Alright guys, so this is going to be our lecture on meiosis. So this is going to be a pretty short packet. In terms of material though, in terms of understanding things, this might be one of the harder things to grasp if you guys have never talked or studied meiosis before.
Alright, so we did talk about one type of self-division already. in our bodies what's called mitosis right so the idea of mitosis before i discuss meiosis so that we don't have to confuse them okay and kind of the main reasons why i put these in separate packets and i put an integumentary system lecture in between them right so that you can kind of clear ahead about what mitosis is and you don't confuse it with meiosis all right so before we talk about my meiosis let's talk about you mitosis right this idea right where we have this ability to replicate ourselves so the first cell we're going to draw here and again this is just review right okay we're going to draw a parent cell and this parent cell okay let's say has 46 chromosomes right through the process of cell division okay involving what well involving mitosis you And cytokinesis. So mitosis, again, is the division of the chromosomes, the division of the nucleus. And cytokinesis is the division of the cytoplasm.
So if this occurs, then what do you have? Well, then you end up with two cells. And these two cells will also have 46 chromosomes.
and we call these ones the daughter cells okay i'll just put dc for daughter cell so you guys recall the questions i asked you guys okay back in chapter two when we did mitosis are the two daughter cells identical to one another after cell division the answer is yes right are the two daughter cells identical to the original parent cell from which they came from okay again the answer is yes all right so we don't change anything Okay, all we're doing is we're going from one parent cell and we end up with two daughter cells. Okay, so we multiply. Okay, and if these two daughter cells were to go another round of cell division, well, they're going to end up with four cells.
And if those four cells underwent another round of cell division, well, they're going to end up with eight cells and 16 and 32 and 64. Okay, 128, 256, so forth and so on, right? So if I want to increase the number of cells, maybe these cells got lost. Okay, because I need to replace them. Think about your epidermis, right? And the cells in the stratum corneum start to flake off.
Well, we have cells in the stratum basal undergoing, okay, rounds of cell division to replace those cells that get lost on the apical surface of that epithelium. Or what if we have a sunburn, right? Well, we can replace the cells that got lost, that got damaged, okay, using cell division. So anytime I want to replace any type of aged, lost, or damaged cells, right, we have...
the ability via mitosis so i don't change anything i just double the number of cells now when we talk about meiosis now okay so this is mitosis right if i talk about meiosis right it's way different so one question i'm gonna ask you okay with mitosis okay or cell division involving mitosis here okay where does this occur in your body where does mitosis happen in your body it happens pretty much everywhere right in your skin there's mitosis right um liver cells that get old right mitosis right when you're growing and your body has to get okay bigger your bones have to grow longer you know that's mitosis so anywhere again you replace lost damaged cells or if the individual is growing that's mitosis so what's meiosis okay and how is meiosis okay different from mitosis well you Meiosis doesn't happen everywhere in your body. It only happens in one part of your body. Okay, now before I can ask you where that is, well then what does meiosis do? Why would I want to have a different kind of cell division?
Well, meiosis produces our gametes. Okay, this is how we produce our gametes. Okay, what are the gametes in our body? Well, if you're a male, what's the male gamete? Well, that's a sperm cell.
right well if you're a female right what's the female gammy well that's the oocyte okay which is the fancy name for the egg so if i want to produce the sperm or the egg okay you use a spec special type of cell division called meiosis. So I'm going to ask you the question though, well, where in your body does meiosis occur then? Well, again, if you're talking about a male, okay, that's occurring in the testes, right? Testicles. If you're a female, well, then that meiosis is occurring where?
Well, within the ovaries. So only in the testes in males and the ovaries in females is meiosis occurring, nowhere else. It can't be like... You know, I'm a keratinocyte in the stratum basal of this guy's epidermis, you know, and I've been doing this for the last X number of years, and I'm just tired of doing mitosis over and over and over again.
Well, I'm going to give something, try something new and do some meiosis in the skin. Well, no, you don't make sperm or egg in your skin, right? Meiosis only occurs within the primary gonads, the testes and the ovaries, right? So why does that have to happen?
Why do I need a special type of cell division to produce? the gametes. Why can't I just use mitosis to make the sperm or to make the egg? Well, let's pretend we didn't use meiosis, right? So what I'm about to draw is wrong, all right?
But if we were to use mitosis in the sperm or in the test of the ovaries to make your gametes, you're going to see what's going to happen here, all right? So let's say we start off with a cell, all right? And the cell has 46 chromosomes, right?
I'm going to use, okay, mitosis now. okay to make our gametes well what's mitosis gonna do it doesn't change anything okay look over to the left okay all we do is produce two identical daughter cells so what's gonna happen now right is you're gonna produce either let's okay let's say let's happen this let's make let's make it distinctive here okay put a tail in it okay so you're gonna make sperm cells okay that have 46 chromosomes though because we don't change anything all we do is replicate right So now you have this sperm cell that has 46 chromosomes, okay? I'm assuming mitosis is also making the eggs in the ovaries, okay? And now I have an egg or an oocyte that has 46 chromosomes because mitosis make that one also. So when the sperm fertilizes the egg, right, you're going to get a zygote, right?
