Meiosis (Updated)

Captions are on! Click the CC button at bottom right to turn off. Follow us on Twitter (@AmoebaSisters) and Facebook! Have you ever wondered how two siblings can
have the same mom and dad still look so different? Well, today were going to talk about a
process that makes that possible----a process called meiosis ----not to be confused with
mitosis, which sounds unfortunately similar. Mitosis makes identical body cells like your
skin cells and stomach cells. Recall from our mitosis clip that since it
makes identical body cells, mitosis is important for growth and for repair of damage or to
replace worn out cells.

But NOT meiosis. Meiosis is a process that contributes to genetic
variety. Meiosis also doesnt make body cells. It makes sperm and egg cells; otherwise known
as gametes, the fancier word.

You might recall that humans have 46 chromosomes. Thats how many chromosomes most body cells
in your body have. But there are some human cells that dont
have 46 chromosomes. Human sperm cells and egg cells have 23 chromosomes.

Why the number difference? Well, if a sperm cell has 23 chromosomes and
an egg cell has 23 chromosomes, when they come together that makes 46 chromosomes. That will allow a newly formed fertilized
egg to develop into a human. Meiosis is what we call a reduction division
because you have a starting cell that has 46 chromosomes and your ending cells---the
sperm and egg cells---have only has 23 chromosomes. Before we start getting into the stages of
meiosis to make gametes, we have to remember what happens before meiosis can start.

Actually, this also happens before mitosis. Its the stage known as interphase. If you remember interphase, its when the
cell is growing, its replicating its DNA, its carrying out cell processes. Just like mitosis, interphase happens before
meiosis is going to start.

So the starting cell has 46 chromosomes, and
you have to duplicate those chromosomes in interphase before meiosis starts. That basically means youre duplicating
your DNA, since chromosomes are made of DNA. And protein. Ready for the tricky part? Because we tend to count chromosomes by the
number of centromeres present, when the 46 chromosomes duplicate, we still say there
are 46 chromosomes because the sister chromatids are still attached and were counting by
centromeres.

So 46 chromosomes here, they replicate in
interphase, and you still have 46 chromosomes in this picture. But you went from 46 to 92 chromatids. Little tricky there. We have a detailed video that explains these
chromosome numbers before and after replicating in interphase that can be useful for understanding
meiosis.

Ok so interphase checklist done---now we can
move into meiosis. You might remember the mitosis stages PMAT;
the p was for prophase, the m for metaphase, the a for anaphase, and
the t for telophase. The good news is that in meiosis, you still
use those terms, but because meiosis is actually a reduction division, youre going from
46 chromosomes to 23. Which means you actually divide twice.

So instead of mitosis where you divide one
time and do PMAT one time, in meiosis, youre going to divide twice and therefore do PMAT
twice. Because of this, in meiosis, you put numbers
after the phases to indicate whether youre in the first division or the second division. So lets dive right in. So lets start with the very first step
prophase I.

One thing I like to remember about prophase
is pro, this actually means before. It kind of helps you remember that it comes
before all the other stages start. This is where the chromosomes are going to
condense and thicken. They are actually going to line up with their
homologous pairs.

The word homologous means that the chromosomes
are approximately the same size and that they contain the same types of genes in the same
locations. They are going to match up. It is during this prophase 1 that this amazing
process occurs called crossing over. I know crossing over probably sounds like
something very different, but this is a really awesome process because when these chromosomes
are lined up in homologous pairs, they have a way that they can transfer their genetic
information and exchange it between each other.

Its kind of like these chromosomes flop
over each other and they do a little genetic information exchange here. It makes for what we call---recombinant chromosomes---which
can eventually contribute to the variety that we were mentioning that siblings can have
even when they have the same parents. More about that later. Now we move into metaphase I.

In metaphase I, think if the M as standing
for middle. The chromosomes are going to be in the middle
of the cell. Its a little different, though, from mitosis
because theyre still going to be in pairs in the middle of the cell so its not a
single file line; they are in pairs in the middle. During anaphase I, think A for away because
the chromosomes are going to be pulled away by the spindle fibers.Then, we end with telophase
I, where you have two newly formed nuclei and it becomes obvious you will end meiosis
1 with two new cells.

Cytokinesis follows with splitting the cytoplasm. But were not done yet. On to meiosis 2! The very first step in meiosis II is prophase
II. Its not going to be nearly as eventful
as it was in prophase I because they are not going to have homologous pairs.

They also are not going to have that amazing
process called crossing-over. That doesnt happen again in prophase II. You have your chromosomes and the spindles
starting to form like in prophase I but prophase II is not nearly as eventful of having that
process of crossing over. In metaphase II, remember think m for middle,
the chromosomes are going to line up in the middle.

This time, though, they are in a single file
line. They are not in pairs like they were in metaphase
I. Anaphase II, remember A for away, but this time its the chromatids that are getting
pulled away by the spindle fibers. Chromatids are getting pulled away to opposite
sides of the cell.

In telophase II, nuclei reform and the 2 cells
are each going to divide so you can see here that 4 cells are going to be formed. Cytokinesis will follow to completely split
the cytoplasm. Now keep in mind that meiosis in males produces
sperm cells and in females, it produces egg cells. Because of independent assortment and also
crossing over, youre going to have variety.

For example, in a male the four sperms cells
that are produced each time, they are all different from each other. They are also different from the starting
cell because the starting cell had 46 chromosomes and the ending cells only have 23. So they are not identical to the original
and they are not identical to each other. This is going to lead to variety.

A reason why two siblings with the same parents
can look different from each other---they still developed from an unique egg and unique
sperm cell that came together. One last thing to think about. Scientists are often looking into the process
of meiosis because sometimes the chromosomes dont separate correctly. Its called nondisjunction when a cell can
receive too many or too few chromosomes in the separation.

This contributes to some genetic disorders,
which is something scientists continue to study. Well thats it for the Amoeba Sisters, and
we remind you to stay curious..

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