daughter cells at the end of meiosis one so at the end of meiosis one each daughter cell is now haploid we have already reduced the genetic information okay and now we will go through a second round of meiosis meiosis ii to separate those sister chromatids the same as we would during mitosis so we won&#39;t spend too much time on the specifics because they they are very much the same features as mitosis they just look somewhat differently so for example prophase one the chromosomes are still condensing the nucleus is still dissolving but in this case we have homologous chromosomes pairing up and crossing over occurring during meiosis one and during metaphase all right instead of what we would see with mitosis where all of our chromosomes are lined up okay all in a single file line we see those homologous chromosome pairs lined up so we see the duplicate chromosomes here okay the microtubules the spindle fibers are still attaching they&#39;re still pulling apart and we can really see that during anaphase is one of our major differences okay so here is meiosis one okay metaphase those homologous chromosomes are paired up anaphase the entire chromosome is separated okay then in meiosis ii and mitosis the chromosomes themselves line up not in pairs this time and this allows for the chromatids to be separated during anaphase or anaphase ii if we&#39;re talking about meiosis and so anaphase very similar same with telophase okay important to recognize that we are still going through telophase and cytokinesis even at the end of meiosis one where we will now have two daughter cells okay and remember each of these daughter cells is now haploid meaning they each only have one set of chromosomes the chromosomes still have duplicated dna so we still write them as x&#39;s but they are still one chromosome so haploid okay meiosis ii very very similar to mitosis the nuclear envelope dissolves again the spindle fibers form the chromosomes align along that metaphase plate during anaphase ii the chromatids are separated from each other and then telophase ii that nucleus reforms following followed by cytokinesis okay so let&#39;s take a look at this visually if we review meiosis or excuse me if we review mitosis we start with a diploid cell the genetic information is duplicated so we draw our chromosomes as x&#39;s because each chromosome now has two sister chromatids those chromosomes line up during metaphase and then during anaphase the chromatids are pulled apart from each other resulting in two diploid daughter cells which are genetically identical to the diploid parent cell if we look at meiosis during prophase one we get this we still have the duplic we have still duplicated the dna so we still have our chromosomes in x&#39;s but at this stage we&#39;re putting our homologous chromosomes together what that means is in a human with 46 chromosomes during mitosis that metaphase plate will basically have 46 lines and during meiosis 1 it will have 23 lines because our chromosomes stay together in homologous pairs okay crossing over has already occurred that occurs in prophase one so then in anaphase one we now separate chromosomes and notice that some of these chromosomes have new genetic material because of crossing over but each one now has half the number of chromosomes they started with and so we have now created two haploid cells each of those haploid cells will now undergo another round of meiosis where we will end with four haploid cells because these chromosomes with their two sister chromatids split apart giving one chromatid and each of the four daughter cells so if we again compare mitosis whose goal is growth and repair with meiosis whose goal is production of gametes for sexual reproduction mitosis has one cell division whereas meiosis goes through two mitosis starts with one diploid cell and ends with two diploid cells meiosis starts with one diploid cell and ends with four haploid cells mitosis the daughter cells are genetically identical and this makes sense okay this is the process that say we use for when we have a scrape or when we have a cut on our knee right we don&#39;t want the skin the new skin cells to somehow be genetically different they are already doing their job we just need more of them okay but in meiosis where the goal is to make a new organism we do want some variation so that that new organism can adapt to a changing environment so now let&#39;s look at a quick bioflex video kind of summarizing the process of meiosis and if after this lecture you&#39;re still a little uh confused i would encourage you to watch both the mitosis and the meiosis bioflix videos back to back to help you compare and contrast the two different processes humans produce gametes eggs and sperm through the process of meiosis here we&#39;ll follow the production of male gametes by focusing on this cell as it goes through meiosis let&#39;s begin in the nucleus where genetic information is stored in chromosomes most of a person&#39;s cells are diploid with two sets of chromosomes one set is from their mother shown here in red and the other set is from their father shown in blue each maternal chromosome has a corresponding paternal chromosome these matched pairs are called homologous chromosomes during interphase chromosomes are duplicated each chromosome now consists of two identical copies called sister chromatids zooming in we see that each sister chromatid is made up of dna wound around histone proteins each strand coils up into a tight helical fiber as meiosis begins a spindle forms and duplicated centrosomes start to migrate toward opposite poles of the cell back in the nucleus the chromosomes are condensing in meiosis homologous chromosomes stick together in pairs the close association of homologous chromosomes allows segments of non-sister chromatids to trade places this recombination of maternal and paternal genetic material is a key feature of meiosis after the spindle forms and the nuclear envelope breaks down microtubules from opposite poles attach to each chromosome of the homologous pair resulting in a tug of war at metaphase one the chromosome pairs are positioned in the middle of the cell the next stage begins when homologous chromosomes separate from each other and move toward opposite poles each chromosome still consists of two sister chromatids this cell began meiosis with 46 chromosomes but each daughter cell now has only 23 chromosomes in meiosis ii microtubules from opposite poles attach to the chromosomes which then move to the center of the cell next the sister chromatids separate becoming full-fledged chromosomes that move to opposite poles nuclear envelopes reform and each daughter cell divides into two cells we started with a single diploid cell and now that meiosis is complete we have four haploid cells cells with a single set of chromosomes these haploid cells mature into gametes that can contribute to the next generation so now that we have a good feeling for the process of meiosis how it&#39;s similar to and different from the process of mitosis let&#39;s look at what happens when things could go wrong so we&#39;ve talked in previous lectures about mutations and how changes in a particular gene depending on where that gene is can lead to some pretty serious differences