If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content
Current time:0:00Total duration:9:45
AP.BIO:
IST‑1 (EU)
,
IST‑1.F (LO)
,
IST‑1.F.1 (EK)
,
IST‑1.H (LO)
,
IST‑1.H.1 (EK)
,
IST‑1.H.2 (EK)
,
IST‑1.H.3 (EK)

Video transcript

in the last video we had just started to get into meiosis and to be more precise meiosis wanted to be even more precise in that prophase one but we spent a good bit of time on prophase one because some interesting things happened some things happen just like prophase in mitosis where the nuclear envelope disappears or starts to disappear you have the the chromosomes for going into their dense form that has kind of this classic shape that you could see from a microscope but what was unique or what was interesting about meiosis 1 and prophase one in particular is that you have this chromosomal crossover that it's a pretty typical thing to happen in meiosis 1 and it happens an affair or tends to happen in a fairly clean way where homologous sections of these homologous pairs cross over so they tend to they these sections of the chromosome tend to code for the for the for the same genes they're just different variants of those same genes they might have different alleles and once again this just adds more variation as we get into sexual reproduction so it's a kind of a neat thing that happens here but now let's continue with meiosis and in particular meiosis 1 and you could guess what the next phase is going to be called it is metaphase 1 metaphase metaphase 1 and it has some similarities with metaphase in mitosis so in metaphase 1 so let me draw my cell so this is the cellular membrane right over there I have my centrosomes which are now going to play more significant roles the nuclear membrane is now gone and just like in metaphase in mitosis my chromosomes are going to line up along the at along here I'll draw it kind of this up/down axis so let's do that so you have this one right over here this is one chromosome to sister chromatids and we had the chromosomal crossover so it has a little bit of pink here I'm going to take a little bit of time to switch colors a little bit more frequently and then you have the one at least most of which you got from your mother and but there's been a little bit of chromosomal crossover over here as well so draw that let me draw that and then you have this one and just for the sake of so you have this one this chromosome from your father it is replicated so it's now two sister chromatids and this one from your mother and I'm not going to show the chromosomal crossover here maybe didn't happen over here no homologous recombination over here so these are I guess shorter now let me draw the centromeres the centromere is I've started doing in this blue color so the centromeres the centromeres and then the centrosomes you have these microtubules that start they can push the centrosomes away from each other but they also attach at the kinetic ores to the chromosomes to the chromosomes just like that so and these are you know the microtubules you'll see people just talk about oh these connect and they and they able to move things around I find this incredible that you just have a bunch of proteins through just kind of chemical and-and-and and thermodynamic processes are able to do really interesting things like like move chromosomes for two different parts of the cell so that we eventually can get these gametes that can participate in sexual reproduction this is an amazing thing and you know it's it's kind of it's it's developed over billions of years of evolution but it's it's just it's just mind-boggling to think about the complexity and not all of this is completely understood exactly how this works I mean you have these kind of motor proteins that help move the chromosomes along the these these microtubules can elongate and shorten in interesting ways so it's a really fascinating process but anyway this is what's happening in metaphase one now you can probably guess what happens after that we then move to anaphase one so let me we now go to anaphase one I'll write that over here anaphase anaphase one and just like in anaphase in mitosis over here the chromosomes start getting pulled apart except for one significant difference and this is actually a very significant difference in mitosis mitosis the sister chromatids get pulled apart the sister chromatids get pulled apart to become two daughter chromosomes that does not happen in anaphase one in anaphase one the sister chromatids stay together it's the homologous pairs that get pulled apart so let me draw that so this homologous pair up here gets pulled apart the two sister chromatids do not get pulled apart here so you have this one is getting pulled on to this side so this one's getting pulled on to this side it has a little bit from the original so a little bit of that right over there and then you have this one getting pulled on this side so draw it the best I can the colours alright so it looks like that oh that's nice to have that is kind of easy to keep track of because these switch colors like that and then you have this one getting pulled on this side this one getting pulled on this side and finally finally this one getting pulled on to that side and let me draw the centrosomes so that's my centrosome and once again it's pulling it's or I guess you could say the chromosomes being moved and these things are pushing each other apart the two centrosomes might be pushing apart to get to the opposite ends of the actual cell but they're bringing there's all sorts of interesting mechanisms that are bringing along these microtubules bringing the chromosomes once again splitting the homologous pairs and how they split is is random you know this this pink one could have been on the right side this orange one could have been a left side or vice versa and once again this adds via this adds this adds more kind of variation amongst the gametes so even all of the all of the resulting gametes that get produced they all will have different genetic information so this is anaphase one you're pulling these apart and then you could imagine what happens in telophase one so telophase 1 telophase telophase 1 telophase 1 and this is this is fairly analogous to what happens in mitosis in telophase so now you have your cytokinesis is beginning so it actually might even begin earlier in mitosis it happens is early as anaphase at least the cytokinesis is starting but you're starting to see that the these two things the the the homologous pairs are fully split apart and they're at opposite ends and actually they could begin to they can begin to unravel into their chromatin state so this one begin to unravel into its chromatin into its chromatin state as a little bit of the magenta whoops it has a little bit of the magenta right over here this is unraveling as well this is unraveling like that once again into our score button state the cellular the new and let me do the other ones as well so this is this one right over here the beginning to unravel this one over here beginning to unravel it's got a little bit of orange on it it's got a little bit of orange on it the nuclear membrane begins to form again the nuclear membrane begins to form again in some ways it's reversing what happened in prophase one where the nuclear membrane disappeared and the and the chromosomes condensed and let me draw let me draw the centrosomes which are outside of the nuclear membrane just like that and the microtubules are also dissolving the microtubules are also dissolving and you have your cytokinesis so your cytokinesis so these separate these separate into two cells so once again when we did the overview of of meiosis we said look the first phase of meiosis you go from a diploid a diploid germ cell to two haploid cells and these aren't quite our end product yet this right over here what we have just gone through what we have just gone through all of this combined that we have just gone through this is meiosis one and in the next video we're going to go through meiosis two whoops I didn't mean to do that this is so let's see all of this is meiosis one let me write that in a different color and bold so this is all meiosis meiosis one here and you can see each of these cells now have a haploid number then they now have the haploid haploid number of two chromosomes each now each of those two chromosomes do have two sister chromatids and as we'll see meiosis two which is very similar to mitosis is going to split up the sister chromatids from each of these chromosomes which gives us two daughter chromosomes so we're going to see that over here so your haploid number here is two you have two chromosomes here and you have two chromosomes there and we'll explore meiosis two in the next video
Biology is brought to you with support from the Amgen Foundation
AP® is a registered trademark of the College Board, which has not reviewed this resource.