Meiosis: Where the Sex Starts - Crash Course Biology #13

Key Takeaways

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 share genetic information and then that fertilized egg splits in half and then those have 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 just I don't know how to impress you but riddle me this my friends if sexual reproduction begins with sex cells the sperm in the egg where did the sperm and egg come from oh dude so how do a 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 at all 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 answer to these questions and a lot of other of life's mysteries is meiosis [Music] in the last episode we talked about how most of your cells your body or somatic cells grow themselves through the process of mitosis mitosis replicates a cell with a 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 be half of your cells would be clothes 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 group 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 allele 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 and 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 kinds of cells requires a process that's very similar to mitosis but with a totally different outcome meiosis that's when 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 then it goes through another out of the stages again and they have the same names conveniently except with the 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 whatever you know what it depends on they're diploid cells called either primary oocytes work Ron spermatocytes depending on what kind of gamete they make can 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 want to 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 baby makers so 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 DNA in the nucleus begin to duplicate leaving two copies of every strand jog your memory about how DNA does this we did a whole episode on and 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 row P proteins called microtubules and that brings us to the first round of meiosis prophase 1 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 into chromosomes each single chromosome is linked to its duplicate copy to make an x-shaped double chromosome leave this in mind once attached each single chromosome it's called a chromatid one on each side of the X each double chromosome has two chromatids here meiosis prophase one includes two additional and very important steps crossover and homologous recombination rather 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 cross over and while they're all tangled up they trade sections of DNA that's the recombination the sections that they're trading 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 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 segments 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 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 puffs 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 they're your X chromosome 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 hrunta dand doesn't have a lot of genetic information on it there's prophase the X wants nothing to do with the little wide 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 one and in mitosis you might recall that all of the chromosomes lined up in a single row powered by motor proteins and we're then told in half but not here in meiosis each chromosome lines up - 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 one final phase of the first round telophase one rolls out in pretty much the same way as mitosis the nuclear membrane reforms the nucleoli form within them the chromosomes fray out back into chromatin creased forth between them 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 not as 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 exes so 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 and a separate single strand chromosomes because of this there is no DNA replication involved in prophase two instead the DNA just clumps up again into chromosomes and the infrastructure for moving them the microtubules are put back in place in metaphase to the chromosomes are moved into alignment into the middle of the cell and in anaphase two the chromatids are pulled apart into separate single chromosomes chromosomes uncoil and chromatin the crease forming cleavage in the final separation of cytokinesis then mark the end of telophase two from one original cell with 46 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 from making girls and half will be from 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 one more of the inner goodness of a cell the cytoplasm organelles heads into one of the cells that get split off then to the other one telophase 2 when it's time to split again the same thing happens with more stuff going into one of the cells than the other big old 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 and plants those polar bodies actually also get fertilized too and they become the endosperm that's the starchy protein II 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're 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 say that you know more about the miracle of reproduction a miracle it's science thank you for joining us here on crash course biology / meiosis if you want to re-watch anything you can do that now just click the annotations that are on top of the words next to me just do it otherwise congratulations on learning stuff and being smarter thanks to everybody who helped make this video if you have any questions please ask them down in the comments below or on Facebook or Twitter we will see you next week