Crash Course: Biology
Meiosis: Where the sex starts
- Reproduction! Always a popular topic, and one that I don't mind saying that I am personally interested in. 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. And if you're not suitably impressed by the fact that we all come from one single cell, and then we become this, then I don't, I don't know how to impress you. But riddle me this, my friends, if sexual reproduction begins with sex cells, the sperm and the egg, where do the sperm and egg come from? Ah, 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? And for that matter, why aren't all of our sex cells the same? Like why, are my brother John and I, different? Sure, we both wear glasses, and 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 than he is, 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. The answer to these questions, and a lot of other life's mysteries, is meiosis. (Crash Course intro song plays) In the last episode, we talked about how most of your cells, your body or somatic cells clone themselves through the process of mitosis. Mitosis replicates a cell with the complete set of 46 chromosomes into two daughter cells that are each identical to each other. But of course, even though the vast majority of your cells can clone themselves, you cannot clone yourself. And for good reason, actually reasons. 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, or possibly b) half of your cells would be clones from your mom and half would be clones from your dad, and you would look really weird. But that's not how we roll, we do things a better way, where all of your body cells contain the same mix of DNA, 46 chromosomes grow up into 23 pairs. One in each pair from your mom, and one from your dad. Those pairs of chromosomes are pretty similar, but they're not identical. They contain versions of the same genes, or alleles, in the same spot for any given trait. Since they're so similar, we call the pairs homologous chromosome pairs. Homologous is a word that comes up a lot in genetics, it just means that two things have the same homo-relation logos, even if they are a little bit different. However there are some very special cells that you have, that have only one half of that amount, 23 chromosomes, those are sperm and egg cells. These are the haploid cells, they have half of a full set of chromosomes. And they need each other to combine to make the complete 46. Creating those kind of cells requires a process that's very similar to mitosis, but with a totally different outcome, meiosis. That's when a specialized diploid cell splits in half, twice, producing four separate cells, each of which is genetically distinct from the others. Meiosis is a lot like mitosis, except twice. It goes through the same stages as mitosis, prophase, metaphase, anaphase and telophase, but then it goes through another round of the stages again, and they have the same names, conveniently, except with a two after them. They're like sequels. And just as with the Final Destination movies, the sequels have pretty much the same plot, just some new actors. The raw materials for this process, are in your ovaries, or your testes depending on, you know, you know what it depends on. They're diploid cells called either primary oocytes, or primary spermatocytes depending on what kind of gamete they make. 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. Here you might wanna go watch the previous episode about mitosis again, because that's where we go into detail about each stage of the process. Once you're done with that, we can start making some babymakers. 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. Long strings of DNA in the nucleus begin to duplicate, leaving two copies of every strand. To jog your memory about how DNA does this, we did a whole episode on it, you can watch it and come back. A similar process takes place with the centrosomes, 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. And that brings us to the first round of meiosis prophase I. This is nearly the same as in mitosis, the centrosomes start heading to their corners of the cell, unspooling the microtubules, and the DNA clumps up with some proteins in the chromosomes. Each single chromosome is linked to its duplicate copy to make an X-shaped double chromosome. And keep this in mind, once attached, each single chromosome is called a chromatid, one on each side of the X. Each double chromosome, has two chromatids. Here meiosis prophase I includes two additional and very important steps, crossover and homologous recombination. Remember that 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. Each double chromosome lines up next to its homologue, so there's your mother's version lined up right next to your father's version of the same chromosome. Now if you look, you'll see that these two double chromosomes each with two chromatids, add up to four chromatids. Now watch, one chromatid from each X, gets tangled up with the other X, that's crossover. And while they're all tangled up, they trade sections of DNA, that's the recombination. 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 chromosome's genes for that trait. This is important, what just happened here, creating new gene combinations on a single chromosome. It's the whole point of reproducing this way. 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. Remember that one of the pillars of natural selection is variation, and this is a major source of that variation. What's more, since all of the four chromatids have swapped some DNA segment at random, that means that all four chromatids are now different. Later on in the process, each chromatid 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. Same for sperm and men. And that's why my brother John and I look different, even though we're 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. But then of course, there is that one pair of chromosomes that doesn't always go through the crossover or recombination. That's the 23rd pair, and those are your sex chromosomes. If you're female, you have two matched, beautiful, fully capable chromosomes there, your X chromosomes. Since they're 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. 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 chromatids, half of the four resulting sperm will be X, leading to female offspring, and half will be Y, leading to male offspring. Now what comes next, is another kind of amazing feat of alignment. 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, in meiosis. Each chromosome lines up next to its homologous pair partner that it's already swapped a few genes with. Now the homologous pairs get pulled apart and migrate to either end of the cell, and that's anaphase I. The final phase, of the first round, telophase I, rolls out in pretty much the same way as mitosis. The nuclear membrane reforms a nucleoli form within them, the chromosomes fray out back into chromatid. 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. We now have two haploid cells, each with 23 double chromosomes that are new, unique combinations of the original chromosome pairs. And these new cells, the chromosomes are still duplicated and still connected at the centromeres, they still look like X's. But remember, the aim is to end up with four cells, so it's time for those sequels. 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. Because of this, there's no DNA replication involved in prophase II, instead the DNA just clumps up again into chromosomes and the infrastructure for moving them, the microtubules, are put back in place. In metaphase II, the chromosomes are moved into alignment into the middle of the cell, and in anaphase II, the chromatids are pulled apart into separate, single chromosomes. The chromosomes uncoil into chromatid, the crease forming cleavage in the final separation of cytokinesis then mark the end of telophase II. From one original cell with 46 original chromosomes, we now have four new cells with 23 single chromosomes each. 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. 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. 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. 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. 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. In plants, those polar bodies actually also get fertilized too, and they become the endosperm. That's the starchy, proteiny stuff that we grind into wheat or pop into popcorn, and it's basically the nutrients that feed the plant embryo, the seed. And that's all there is to it, I know you probably were really excited when I started talking about reproduction, but then I rambled on for a long time about haploid and diploid cells, but now you can't say that you know more about the miracle of reproduction-- it's not actually a miracle, it's science!
Biology is brought to you with support from the Amgen Foundation