And what's that zygote going to have now? How many chromosomes... is that zygote gonna have?
It's gonna have 92 chromosomes, right? Is that even human, right? Is that gonna be viable?
Is that embryo gonna make a fetus? Is that fetus gonna develop, okay, okay, into, right, a baby? The answer is no, okay? That's gonna be embryonic lethal. That's never gonna make it full term in terms of developing, okay, into a viable human being.
So by making sperm cells that have 46, okay, and eggs that have 46, because we didn't use meiosis, we use mitosis instead, because we didn't change anything. Well, then you start making now zygotes that have the improper number of chromosomes, you make gametes that have the improper number of chromosomes. So what type of meiosis? Okay, do I need? Okay, what do you think meiosis is going to do to make sure that I don't make sperm with 46 chromosomes and eggs with 46 chromosomes?
Well, meiosis is going to cut that 46 in half, right? So if we do the proper, okay, steps of meiosis, and I start off with a cell that has 46 chromosomes, okay, but instead of using mitosis to make my sperm or the egg, I'm going to use meiosis. Well, what are you going to end up with? Well, you're going to end up with now gametes, right?
that only have 23 chromosomes, right? You want to make gametes that have half the number of chromosomes. So that what? So that now the sperm cell that has 23 chromosomes, okay, fertilizes his egg, okay, that also only has 23 chromosomes. Well, then what do you make?
Well, then you make a zygote, right, that now has only 46 chromosomes. Again, this is where we started. Everyone listening to this lecture, this is where you started your life right there.
Okay, as a fertilized egg, a zygote. And that zygote through multiple rounds now of mitosis, okay, made the 100 trillion cells that are present in your body today. All 100 trillion cells came from that one zygote.
Through the vases what? Well, one sort of cells from two, two to four, four to eight. via the process of mitosis now.
So meiosis makes your gametes, okay, that have half the number. Okay, so if I'm an organism that has 46, meiosis is going to cut it down to 23. What if there was an alien that had 50 chromosomes, right? Well, then how many chromosomes are going to be in their gametes? 25. Half of 100, okay, did I say 50?
Yeah, 50. Half of 50 is 25, okay? If I'm another alien species that has 100 chromosomes, well, meiosis is going to cut it down to 50. So the job of meiosis is to cut your chromosome numbers, in this case total chromosome number 46, okay, down to 23. So that when this sperm that has 23 chromosomes fertilizes this egg that has 23 chromosomes, well then we produce a zygote that has 46. And then from there we can then build, okay, and multiply, okay, to produce then the 100 trillion cells that are in your body today. All right, so I think that's a good intro, okay, to start.
this packet here and again it's not that long it's only seven slides okay but in terms of if you've never heard this before okay this may be a little challenging for you all right so the life cycle of multi-celled organisms okay again it can change from having x number of chromosomes to y number chromosomes back to having x number of chromosomes all right so in this figure here you see what well you see here here's a male okay and here is a female Now, in the testes of this male, okay, we're producing sperm via the process of meiosis, right? Via, okay, or present in the ovaries of this female, right? We're having meiosis, and she's producing her eggs. So, one thing I want to introduce to you guys, we as adults are diploid, okay? What does it mean by diploid?
Well, we have two copies of every chromosome. We have two sex chromosomes. We have two chromosome ones, we have two chromosome twos, we have two chromosome threes, we have two chromosome fours, all right?
We have two copies of every chromosome. So again, if I highlight just one chromosome, right? Let's say chromosome one, just to make sure, okay?
We're just making it simple here, okay? So here's chromosome one, I'll shade it in. Well, this is the chromosome one that you got from your dad, right? And then here's another chromosome one here.
Well, this is the chromosome one, okay, that you got. from your mom and what's the name guys we give hey to matching chromosomes in this case chromosome one chromosome one from dad and chromosome one from mom well we call these homo okay homologous chromosomes all right homo means same right so let me ask you these questions because we talked about this when we did right the cell lecture you Are homologous chromosomes identical to one another? Okay, is chromosome one from dad, okay, identical to the chromosome one that we got from mom?
The answer is no, right? I describe them as being a pair of shoes, where, okay, this chromosome one from dad is kind of like the shoe that we got, okay, is the left, okay, foot, okay, and the chromosome one that we got from mom is the right foot, okay? They look very similar.
They have the same colorways, right? But what? We know that the left shoe and the right shoe are not identical because if they were identical, you can just arbitrarily pick any shoe, put it on either your right foot or left foot and it would fit.
So they're not the same. They're very similar, but they're not identical. Okay.
