Biology: Meiosis: Prophase I

URGENT!!!

09/18/2011

~ Rachelle1

What is the difference between bivalent and tetrad?

...?

11/18/2010

~ erik2

soo if you're a freshman in katy texas this video will no be very helpful except for the last ten seconds.terms in here are we have not bothered discussing and very detailed.
which is good but it was not summarized in places where i needed it most.

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Cell Reproduction
Meiosis
Meiosis: Prophase I

You know, if you guys really understand this whole concept of what a homologous chromosome pair is, meiosis is
going to be easy for you. Before we do that, before we go on and take a look at the stages of meiosis, just like in
mitosis, get the idea and then worry about the big words and the terms. It will fit into place for you, I promise.
We have established the fact that you need to visualize sexual reproduction as the combination of genetic material
from a mother and a father. You can visualize the fact, that being said, you have to half the amount of genetic
material from the mother and the father. Therefore, you have to go from a situation where the gamete has half the
amount of DNA, if that is the male, and the female gamete has to have half the amount of DNA.
The dilemma that we run into is, which DNA do we send? We know that we can send half, but how do we split up our
DNA? So we find out, that in order to sexually reproduce, we have to explain this with this idea of homologous
chromosomes. It is not something that we are making up, this is the way it works.
You, as humans, have 46 chromosomes, actually 23 pairs. Here is the thing. What makes two chromosomes
homologs? Well, let’s come up with kind of a generic definition of homologous chromosomes. Homologous
chromosomes, are chromosomes that have genes on them that control the same traits, that have DNA that does the
same job. Or, homologous chromosomes are chromosomes that control the same functions. So, for example, if
these are two chromosomes taken from my body, and this particular gene right here, that I just detached, is the gene
for eye color, then its homolog in the exact same spot, will have the gene for eye color, and since these chromosomes
are doubled, which they normally are – remember they don’t double until they are ready to replicate, I am removing
the genes for eye color. So in active meiosis, I literally, because, I have doubled chromosomes, have two each of
these. So the point here is this, even if I were to make believe that my chromosomes only were single stranded, like
so, and I ignored the double strand part, remember, this gene is homologous to this gene. This gene is homologous
to this gene, why? Because this chromosome is homologous to this one. I hope you are getting the feel for
homologous chromosomes, because if you are, the next thing I want to tell you about, prophase of meiosis is going to
be easy for you. Guess what? The rest of it is too. So, got it? 23 homologous pairs, they work together, or like I
said, if you are a wasp, five homologous pairs to give you a total of ten chromosomes. Homologous pairs is
everything, and understanding meiosis and eventually, understanding genetics.
So, what is going to happen? Well, here is the scoop. Meiosis is based on mitosis, evolutionarily. Mitosis came first;
therefore, if you guys know mitosis and if you haven’t learned mitosis yet, I want to learn it, because you got to really
understand mitosis before you understand meiosis. So become one with mitosis. If you are one with mitosis, let’s go
on.
In mitosis, you remember the first thing that happened, we had a jumble of chromatin and the chromatin went through
super coiling and formed double chromosomes. The chromosomes are tightly packed genetic material around protein
super coiled. All of that’s fine, well and good. Then in mitosis, you remember, that we had to then line these up, and
it was very simple. We split them. That was the end of mitosis.
But now we have a different story. Remember that we have established the fact that the first thing we have to do is
somehow make sure that these homologous pairs split and get separated from each other by a cell membrane. That
being said, do you see anything wrong? Yeah, because it is based on mitosis, they are still doubled. So we are going
to have to go through two divisions in meiosis, rather than one.
So this first phase that I want to tell you about is going to be called is going to be prophase one. Hey, what do you
know? We even use the same names; prophase, metaphase, anaphase, telophase, and we could throw in a
prometaphase, if you want to, but we are going to use the same phase names. We are going to call them prophase
one, metaphase one, anaphase one, telophase one. What do you think comes next? Prophase two, metaphase two,
anaphase two, so again, mitosis, you get mitosis, you’ve got meiosis.
Let’s see what happens in the first phase of meiosis. First phase of meiosis is the most important phase. If you are
going to study a phase and say, “I want to know meiosis,” this is the one that you’ve got to study. Wait till you see
what happens here. Big time. Here is what happens. Number one, a synapsis occurs. Synapsis, or we can say that
the chromosome synapse, synapsis. What that means, is that the homologous pairs associate with each other in the
nucleus. So, let’s get away from this for a second and let’s take a look at a typical interphase. There is a typical
meiotic interphase. It looks just like a mitotic interphase, you’ve got your centrosome, and you’ve got your centrioles
and an animal cell, the nuclear membrane is cool, and then what starts to happen is, it starts to divide and the nuclear
membrane breaks down and the spindle begins. But I want to show you this. This is not mitosis. This is meiosis. The
homologs associate. This is, like, the secret to sex, because these homologs are starting to get together.
So they get together and they form these places where they bud into each other. This is going to be important in a
second. In fact, they kind of crisscross, like so. We have a name for those crisscrosses, as you can imagine, they are
important. That crisscross place where they crisscross, a single one, is called a chaisma, together, or if it happens
five times, plural, chaismata. Where they crisscross, they form chaismata. This is in prophase. What are you doing
here, guys? You are organizing. When I used to go to elementary school, we used to be in the school yard all of the
time. I love to use this analogy. It was time, lunch was over and Sister Mary Camillus, used to take out here bell and
ring it. We, this is wild, used to have to freeze in our places and stop, just like this. Then we would wait for Sister
Mary Camillus to ring the bell again and we had to find a partner, we had to go in that school yard and find our
partner, because we were about to get in line. I am kind of giving away what happens next. She didn’t call it the
metaphase plate, but do you get, all right. So the first thing we had to do was pair up.
Now, chromosomes, when they pair up, do something that we never did. They kind of like exchange, they exchange
parts, and I am going to show you that next, because a process, a very unusual thing happens here. Sometimes in
these chaismata, chromosomes switch genetic material. Now remember since these are homologous, this is not a
mutation, this is not bad news. This is normal, because they might just switch some of their genetic material. What is
going to happen is – I will get a little ahead of myself because this is so cool. Look when these things eventually
separate – holy mackerel look what you did. Some of those genes from this chromosome went onto this one and
some of the genes from this chromosome went over here. That is called crossing over. I have to make believe that I
haven’t told you about that yet, because we are still in prophase.
So crossing over is going to occur. Guess what? I wanted to spend the most time here on prophase, because this
takes up 90 percent of the entire meiotic cycle. 90 percent of meiosis is in prophase one. Significant events, I can’t
say this enough. Pairing of the homologs. And you know what? We even have a name for that. The process is call
synapsis, but I want to show you something. I am going to spread these out. There are now, let’s count numbers.
How many strands? What are each of these strands called? Each strand is called a chromatid right, remember that.
Nothing is going to change. The four of these strands together, is referred to as a tetrad. Now I have a word I can
use. What is significant in meiosis one, in prophase one? Number one, tetrad formation, never happens in mitosis.
Number two, crossing over at the chaismata, both of these events become very important later on.