High school biology
Phases of meiosis II
Meiosis is a process that creates sex cells (gametes) with half the number of chromosomes of the parent cell. Meiosis has two stages: meiosis I and meiosis II. In meiosis I, homologous chromosomes pair up and exchange genetic material (crossing over). In meiosis II, sister chromatids separate, creating four haploid cells.
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- So, in Telophase 1, the Chromosomes start to unravel or whatever, but then in the next stage, Prophase 2, they condense again? Why didn't they just stay condensed, as they were in Anaphase 1 (before Telophase 1)? Basically, what is the point of them not staying condensed? Thanks(45 votes)
- Cells have other functions than just to reproduce, they have jobs! When you go on a trip you pack up your suitcase, right? Nice and tight and compact, but we all know how frustrating it is to live out of a suitcase so we unpack it to make it easier to access. It would be difficult for proteins to be synthesized if the DNA was still tightly packed, so the cell unravels the chromosomes so it can do its job creating hormones, proteins, etc.(83 votes)
- i don't get what sal said about starting meiosis II with a haploid number and ending it with a haploid number as well :( please clear this up thank you(12 votes)
- In other words: two sister chromatids, when attached at the centromere, are a single chromosome. So while there are still technically 2n (diploid number) 'sister chromatids,' there are only n (haploid number) chromosomes. When anaphase II splits the sisters apart, they are still chromosomes so there are haploid number of chromosomes at the end.(6 votes)
- if there is no replication of DNA in interphase 2 thn why it is said that it is a S (synthesis) phase? nd when the centrosomes duplicate.. between meiosis 1 and 2 ?(9 votes)
- interphase 2 isn't exactly what you would call "interphase"...
It's just a period where the cell grows and gets ready for meiosis 2.
hope this helps you ;)(9 votes)
- Why does the two cells stay connected through out meiosis-2?(6 votes)
- apoorva, thats because there are two divisions of the cell
-karyokinesis ( karyo - nucleus kinesis - division i.e, prophase, metaphase...)
-cytokinesis ( cyto- cytoplasm )(4 votes)
- does chromosomal crossover happen in mitosis prophase as well?(6 votes)
But recall what mitosis is - here chromosomes are homologous and crossing over cannot result in new combinations of chromosomes/genes.(4 votes)
- Are centromere and kinetochores at the same place ?
And what the diffrence bietween them?(4 votes)
- The centromere is a specialized region of a chromosome — it acts as the site for kinetochore assembly. It is made of chromatin (DNA plus associated proteins and RNAs).
A kinetochore is a protein complex that assembles on centromeres — its primary function is to connect (one or more) microtubules from the spindle to the chromosome. It is also involved in regulation of that attachment.
You might find it helpful to think of centromeres as being the foundations for kinetochores.
Does that help?(7 votes)
- how does the centrosome become two again in prophase II?(6 votes)
- Interphase takes place between two meiotic divisions.
The same way it duplicates in the mitosis - during interphase.
So, after meiosis I, interphase takes place and S phase during which centrosome is duplicated. Shortly after proceeds prophase II. Now you have two centrosomes. :D
- How will we know what type of gamete the four haploid cells will be?(4 votes)
- You don't really know precisely because you don't know how the crossing over occurred. All you know is that the gamete has genes from the parental gametes.(4 votes)
- What is the point in meiosis 2, if meiosis 1 already turned the diploid cell into a haploid and in meiosis 2 you start with a haploid cell and end with a haploid cell? Why is meiosis 2 necessary for meiosis 1, for 'fertilisation'...thanks.(4 votes)
- After meiosis I, those n=23 chromosomes have two chromatids. Meiosis II just separates them into single chromatids.(3 votes)
- I have a question, please.
I dont get why or how meiosis II occurs after fertilization, shouldn't it not be before? (to produce the gametes).
