Meiosis: Where the sex starts발음듣기
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Meiosis: Where the sex starts
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출처 : 칸아카데미
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The kind of reproduction that we're most familiar with is, of course, sexual reproduction where sperm meets egg, they share genetic information, and then that fertilized egg splits in half, and then those halves split in half, and so on, and so on, and so on to make a living thing with trillions of cells that all do specialized things.발음듣기
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If you're not suitably impressed by the fact that we all come from one single cell and then we become this, than I don't know how to impress you. Riddle me this my friends.발음듣기
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If sexual reproduction begins with sex cells, the sperm and the egg, where do the sperm and the egg come from?발음듣기
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Aah dude! So how do sex cells form so that they each have only half of the genetic information that the resulting offspring will end up with?발음듣기
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We both kind of look like a tall Doctor Who but, you know, we have different color hair, and different noses, and I'm way better at Assassin's Creed then he is.발음듣기
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So, why aren't we identical? As far as we know, we both came from the same two people with the same two sets of DNA.발음듣기
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(music) In the last episode, we talked about how most of your cells, your body or somatic cells, clone themselves through the process of mitosis.발음듣기
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Mitosis replicates a cell with a complete set of 46 chromosomes into 2 daughter cells that are each identical to each other.발음듣기
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But, of course, even though the vast majority of your cells can clone themselves, you cannot clone yourself and for good reason, actually, reasons.발음듣기
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If mitosis were the only kind of cell division we were capable of, that would mean, A - you would be a clone of one of your parents, which would be awkward to say the least.발음듣기
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Or possibly, B - half of yours cells would be clones from your mom, and half would be clones from your dad, and you would look really weird.발음듣기
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Where all of your body cells contain the same mix of DNA, 46 chromosomes grouped into 23 pairs.발음듣기
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However, there are some very special cells that you have that have only one half of that amount, 23 chromosomes.발음듣기
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They have half of a full set of chromosomes and they need each other to combine to make the complete 46.발음듣기
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Creating those kinds of cells requires a process that's very similar to mitosis, but with a totally different outcome, meiosis.발음듣기
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That's when a specialized diploid cell splits in half twice, producing 4 separate cells, each of which is genetically distinct from the others.발음듣기
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It goes through the same stages as mitosis; prophase, metaphase, anaphase and telophase, but then it goes through another route of the stages again and they have the same names, conveniently, except with a 2 after them. They're like sequels.발음듣기
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And just as with The Final Destination movies, the sequels have pretty much the same plot, just some new actors.발음듣기
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And the raw materials for this process are in your ovaries or your testes, depending on, you know, you know what it depends on.발음듣기
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Their diploid cells called, either primary oocytes or primary spermatocytes depending on what kind of gamete they make.발음듣기
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Men produce sperm, you may have heard, and they produce it throughout their adult lives, whereas women are born with a certain amount of eggs that they'll release over many years after puberty.발음듣기
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Here you might want to go watch the previous episode about mitosis again because that's where we go into detail about each stage of the process.발음듣기
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Now, just like with mitosis, there's a spell between rounds of cell division where the cell is gearing up for the next big split and this is called interphase, when all the key players are replicating themselves.발음듣기
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The long strains of DNA in the nucleus begin to duplicate leaving two copies of every strand.발음듣기
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To jog your memory about how DNA does this we did a whole episode on it and you can watch it and come back.발음듣기
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A set of protein cylinders next to the nucleus that will regulate how all of the materials will be moved around along these ropey proteins called microtubules.발음듣기
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The centrosomes start heading to their corners of the cell, un-spooling the microtubules and the DNA clumps up with some proteins into chromosomes.발음듣기
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Each single chromosome is linked to it's duplicate copy to make an X shaped double chromosome.발음듣기
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Now, keep this in mind, once attached, each single chromosome is called a chromotid, one on each side of the X.발음듣기
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Here in meiosis prophase I includes two additional and very important steps, crossover and homologous recombination.발음듣기
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Remember, the point here, is to end up with four sex cells that each have just one single chromosome from each of the homologous pairs, but unlike in mitosis where all the copies end up the same, here, every copy is going to be different from the rest.발음듣기
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Each double chromosome lines up next to it's homologue, so there's your mother's version lined up right next to your father's version of the same chromosome.발음듣기
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Now, if you look, you'll see that these two double chromosomes, each with two chromotids, add up to four chromotids.발음듣기