And that's how we describe homologous chromosomes. All right. So what do these look like again?
Right. When they replicate. So prior to cell division, right, they have to replicate. So you have what? Well, here's chromosome one from dad replicated.
Okay, and they're connected where? What do we call this structure again? Well, we call that a centromere.
Okay, here's chromosome one from mom replicated, again, connected at their centromere. So again, we still call these homologous chromosomes, right? But what do I call this?
Okay, what are these called? What are these called here? Okay, I'm just going to label one of them because I'm running out of space.
We call them sister chromatids. Okay, our sister chromatids. identical so when i replicated this chromosome one for mom did i make an identical copy of that chromosome one for mom the answer is yes so these sister chromatids are identical okay what do i name these two here okay same deal sister chromatids okay are these identical yes this search to chromatid from one from from dad is identical to that one also from dad so when you make a copy you make a perfect copy okay Alright, so why do we consider humans diploid?
Why as adults are we, okay, the word we use is diploid, okay, and we symbolize it to N, all right? N refers to the ploidy number. How many sets of each chromosome do we have?
So here I see how many copies of chromosome one. I see two, okay? If I were to draw chromosome two from mom and chromosome two from dad, okay? Well, here's two from dad. Here's two from mom, all right?
How many chromosome twos do we have? Well, we have two. One from mom, one from dad, okay? If I were to draw chromosome three, how many chromosome threes do we have? Okay, whoops, I replicated this one.
Okay, how many chromosome threes do we have? Well, we're gonna have two. Okay, we're gonna have a chromosome three from dad, how much chromosome three from mom. So the reason why we say we are diploid is because we have two copies of every gene. Okay, so what's our diploid number?
Well, 2n equals 46. Alright, so what's the job of meiosis then? Well, our job of meiosis is to take this, okay, diploid number and cut it in half. Okay, we cut it down into our haploid number, okay, symbolized by n. Well, half of 46 then is 23. So the job of meiosis is to go from 2n to n, to go from diploid to haploid, to go from 46 to 23, okay?
So let's say we only had three pairs of chromosomes, okay? So here's three from dad, okay? This is three from mom, okay?
We know that we have more than six. We have 46. okay but instead of looking at all 23 pairs i'm going to get just three pairs chromosome one chromosome two chromosome three so what meiosis is gonna do okay if this was me right and i'm making a sperm cell now right and this is what i have to choose from okay and here's the nucleus of that sperm cell well i'm gonna pick between the chromosome one that i got from my dad or the chromosome one that i got from my mom right so which one am i gonna pick okay so let's say i picked chromosome one for mom so i'm not gonna put both chromosome ones i'm just gonna put one of the two i gotta pick one of the two So I pick this one, okay? Then I go to chromosome two, okay?
Which one of these two am I going to package in my sperm cell? Chromosome two that I got from my dad or chromosome two that I got from my mom, okay? So let's say, okay, I'm going to put chromosome two from my dad that I got, okay? I'm going to pick this one and then I'm going to go to chromosome three, okay?
I'm going to pick between these two, all right? So let's say I put chromosome three from mom in here. So I pick this one. So you're going to cut it in half. Right?
So I have two to choose from, pick one. I have two to choose from, pick one. I have two to choose from, pick one. So me as an organism, okay, I have six chromosomes. I have one, okay, let's label them here.
I have one, two, three, four, five, six. Meiosis goes from six, I'm an alien organism right now, okay, to three, right? I picked this one, which would be chromosome, okay, the second one that I drew here. Okay, this one would be three.
Okay, and then this one here would be six. Okay. So of the six chromosomes, I picked two, three and six. That's exactly what meiosis is all about.
Okay, taking the full complement of your chromosomes. In this case, I'm an alien organism with six chromosomes. And meiosis is going to cut that in half down to three.
I go through each one, pick one. Okay, pick one, pick one. I pick this one, I pick this one, I pick that one. I'm never going to package both of them in the same sperm cell.
For example, I'm not going to put one from dad and one from mom in here. There's a problem with meiosis then if that happened. So hopefully, guys, you can understand about what we're doing here. Why is it different from mitosis? Why it only occurs in the testes and in the ovaries?
And why we need to do it. Okay, why do we have to even do meiosis in the first place? Because we've got to cut that diploid number of 46 down to 23 in our gametes.
So that when a sperm that has 23 chromosomes fertilizes an egg that had 23 chromosomes, we then produce a zygote that has 46 again. Because if we don't have meiosis, we're going to just double the number of chromosomes every generation. Okay, and then through the process of mitosis, okay, we can now replicate.
okay and produce the 100 trillion cells that are present in our body as an adult today okay all right so let's go back to this figure here so here we see two diploid okay adults here's a diploid male he has 46 chromosomes here is a diploid female she has 46 chromosomes so when meiosis within the testes occurs well then he produces Okay, a haplogamy. This gammy only has 23 chromosomes, right? When in her ovaries, she has my... she starts producing oocytes or eggs that only have half of 46 they only have 23 so when this sperm that has 23 fertilizes that egg that have 23 well then you produce a zygote that is now diploid again this this zygote has 46 chromosomes okay chromosome one from dad chromosome one from mom chromosome two from dad chromosome two from mom chromosome three from dad chromosome three from mom so forth and so on okay and Let's say this zygote got an X from dad and an X from mom.