and complications and development if it&#39;s a mutation in a control gene that can stop or speed up the production of protein now we want to look at larger scale chromosomal art alterations and so remember when we&#39;re talking about the chromosome we&#39;re talking about hundreds if not thousands of genes usually when this happens we get severe impacts most often these result in miscarriages some chromosomal alterations the zygote can survive but with a variety of developmental disorders so the first type of chromosomal issue that we&#39;ll talk about is a fairly common one called non-disjunction during non-disjunction the problem occurs during meiosis ii during the process of anaphase where the sister chromatids do not separate this results in one of the gametes having too many chromosomes some of you might be familiar with trisomy 21 where you have three copies of the chromosome and in the other gamete you have one too few usually this is this always results in a miscarriage the only exception is if the the issue arises in the sex cells and one of the x&#39;s is lost but one x still remains additions of chromosomes are slightly better odds so interestingly the odds of survival vary based on which chromosome has had an extra so for example chromosome number 16 is quite bad almost 30 percent of spontaneous abortions or of miscarriages are accounted for with issues in chromosome number 16 okay notice that we know that based on trisomy 21 down syndrome that having alterations in this chromosome are survivable but we still have a significant chunk of those which do not survive maybe this has to do with when it when the aberration presents or other developmental issues and talking about trisomy 21 and so this is when we have one extra chromosome number 21 okay rather interestingly the odds of non-disjunction go increase with the age of the mother okay and so the the biological reason for this is as that zygote okay so going all the way back to development as the as the zygote is developing the embryo is undergoing mitosis in in human females the egg cells are already starting to undergo meiosis meaning that when born all of the egg cells that you will ever have are already there waiting to finish meiosis now we see this the strong uptick in non-disjunction events after the age of 35 which is when most doctors would consider a pregnancy to be more high risk of non-disjunction and this is because those cells those gametes those the sister chromatids they have been together for 35 years and so it&#39;s just more likely that during anaphase ii they stay together resulting in non-disjunction events okay now moving away from anaploidy which is just having one extra chromosome we also have organisms that are polyploids meaning they have an extra set of chromosomes now for whatever reason extra genetic material seems to be very bad excuse me for animals plants are much more able to handle having duplications of the entire genome okay even having triploids tetraploids if you have ever eaten strawberries from the grocery store and those are octopoids meaning they have eight full sets of chromosomes okay now moving beyond uh non-disjunction events and looking at other alterations in the structure of chromosomes okay one of those could be a deletion where during the course of meiosis a an entire section of the gene of the chromosome gets deleted okay this can be quite a problem if there are control genes there or if there are important developmental genes there not always a good thing you can also have duplication where let&#39;s say an entire section of the chromosome suddenly gets duplicated okay we will look at this a little bit further later in the in the lecture because sometimes this can be beneficial because having this duplication now means that you can conserve the original copy and allow the extra copy to accumulate mutations and this can result in new functions or different functions or slightly better functions which we&#39;ll see in a little bit okay other ways that chromosomes can be altered you can have an inversion where this the stretch of dna is swapped on the same chromosome okay this can be pretty detrimental to an organism you can also have translocations translocations are essentially crossing over but for non-homologous chromosomes for example if you took this stretch of dna from chromosome 2 and put it onto chromosome 15 and then you take this stretch of dna from chromosome 15 and put it on to chromosome 2. okay it does not always result in miscarriage some effects of translocation can be minor but we&#39;ll talk about in a minute some can be pretty major okay so going back to those duplications and how they can be beneficial is that we basically have now an extra copy okay so if you have gene a and that&#39;s duplicated into two a&#39;s now evolution mutations natural selection can start acting on that second copy without losing the function of the original copy okay so if we take for example this deep sea fish we know from genetic research that they had a digestive enzyme which became duplicated hey so the first duplicate continued doing that function continued producing the protein for this enzyme and then the second one became slightly modified into an antifreeze protein this would allow these fishes to move into a new environment a new colder habitat where they could where they were better adapted perhaps had more fitness had a new food source or fewer predators but in some way this new function of the duplicate provided a beneficial outcome we know that in yeast the entire genome has been duplicated and even if in our own hemoglobin okay specifically the beta hemoglobin having the duplicate has allowed for sub functionalization of these of the beta chains to allow for different sections of hemoglobin to carry oxygen more efficiently okay so this would be an example of sub functionalism where it&#39;s still serving the same function [Music] just better if we go back to the translocations we talked about how these can be have minor consequences some of them can have major consequences and there you&#39;ll notice with a lot of these diseases there&#39;s a lot of cancers here and so for example uh larcel lymphoma if you take a chunk of chromosome 5 move it over to chromosome 2. leukemia taking chromosome 12 moving some of that over to chromosome 1. [Music] and these cancers are much different than the cancers that we&#39;ve talked about previously because these are not the result of mitosis these are the result usually of meiosis meaning it&#39;s not just a single cell it&#39;s every cell in your body has the genetic information translocated and so it&#39;s not as simple as identifying the one spot where there&#39;s an issue it&#39;s every cell okay and so treatment of these cancers is much different than cancers that we&#39;ve previously talked about okay so this is our lecture on meiosis so hopefully you learned that meiosis is a type of cell division that only occurs in eukaryotes that produce sexually that should say sexually okay hopefully you can now compare and contrast meiosis with mitosis and you can discuss somewhat the causes and effects of different chromosomal alterations

Transcript for:Meiosis for General Biology

Transcript for:
Meiosis for General Biology