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That's crossover and while they're all tangled up, they trade sections of DNA. That's the recombination.발음듣기
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The sections that they're trading are from the same location on each chromosome, so one is giving up its genetic code for, like hair color or body odor and in return, it's getting the other chromosomes genes for that trait.발음듣기
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Now this is important, what just happened here, creating new gene combinations on a single chromosome.발음듣기
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Life might be a lot less stressful if we could just clone ourselves, but then we'd also clone all our bad gene combinations and we wouldn't be able to change and adapt to our environment.발음듣기
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Remember, that one of the pillars of natural selection is variation and this is a major source of that variation.발음듣기
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What's more? Since all of the four chromotids have swapped some DNA segment at random, that means that all four chromotids are now different.발음듣기
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Later on in the process, each chromotid will end up in a separate sex cell and that's why all eggs produced by the same woman have a slightly different genetic code.발음듣기
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The same for sperm in men and that's why my brother John and I look different even though we're both made from the same two sets of DNA, because of the luck of the genetic draw that happens in recombination, I got this mane of luscious hair and John was stuck with his trash brown puff and don't forget about my mad, Assassin's Creed skills.발음듣기
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But then, of course, there is that one pair of chromosomes that doesn't always go through the crossover or recombination and that's the 23rd pair and those are your sex chromosomes.발음듣기
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If you're female, you have two matched, beautiful, fully capable chromosomes. They are your X chromosomes.발음듣기
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Since they are the same, they can do the whole crossover and recombination thing, but if you, like me, are a male, you get one of those X chromosomes and another from your dad that's kind of ugly and short and runted and doesn't have a lot of genetic information on it.발음듣기
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During prophase, the X wants nothing to do with the little Y because they're not homologous, so they don't match up and because the XY pairs on these chromosomes will split later into single chromotids, half of the four resulting sperm will be X, leading to female offspring and half will be Y, leading to male offspring.발음듣기
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This is metaphase I and in mitosis you might recall that all of the chromosomes lined up in a single row powered by motor proteins and were then pulled in half, but not here.발음듣기
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In meiosis, each chromosome lines up next to it's homologous pair partner that it's already swapped a few genes with.발음듣기
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Now, the homologous pairs get pulled apart and migrate to either end of the cell and that's anaphase I.발음듣기
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The final phase of the first round, telophase I rolls out in pretty much the same way as mitosis.발음듣기
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The nuclear membrane reforms, the nucleoli form within them, the chromosomes fray out back into chromatin, a crease forms between the two new cells called cleavage and then the two new nuclei move apart from each other, the cells separate in a process called cytokinesis, literally again, cell movement and that is the end of round one.발음듣기
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We now have two haploid cells, each with 23 double chromosomes that are new, unique combinations of the original chromosome pairs.발음듣기
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In these new cells, the chromosomes are still duplicated and still connected at the centromeres.발음듣기
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Here, the process is exactly the same as mitosis, except that the aim here isn't to duplicate the double chromosomes, but instead to pull them apart into separate single strand chromosomes.발음듣기
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Instead, the DNA just clumps up again into chromosomes and the infrastructure for moving them, the microtubules are put back in place.발음듣기
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In metaphase II, the chromosomes are moved into alignment into the middle of the cell and in anaphase II, the chromotids are pulled apart into separate single chromosomes.발음듣기
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The chromosomes uncoil into chromatin, the crease form in cleavage and the final separation of cytokinesis then mark the end of telophase II.발음듣기
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From one original cell with 46 chromosomes, we now have four new cells with 23 single chromosomes each.발음듣기
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If these are sperm, all four of the resulting cells are the same size, but they each have slightly different genetic information and half will be for making girls and half will be for making boys, but if this is the egg making process, then it goes a little bit differently here and the result is only one egg.발음듣기
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To rewind a little, during telophase I, more of the inner goodness of a cell, the cytoplasm, the organelles heads into one of the cells that gets split off then to the other one.발음듣기
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In telophase II, when it's time to split again, the same thing happens with more stuff going into one of the cells than the other.발음듣기
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This big ol' fat remaining cell becomes the egg with more of the nutrients and cytoplasm and organelles that it will take to make a new embryo.발음듣기
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The other three cells that were produced, the little ones, are called polar bodies and they're totally useless in people, though they are useful in plants.발음듣기
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In plants, those polar bodies actually, also get fertilized too and they become the endosperm.발음듣기
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That's the starchy, protein-ey stuff that we grind into wheat, or pop into popcorn and it's basically the nutrients that feed the plant embryo, the seed.발음듣기
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