Well, then this is going to be a baby girl. But if this zygote got an Y from dad and an X from mom, well, this is going to be a baby boy. All right? And then through the process of mitosis, this one cell we call a zygote, this embryo divides, divides, divides, divides, divides, divides via mitosis to produce the 100 trillion cells that are in our bodies today as an adult.
Okay? So humans as adults are diploid. We have two copies of every chromosome. Chromosome that we got from our mom and dad all the way down the line. And what?
Our gametes are haploid. Half of that. They only have one copy of every chromosome. One copy of chromosome one.
One copy of chromosome two. One copy of chromosome three. One copy of chromosome four. So forth and so on. So when they come together, two copies of chromosome one.
Two copies of chromosome two. Two copies of chromosome three. two copies of chromosome four so forth and so on okay so what we're going to highlight in this lecture is what happens here how do i go from being having 46 diploid how do i produce now gametes that are haploid only have 23 chromosomes all right so here i like to call this a roadmap okay of what meiosis is all about i like to call it a roadmap Because it kind of outlines exactly what happens to produce those haploid gametes. So here in this figure, we see a cell that has two copies, let's say, of chromosome 1. So let's say the blue one is chromosome 1 that you got from your dad. The red one is chromosome 1 that you got from your mom.
So this is a diploid cell, two copies of chromosome 1. Now, what has to happen with meiosis is pretty much the same that has to kind of happen before mitosis. Okay, and what had to precede mitosis? What did you have to do before you did mitosis? Okay, I'll give you about five seconds.
What did you have to do? You had to replicate your DNA. Okay, it's the same thing here. Before you do meiosis, you have to replicate your DNA. Okay, so here, okay, here's meiosis over here on the right.
This is before meiosis, right? So before meiosis, we replicate our DNA, okay? So look at mom's chromosome one, replicated. Dad's chromosome, replicated, okay? So again, we have these sister chromatids here, all right?
We have those sister chromatids. So again, these are still considered homologous. These are sister chromatids, okay, here and here, okay?
Now we can enter meiosis. So someone tell me what happens here. Well, before I even do that, one of the big things in terms of difference between mitosis and meiosis, okay, is that one, well, the daughter cells are not identical to the original parent cell, right?
The daughter cells are not identical to one another either. And when I talk about meiosis, there's actually two successive rounds of cell division, okay? There's actually a meiosis one, okay? And this one's going to have a prophase one, a metaphase one, right? And an anaphase one.
and right after that in cytokinesis and right after that okay we're not done we can then go to a meiosis two which is gonna have a prophase two a metaphase two an anaphase two and a telophase two and then we're done so there's a meiosis one and there's a meiosis two okay in this whole thing we're calling meiosis so can someone tell me okay virtually i guess What happens guys between, okay, in meiosis 1? Okay, so here I see my replicated homologous chromosomes, right? So what happened between here and here? What just happened from here to here?
What did I separate? Well, you separated homologous chromosomes. So the job of meiosis 1 is to what? Separate homologous chromosomes. Remember that, right?
Don't just memorize that, understand that. Okay, because remember, we want to now go from having two copies of chromosome one to cells that only have one copy. copy of chromosome 1. And the first step to do that, we're going to separate homologous chromosomes first.
Okay? Now, is this something you want to slap a tail on and call it a sperm cell? Is this something you want to call an egg?
No, because it still has two copies of chromosome 1 in it, right? It still has a sister chromatids attached at their centromeres. So what do I have to do, okay, to make this done, to make it complete?
Well, you still have to separate sister chromatids. You still have to separate sister chromatids. And that's exactly what happens in meiosis II. The job of meiosis II is to what? Separate sister chromatids.
Separate sister chromatids. Then you're done. Look at these cells.
How many chromosome 1s do you see in this sperm or this egg? Just one. We started off with how many? Two.
Now we're down to one. Remember, the job of meiosis is to cut it in half. So if I start off with two, cutting it to one, how many do I see in here?
One. How many do I see in here? One. How many do I see in here? One.
So we first separate homologous chromosomes in meiosis one. We then separate sister chromatids in meiosis two. Now we produce haploid gametes. These are the things that only have one copy of chromosome one.
Now this is an overly simplified figure here, right? Because how many pairs of chromosomes? Okay, do we typically have when this is happening? We have 23 pairs, right, to make our 46 chromosomes.