Thank you in advance.(4 votes)
- Meiosis II occurs before fertilization to produce the gametes like you suggested.(2 votes)
- [Voiceover] In the last video, we had completed meiosis I, and now we're ready to go into meiosis II. And you might be wondering, "Well, hey, after mitosis, we went back into our interphase, "is there kind of a rest period "between our two phases of meiosis?" And the answer is, sometimes. There can be a rest period where you have an interphase II, and that will depend on the type of cell and the species and all of that, but it is possible, so I'll actually put that over here. So we could have an interphase II. So interphase... interphase II, which you could kind of view as a rest period. But then we get into meiosis II, which will allow us to complete all of meiosis. So you can imagine meiosis II starts with prophase II. And in prophase II, now I'm dealing with two cells here, so in prophase II, and I'm gonna do it for both of the cells that I have after I finished meiosis I, so in prophase II, so let me... that's one of the cells. I'm not gonna have space to draw it properly, so let me draw it, so let me draw this one first. So this is one of the cells right over here, and then this is the other cell right over here. In prophase II, just like in prophase I, and just like in prophase in mitosis, and let me write the phases here, this is prophase II we're talking about, prophase II. Your nuclear envelope dissolves again, so let me show a dissolving nuclear envelope. So your nuclear envelope dissolves again, and your chromosomes once again condensing (mumbles) I guess you could say into their denser form, so it's gonna look like this, this, and this. On this side, it has a little bit of the magenta that was from the chromosomal crossover back in prophase I, and then you have this character right over here that is shorter. You have this chromosome, right over here, and it had a little orange section from the chromosomal crossover just like this, and then you have the shorter orange chromosome just like that, so they have condensed, and you've actually, each of these cells now will have duplicate centrosomes. So the centrosomes have replicated and they will start to migrate to opposite ends of the cell. So once again, very strong analogy, especially to frankly, prophase from mitosis. So now let's keep going. We're now ready to go to metaphase II. Metaphase II. Metaphase II. And let me draw my two cells. This is one of them and then this is... the other. Let me draw an arrow here so you can see that we are entering into another phase. So we are entering into another phase. Metaphase II. And you can just imagine, it's very similar to what happens in metaphase in mitosis. And actually all of meiosis II is very similar to what happens in mitosis. So in metaphase II, our centrosomes have migrated to the poles. So our centrosomes have migrated to the poles. And I'm gonna do it twice, because I'm now dealing with two different cells. My nuclear membrane is now disappeared. And I have my now dense chromosomes lining up along the equator here. So this magenta one, it'll line up here over here. So it might look like that. And actually, let me draw all the magenta ones now since I have my magenta color selected. So this is the longer one in this one. And this had a little bit of orange in it. Let me, has a little bit of the orange here, and then I had the shorter orange chromosome. The shorter orange chromosome on this cell had the longer, the larger orange chromosome. So then it had a little bit of pink on it. And of course, you have your microtubules, that are... I've been doing that in blue, so I'm gonna continue to do it in blue, that are pushing the centrosomes apart, but are also attaching to the chromosomes at the kinetochores. At the kinetochores. So there you go, and remember this right over here, where the two sister chromotids attached, those are our centromeres. So let me just draw it all out like this. So it might go, it might be something like that. And now we're ready to anaphase II, and you can imagine what's about to happen. Things are about to pull apart. And once again, this is analogous to what happens in anaphase in mitosis, so let me... So anaphase II. Anaphase II. I'm gonna draw all my cells again, this is taking me twice as long because I have to do it for twice as many cells, so that's one cell there. This is another cell here, this is, I got a centrosome here, centrosome here, centrosome here, centrosome here, and then the key here is this is why it's like mitosis and not like anaphase in meiosis I, is instead of, or like in mitosis, we're now going to split the sister chromatids so they now become two daughter chromosomes. When they're connected, they're just together, they're viewed as sister chromatids that make up one chromosome, but now they get, now they're getting pulled apart. So this one might get pulled in this direction, and then this one might get pulled in this direction. It has a little bit of the magenta right over here and then one of the sister chromatids, which would now be a daughter chromosome, going in upwards in one of them, going downwards. And let me draw all the microtubules here. All the microtubules that are doing, that are super involved in all of this work of getting things to the right sides of cells, and this is gonna happen in this cell as well. So in this cell, so this one... might be going down here, and this one is moving up here, and this one had a little chunk of orange on it, so let me draw that. Little chunk of orange. And then once again, one of the formerly sister chromatids, now daughter chromosomes going up there, and now going down over here. Over here. And let me draw the microtubules that are really... well, I've said it multiple times, super involved in actual the movement going on. They're elongating, they're these motor proteins that are moving the chromosomes along, once again, they're connected at the kinetochores right over here. Connected at the kinetochores. Right over there. And now we're almost done, we're ready to move into telophase II. So we're now going to go into telophase II. Telophase II. Where my two cells are now becoming four cells, so telophase II, I'm gonna show the cytokinesis starting to happen. So telophase II. So turning into four cells, trying to show the cytokinesis happening on... In this cell up here, I have this character, and has a little bit of magenta right over here. That's this right over there and then you have the shorter magenta one. And actually, they are starting to, they're starting to unravel into their chromatid form, so maybe I'll draw that a little bit, and then this one, right over here, is starting to unravel into its chromatid form. And so it this that one. Whoops, wanna do that in that magenta color. Starting to unravel into its chromatid form, I wanna do it over here, this one is starting to unravel. And so is this one. So is... (laughs) I'm having trouble changing colors. And so is that one, and then up here, this one's starting to unravel, this one over here, and... this longer, mostly magenta one is also starting to unravel. Also starting to unravel. You start having your nuclear envelope formed again, so your nuclear envelope is forming again. Nuclear envelope is forming. Your microtubules are dissolving. Let me draw the centrosomes, they're outside of the nuclear envelope. Outside of the nuclear envelope. And of course, you're finally dividing the cells, your cytokinesis happens, so now you have your four, your four cells, each have a haploid number. They each have two chromosomes. Remember, you diploid number was four, the germ cell had four chromosomes, two pairs of homologous chromosomes. Now each of your resulting gametes, these are now gametes now, these are gametes, they have a haploid number. But we started with a haploid number at the beginning of meiosis II, so that's why meiosis II is often compared to mitosis. So let me make this clear. This right over here is meiosis II because it preserves the number of chromosomes, just like mitosis. So this is meisosis, meiosis II right over here. We started with a haploid number and we finished with a haploid number just like this, and now these gametes are ready for some fertilization. And it's important to realize now, now these each have two chromosomes, and these are not homologous chromosomes. These are coding for different genes. But then they will each of these have the potential to fuse with, if this is a sperm cell, then this could fuse with an egg and then together they can create a diploid number of chromosomes. It could have the full complement of homologous pairs. But that's what these are for, these are for sexual reproduction.