Biology: Homologous Chromosomes

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Cell Reproduction
Meiosis
Homologous Chromosomes: Thanks, Mom and Dad!

You know we have established the fact that chromosome number in sexual reproduction is going to be a big time
problem. Again, using humans as an example, we somehow have to figure out a way to get this 46-chromosome thing
and work it out so that somehow, we can form gametes whether they’re sperm or eggs, with 23 chromosomes in it
each. Because no human I know, is going to well with 92 chromosomes. So we have this dilemma and the dilemma
is, we need a process to make gametes. So how are we going to come up with that process? Well I have already
told you the name of the process. The name of the process is going to be meiosis, but there is more to that than
meets the eye.
Now, I am going to do a little meiosis action with you here, and I am going to let you take a look and see if you can
figure out what the problem is. I am going to do a simple organism. Let’s say that this is an organism with a mere two
chromosomes. This is its somatic state. What does that mean? Somatic state, its body cell. So we’ll draw, let’s
curve this up so I get more room, if I can fit this all on one page, I’m going to be a very happy boy, okay. So we let’s
see a cell here. We will make this mom. We are going to cross her with dad. He’s got his two chromosomes. There
is dad; he is a little irregular. So dad here and mom and they both have two chromosomes each. You guys know that
the idea of this whole thing is to separate chromosomes. Now, I have kept these chromosomes doubled, and I am
going to work through them doubled. We will see the whole story of meiosis later on, but right now, I want to think
about assorting or sorting these chromosomes.
Now, what did I say I was going to do? I said that I was going to come up with a way to make sure that baby has – we
are going to put baby over here. We want the little bambino over here to have two chromosomes. Why? Because
we want it to be normal. Since this mother of this species has two, and the father has two, all offspring should have
two. So we are going to do a meiotic process here. So mom is going to split her cells, and for those of you who know
a little bit about meiosis, realize I am greatly simplifying this, but that is okay, because we have to show the problem
here. So we are going to have mom put her chromosome right in there and it is not allowed to stick out, but it is. We
will put this chromosome here and then we will have dad do the same thing here. So we will let the chromosomes
hang out.
Now, we have to make that we understand that this is dad’s sperm. Here is the thing. I want to go back. Remember
how I had this? Red and yellow, that was not random. Let’s say, for example, that this chromosome has a gene on it,
this is a very simple organism, all right, and this chromosome has a gene on it to, say, grow a head, and this
chromosome has a gene on it to grow a body. Well, this is good, because mom, that means that she’s got a body and
a head and dad had the same thing, a body and a head. So we are going to do this, we split them; remember we got
to get down one from each parent; I mean that is the key here. So this is good, this sperm can fertilize the egg and
look what we got. We have a normal baby. This sperm gave its chromosome to that egg. See, they are empty and
you have this baby and life is good. We have a gene for making a body and a gene for making a head.
But, I have a question for you. See anything wrong with this scenario? I do, because sperm just swim and eggs just
get fertilized. And I have a question for you. Could this sperm have fertilized this egg? We have a bodiless head,
you see? We ended up with two genes for making a head, but none for making a body and vise versa. We have a
headless body. You get it. I hope you get it. This is a problem. So we come to the second central problem of
meiosis. What is that problem? How are we going to sort chromosomes?
So in summary, let me just say this. As we approach the study of meiosis, there are two dilemmas that I am going to
pose to you. Number one. Problem number one is this – how to divide chromosomes by number? In other words, go
from 2N to N. The second dilemma that I want to pose to you is this – how to sort chromosomes? I am going to lay
part of the answer on you right now.
I think what you guys realize by now is that the situation that I gave you couldn’t happen. It wouldn’t work, because of
the fact that sperm and eggs have no minds as to what chromosomes they carry, so here is an inkling of where we
are going. It shouldn’t be a major intuitively to realize that we really don’t have 46 chromosomes. Instead, we have
23 pairs of chromosomes. Think of this. If I say to you that we have 23 pairs of chromosomes. Now what do we
know? We now know that in order to get a normal egg, if we can merely make sure that the 23 that go to one side,
are paired members to the 23 that go to, say, this side. Then we will make sure that our gametes will have, not just 23
chromosomes, but 23 unrelated chromosomes.
Guess what? This is sex. If this is an egg, what is the male going to carry? Yes, the other members of the pair. We
call those homologous chromosomes. Homologous chromosomes are the secret to the proper running of meiosis.