There are 23 pairs of chromosomes, okay, that are undergoing this simultaneously. To go from 46 chromosomes down to only 23 chromosomes at the very end of meiosis, okay? But I think people's heads would just explode if there were 46 chromosomes over here, all right? So for you to understand this, okay, again, let's take baby steps. Let's start off with just one pair.
And then we'll go to two pairs and three pairs and four pairs. Ultimately, you can picture in your head 23 pairs doing the same exact thing. All right.
Okay, so these things here, guys, we're going to do that. Okay, on our on our pseudo pseudo guys, I'm hearing like whiteboard. Okay, pseudo is false or fake whiteboard.
all right this would be the time in my lecture that i would actually step off to the side okay stop looking at the powerpoint and start writing on on the whiteboard with my dry erase markers all right so we're going to go over here okay and basically just draw okay what i would normally draw on the whiteboard in lecture okay so we're going to start off with a cell called a myocyte okay and this is going to be a diploid cell Alright, so it's gonna have two copies, okay, of every chromosome here. Now before we do that, okay, let's come up with a kind of like a little key here. Erase things.
Let's go. Alright, so our little key here, we're going to actually stick with some of these colors, alright? But we're going to draw our genetic material first in the form of chromatin, much like we did for mitosis, guys, okay? We're also going to draw it in the form of chromosomes, okay?
So, starting off, let's say we're dealing with chromosome 1 from dad, okay? In the form of chromatin, we'll draw it as a line, alright? That's shaded in.
And then, okay, one from mom, right, we're going to draw as a line again, but not shaded in. All right, we're going to draw a second pair, and we'll do two also, right? So two from dad will be shaded in, but blue, okay? Actually, let's use a different color. I think black and this navy blue might be a little too close in color.
Okay, we'll use green, all right? so here now is two from dad right and this here will be two from mom all right so that's what they look like when they're chromatin okay so what about chromosomes so for chromosomes okay much like we drew over there already okay but we're reestablishing our key here this will be one from dad this would be one from mom okay this will be two from dad okay and this will be two from one okay and again if they replicate they will look something like this okay so again they're still homologous chromosomes even if they're replicated okay but if i compare okay these two together again we call these sister chromatids these would be sister chromatids okay so there's a little a little legend or a little key over there okay So here's my myocyte. So we're not dividing it, right?
So should I find chromosomes or should I find chromatin in the nucleus of this myocyte here, right? Well, you should be able to find chromatin, right? We're not dividing it.
So we're gonna have one from dad in here. We have one from mom. Okay, we're gonna have two from dad in here.
Okay, and we're gonna have two from mom. Right again, I'm only drawing two. of the of the 23 pairs of chromosomes okay again i'm even simplifying meiosis right in this little presentation all right so what do we have to do prior again to cell division okay well we have to replicate our dna okay so you have dna replication so here's my myocyte here's my nucleus i'm gonna draw a little bigger so i can actually fit things in here right here's one from dad replicated right here's one from mom replicated here's two from dad replicated right here's two from mom replicated so we have dna replication prior to meiosis right prior so now after we replicated we have then the green light to enter meiosis.
All right, so the first meiosis we're going to talk about though, okay, is going to be meiosis 1. Okay, do you guys remember what happens in meiosis 1? What do we separate? Well, we're going to separate homologous chromosomes, okay?
That's important for you to know because you have to figure out what happens in anaphase of meiosis 1. Well, if you know this, you know what's going to separate that, right? So the first part of meiosis 1 is going to be prophase 1, okay? Now, in prophase 1, it's going to be very similar to what happened in prophase of mitosis, okay, but a little bit different, right?
So here's my nuclear envelope, all right? One thing that's going to happen is that the chromatin will condense to form chromosomes. Okay, so here's dad's one, right? We're going to make it look like chromosomes now. So we're going to draw sister chromatids, again, attached at their centromeres.
We're going to shade this in, so that's dad's one. Okay, mom's one also condenses. Now, instead of me having me, might as well, I'll just erase.
Okay, here's mom's one. Okay, condensed, chromatin condenses to form chromosomes. Okay, and then we have dad's two. condensing to form chromosomes okay and then we have moms too condensing to form chromosomes all right so that's one thing that happens all right another thing that happens is that the nuclear envelope disintegrates so it's very similar to mitosis so far right so nuclear envelope disappears Okay, and then mitotic spindles extend, okay, and push the central somes to opposite poles of the cell.
Okay, so here I'm already drawing it, okay, with the microtubules already pushing the central somes to opposite poles. Now, before I draw the spindle fibers attaching to the chromosomes, okay, another thing that happens in prophase, one, that's different between prophase and mitosis. is that homologous chromosomes pair up.
They form what are called tetrads in prophase I, right? So here I kind of drew dad's chromosome I a little bit far from mom's chromosome I. So what I'm going to do is I'm actually going to draw them right next to one another, okay?
