Meiosis and genetic diversity
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Fertilization terminology: gametes, zygotes, haploid, diploid
Voiceover: So let's talk a little bit about how we all came into being. What we see right over here, this is a picture of a sperm cell fusing with an egg cell. So that's a sperm cell and this is an egg cell or we could call this an ovum. And even this scene depicted right over here, this is the end of an epic competition because this sperm cell is one of one of two to three hundred million that is vying for this ovum. So there's two to three hundred million of these characters and they're all vying for this ovum and the one that you see that's about to fuse for it, this is the winner of this incredibly - remember two to three hundred, 200 million to 300 million sperm are trying to get here so this is a major victory and to some degree we should all feel pretty good about ourselves because we are all the by-product of that one in 200 to 300 million sperm cells that won this race getting to our mother's ovum. So the sperm cell came from our father and the egg cell, this is all happening inside of our mothers, the egg cell is from our mother. Now, once this happens, let's talk a little bit about the terminology. So once these two fuse, or the process of them fusing, we call that fertilization. Fertilization. And it produces a cell that then differentiates into all of the cells of our body, so you can imagine that this is an important process. So let's make sure that we understand the different terminology, the different words for the different things that are acting in this process. So each of these sex cells, I guess we could say, the sperm cell and the ovum, these are each called gametes. So this right over here is a gamete and the ovum is a gamete, the egg cell is also a gamete. And as we'll see, each gamete has half the number of chromosomes as your body cells or most of the somatic cells of your body so outside of your sex cells that might be in your ovaries or your testes, depending on whether you're male or female, these have half the number so let's dig a little bit deeper into what I mean there. So let's just do a blow up of this sperm cell right over here, so a blow up of a sperm cell and I'm not going to draw it to scale, you see the sperm cell is much smaller than the egg cell but just to get a sense, so let me draw the nucleus of this sperm cell, so just like that. If we're talking about a human being, and I'm assuming you are a human being, so that might be of interest to you, this will have 23 chromosomes from your father so let's do them. One, two, three, four, five, six, seven, eight, nine, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22 and for the 23rd one, that's going to be your sex-determining chromosome so if your father contributes an x, you are going to be female, if your father contributes a y, you are going to be male. So these are the chromosomes in the male gamete or I guess I should say the gamete that your father's contributing, the sperm. So this is a gamete right over here and that's going to fuse with the egg, the ovum that your mother is contributing and once again, I'm not drawing that to scale. So this is the egg, and let me draw it's nucleus. So that's it's nucleus, once again none of this is drawn to scale. And your mother is also going to contribute 23 chromosomes. So one, two, three, four, five, six, seven, eight, nine, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22 and then she will contribute an x chromosome for the sex determining so your sex determining chromosomes are going to be xy, you're going to be male, if this was xx, you're going to be female so this is also a gamete here. So a gamete is the general term for either a sperm or an egg. Now once these two things are fused, what do we have? Once they're fused, then we're going to have you could say a fertilized egg but we are going to call that a zygote so let me draw that. I'm going to do this in a new color, and I'm running out of space and I want this all to fit on the same screen so I'll draw it not quite at scale and so let me draw the nucleus of the zygote, I'm going to make the nucleus fairly large so that we can focus on the chromosomes in it, once again none of this is drawn to scale. So you're going to have the 23 chromosomes from your father, so let me do that. One, two, three, four, five, six, seven, eight, nine, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22 and 23, and then the 23 chromosomes from your mother. One, two, three, four, five, six, seven, eight, nine, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22 and 23 so you got that x chromosome from your mother. And as you might have notice, I've drawn them in pairs so you now have a total, let me make it clear, you have 23 chromosomes here, 23 chromosomes in the sperm, you have 23 chromosomes in the egg and now you have 46 chromosomes in the fertilized egg, 46 chromosomes, and now that we have a full contingent of chromosomes and then this cell can now keep replicating, keep splitting and differentiating into all of what makes you, you, we call this right over here, we call this a zygote. So one way to think about it, the gametes are the sex cells that have half the number of chromosomes and the zygote is the cell that's now ready to differentiate into an actual organism that has double the number or that has a full contingency of chromosomes, that has 46 chromosomes, and you see that I've made them in pairs and these pairs, we call these homologous pairs and in each of these pairs, this is a pair of homologous chromosomes. So what does that mean? Well that means that in general, these two chromosomes, you got one from your father, one from your mother, they code for the same things, they code for the same proteins but there are different variants of how they code for those proteins, those traits that you have so gross oversimplification is, let's say that there is a gene on, that one from your father that helps code for hair color well there would be a similar, there would be another variant of that gene on the chromosome from your mother that helps code for hair color as well. So these are homologous chromosomes, these two chromosomes code, in general, for the same things and so the zygote now has, you could say it has 46 chromosomes or you could say it has 23 pairs of homologous chromosomes. And this is, once again, this is the case for human beings. If we're talking about some other species, instead of 23 pairs of homologous chromosomes or 46 chromosomes in total, you might be talking about 10 pairs of homologous chromosomes with 20 chromosomes in general. Now to help biologists, to help clarify when they're talking about the number of chromosomes for a given species, they introduce two words, haploid and diploid. And haploid is referring to when you have half the full contingency of chromosomes. So for human beings, the haploid number is 23. So, this is the haploid number, it is 23. For another species it would be something else and haploid is based on the prefix "hapl", that's the prefix for single, so you have a single member I guess you could think of it, of each of the pairs and now you have both of each pair, you have both chromosomes in each pair or you have the full contingency and this 46 chromosomes, this is called the diploid number for humans. The diploid number right over here. And when people talk in general, and we will speak in general when we start talking about mitosis and meiosis for a given species they will refer to the haploid number, they will refer to the haploid number as n chromosomes and they will refer to the diploid number as just twice that, as two n chromosomes. So hopefully this gets you familiar with some of the vocabulary around fertilization and haploid and diploid and zygotes and gametes and also makes you feel a little bit better about yourself that, just to exist, at least half of your chromosomes had to win an incredibly competitive race.
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