So instead of looking like that, okay, mom and dad's chromosome I's are going to pair up, okay? So here's mom's one, or excuse me, dad's one, okay? And here's mom's one, okay? So they paired up with one another. They're right next, as opposed to these in green, they're kind of far apart.
So homologous chromosomes pair up. during prophase one. So it's kind of the same thing here, okay?
We're going to pair up, okay, dad's chromosome one, I'm sorry, dad's chromosome two, I'm sorry, with mom's chromosome two, okay? Homologous chromosomes pair up, okay? They form what are called tetrads, right? Tetra means four, right? So one, two, three, four.
This right here would be considered a tetrad right there. Now, The mitotic spindles, okay, they are gonna then also, oops, okay, attach, okay, but instead of spindle fiber from each centromere attaching, okay, to the chromosomes, you're only going to get one from each side, okay, so here, let's say we look like this, like that, we're going to grab this one, we're going to grab that one, okay, so that is prophase one, so let's recap what just happened here, right. Chromatin condense to form chromosomes. Okay.
Nuclear envelope disintegrates. Mitochic spindles push central somes to opposite poles. Homologous chromosomes pair up and form tetrads. Okay.
Again, it's time the mitotic spindles, right, okay, attach to our chromosomes here. Now, in metaphase one, okay, I got to give a little bit of a space here. Okay.
I'm actually going to draw two different ways. these two pairs of chromosomes can line up. Okay, so I'm gonna put the word or here.
Okay, if they, if it does the way that I drew it in this prophase one, where are you going to see this? Okay, so you're going to see, for example, right, again, when we think about metaphase, we think about a metaphase plate. Okay, so dad's Chromosome 1 lines up on one side.
Oops, sorry. So before I even do this, one thing to note, homologous chromosomes line up along the metaphase plate. Mom's pair lines up on the other side of the metaphase plate. So what I see here are homologous chromosomes lining up along the metaphase plate.
So that's what happens in metaphase 1. homologous chromosomes line up along the metaphase plate. So also with dads, okay, actually the way I drew this on the bottom is incorrect. Okay, so let me let me correct this here.
Okay, so we're gonna see this actually. There we go. Okay.
So a little, little mistake drawing those spindle fibers. Okay. So mom's chromosome one and dad's chromosome one, homologous chromosomes line up along the metaphas plate. Okay.
Same thing, right over here, right? You're going to see then dad's chromosome two on this side of the metaphase plate. Okay. You can see mom's chromosome two on that side of the metaphase plate.
Okay, so the main thing that happens again during metaphase one, homologous chromosomes line up along the metaphase plate. Now, if these mitotic spindles attached in a different way, okay, it's possible that, okay, we could have seen this. Okay, so here is just another cell.
Okay, here's my metaphase plate. Okay, we could have keep the black ones the same. Okay, so here's dad's chromosome one on this side of the metaphase plate. Here's mom's chromosome 1 on the opposite side, so homologous chromosomes line up. Now, what if I just did this?
What if I just put dad's chromosome 2 down here, and then mom's chromosome 2 up here? So instead of having the shaded one on top, dad's 2, we had the shaded 2 on the bottom. So dad's chromosome 2 now is on this side of the metaphase plate, and mom's chromosome 2 is on the other side of the metaphase plate. Okay, does that still fulfill homologous chromosomes lining up along the metaphase plate? Okay, and the answer is yes, right?
So as long as homologous chromosomes line up, okay, you don't really care how they line up. So here, okay, one and two could have lined up like this on top, or one and two could have lined up like this on the bottom. Okay, so there's two different ways that this could have lined up. Okay, so what I want you to write right now on your lecture slides pertaining to metaphase one is that what?
Okay, non-homologous chromosomes. So this is chromosome one in black, and this is chromosome two in green, right? These are non-homologous.
Black and green are non-homologous. So you're writing in your lab, in your lecture slides right now, under metaphase one, non-homologous chromosomes independently assort during metaphase one. Non-homologous chromosomes independently assort. during metaphase one of meiosis one okay which is which is represented in this picture here right so you can have it lining up homologous chromosomes like what we see on top or you can have the homologous chromosomes lining up what we see on the bottom black and green do not care how each one is lining up okay it's not like dad's chromosome two is looking over over its shoulder saying well how did you line up right so here Here's dad's chromosome 2. It's not looking at dad's chromosome 1 saying, oh, are you on this side? I'll be on the same side as you.
We're buddies, right? Or it's not mom's chromosome 1 looking over and saying, oh, where's mom's chromosome 2, right? They don't care, okay? So we can line up like that, or we can line up what we see on the bottom here.
They are independent of one another. All right, so of these two, okay, I always ask my class which one do you want to follow okay because I don't follow both of them okay which whichever one we don't follow you may want to try at your home to see if you can figure it out okay so traditionally I hope my students want to pick the bottom one here because it's a little bit more complicated right here right here it's dad and dad on the same side and mom and mom the same side here here it's a little switched it's a little bit more complicated okay so we're going to follow this bottom one here you may want to do the top one on your own to see if you can figure it out okay so after metaphase one well then you have what's called anaphase one okay so we're following the bottom one okay so one thing that happens okay are the mitotic spindles that are connected okay and opposite central somes they get longer so we're going to stretch our cell again okay we're going to go from having an a spherical cell to more of an oval cell all right again that's that's because what well that's because okay these mitotic spindles are getting longer and then pushing the central soaps the opposite poles of the cell we're stretching our cell okay now these mitotic spindles here and here these ones will get shorter okay and by getting shorter what's going to happen now is you're going to now pull and separate the homologous chromosomes from one another right so here mom and dad's chromosome one will separate so here's dad's one going this direction Here's mom's one going in the opposite direction, right? These ones will get shorter.
So here's mom's two getting pulled in that direction. Here's dad's two getting pulled in this direction over here, okay? So what happens in anaphase one?
Well, in anaphase one, right, homologous chromosomes, right, separate. Why? Well, what's the purpose of meiosis I?
Separate homologous chromosomes. Well, homologous chromosomes separate in anaphase I, all right? Now, after anaphase I, we're going to have telophase I.
I'm going to combine this already with cytokinesis, okay? So that's cytokinesis, guys, all right? And the telltale sign of cytokinesis, again, is the cleavage fro.
right so i'm going to already draw the cleavage furrow forming right here right so there's our cleavage fro forming all right what else it's pretty much the opposite now right so our mitotic spindles will retract our nuclear envelope is going to reform all right and then our chromosomes would decondense to form chromatin okay so here's dad's chromosome one okay still replicated right here's mom's one okay still replicated up here is mom's two and down here is dad's two okay well then ultimately we know this already okay is that cleavage furrow okay will meet on all sides Okay, we then separate the two cells from one another. All right, we are done with meiosis two. What did we do?
We separated homologous chromosomes. So again, this is not where we would want to stop, right? We wouldn't want to stop here, okay? Because, okay, if I put a slap of flagella on this, this is not something I want to call a sperm cell right now.
It still has two copies of chromosome one in it, or chromatin, right? It still has two copies, right, of two, okay? I want to separate sister chromatids now, okay?
So that leads us now to the next part, okay? So all I'm going to do now is i'm gonna take this right here okay so actually it's just the copy paste right here do a screenshot i'll take this i'll put it over here Okay, so there's our products, guys, of meiosis 1. All right, so now we're ready to do what? Well, now we're ready to enter meiosis 2. Okay, so remind me, what happens in meiosis 2?
What's the whole purpose of this part again? So meiosis 1 separated homologous chromosomes, what's meiosis 2 going to do? Okay, well we want to separate sister chromatids, all right? So we're going to have a prophase 2 right now. Let's draw that, all right?
So each one of these two cells is going to undergo, right, prophase 2. So we'll draw one cell here, okay? We'll draw the other cell here, okay? And we'll progress.
Alright, let's actually do this. Give me myself a little bit more room here. Okay, so here's one cell here.
That's another cell here. Alright, so there's our nuclear envelope. There's our nuclear envelope, right?
And then mitotic spindles, okay? So one thing, again, chromosomes condense to, okay? Chromatin condense to form chromosomes, okay? So here's one from dad, condensing to form chromosomes, okay? Here's one from mom.
condensing to form chromosomes here's two from mom condensing to form chromosomes here's two from dad condensing to form chromosomes okay all right another thing that happens nuclear envelope disintegrates okay so let's go erase that nuclear nuclear envelope In addition to the nuclear rope disintegrating, we're also going to then do what? We're going to now also extend our microtubules. So microtubules from opposite centrosomes push each other far apart from each other, the opposite poles of the cell.
In addition to that, we're going to have mitotic spindles that now attach to the chromosomes. So here we're going to see... mitotic spindles okay attaching to each one of the chromosomes here okay opposite poles let me start to see this all right so that's pro phase one so highlighting that again right chromatin condenses to form chromosomes nuclear envelope disintegrates mitotic spindles push the centrosomes to opposite poles of the cells and then mitotic spindles from both centrosomes opposite centrosomes attached to our chromas chromosomes Alright, so the next one now is metaphase 2, okay? Something has to line up along the metaphase plate, okay?
So what do you think we're going to line up along the metaphase plate, okay? For this one, okay, metaphase 1, homologous chromosomes lined up along the metaphase plate. Why? Because we want to separate them in anaphase. So if I want to separate sister chromatids in anaphase 2, what do you think I have to line up along the metaphase plate, okay, of metaphase 2?
You better line up sister chromatids if you eventually want to separate them, right? So here is a cell, there is the metaphase plate that will not label, okay, but that's a metaphase plate. Okay, just pick something to make an eraser.
Okay, there there's another cell. Here's the metaphase plate here All right, so sister chromatids line up. So we're going to have now dad's one, sister chromatids line up along the metaphase plate.
Down here, mom's one, okay, sister chromatids line up along the metaphase plate. Up on this cell, mom's two, sister chromatids line up along the metaphase plate. And up here, mom's two, I mean dad's two, I'm sorry.
lines up along the metaphase plate. Again, manipulating all this would be these, okay, central somes and their microtubules here. Okay, so what happens in metaphase two?
Sister chromatids line up along the metaphase plate. All right, anaphase two. Well, if sister chromatids line up along the metaphase plate in metaphase 2, well, then we're going to separate sister chromatids in anaphase 2, all right? So one thing, again, the cells will stretch. They'll get longer, all right?
And the reason for that, again, is that we have cells or some central somes from opposite poles, okay? Microtubules growing longer, thereby stretching your cell. Okay, well then these microtubules though here, here, okay, will get shorter.
Well, if they get shorter then, well then you're going to separate sister chromatids. Alright, so this top one here, we're going to separate dad's one, sister chromatids separate. Down here, we're going to separate mom's one, sister chromatids separate.
Back up here, okay, we're going to separate moms two, sister chromatids separate, okay, and then down here, okay, we're going to see dads two, sister chromatids separate, okay. So what happens in anaphase two, sister chromatids separate, all right. Now we're going to combine telophase two with cytokinesis.
All right, so up here we're going to draw something, okay, that has a cleavage furrow forming. Okay, down here, same deal. We'll draw something with a cleavage furrow forming.
All right, so mitotic spindles will retract. Okay, we're going to reform our nuclear envelope. and we're going to decondense chromatin back to chroma some chromosomes back to chromatin okay so these two cells have dads one so instead of looking like chromosomes they're gonna look like chromatin now these cells here are gonna have moms one okay we're gonna draw in the form of chromatin okay these cells up here will have moms two we'll draw in the form of chromatin okay and down here these cells will have dads two we'll draw in the form of chromatin all right Now ultimately, okay, we're going to do what? The cleavage furrow on all sides will come together and meet, okay? And when that happens, we're going to separate our cells from one another, okay?
So here's one cell, here's another cell, okay? So how many copies of chromosome 1 do I see, right, in this first cell over here, right? I only see one chromosome 1, okay, in the form of chromatin.
How many 1s do I see in this one? Just one. Dad's 1. Okay, how many 1s do I see in this one?
Just one. Mom's 1. How about this one? Just one. Mom's 1, right?
How many 2s do I see in this cell here? Well, just one. Well, it just happens to be Mom's 2. Mom's 2. Over here, dad's two, dad's two. So you'll get one black and one green, one black, one green, one black, one green, one black, one green.
You only have one copy now of chromosome one, one copy now of chromosome two. Because remember their initial myocyte, you had two blacks and two greens, okay? Because we had two copies, okay, of blacks and two copies of greens, we were diploid. We were 2n. So by separating homologous chromosomes first, and then separating sister chromatids second, you end up with what?
Well, now these are our gametes, okay? And these gametes that we just produced, okay, are now haploid. They're just n. 1, 1, okay? One copy of chromosome 1, one copy of chromosome 2, again, the form of chromatin, okay?
One black, one green. One black, one green, one black, one green. So is this something now I would slap a tail on and call it a sperm cell?
Yeah. Okay. What if this was in the ovaries? Is this something I would call eggs?
Yeah. So we went from having four chromosomes, right? One, two, three, four, to now only having two, right?
And that's the job of meiosis, okay? Cutting four down. to two in this case in real life cutting 46 down to 23. okay so what these slides explain okay meiosis one meiosis two is pretty much everything that we drew on our pseudo whiteboard all right and this last slide what it does it just compares mitosis with meiosis Again, mitosis, again, occurs where in your body?
It occurs everywhere, right? Anywhere that you have to grow, anywhere that you have to replace cells that were lost, anywhere you have to replace cells that are old, right? And then you have meiosis.
Where does that occur? Well, that occurs only in one part of your body within your primary gonads. If you're female, this occurs in the ovaries.
This is, if you're male, this occurs in your testes, right? So why do I need a special type of cell division in my ovaries or my testes? Well, because that's where you produce your gametes. That's where you produce your...
haploid gametes right instead of having okay two copies of every chromosome you only have one copy of every chromosome instead of having 46 chromosomes you only have 23 chromosomes one copy of chromosome one one copy of chromosome two one copy of chromosome three one copy of chromosome four one copy of chromosome five one copy of chromosome six one copy of chromosome seven one sex chromosome right So hopefully, guys, this clarifies, okay, the difference between mitosis and meiosis, why you even need a special type of cell division, right, called meiosis. All right, so have a good day. I'll see you guys on the next video.