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Have you ever wondered how two siblings who could have the same mom and dad can still
look so different? Well, today we’re going to talk about a process that makes that possible, a process called meiosis – not
to be confused with mitosis which unfortunately sounds very similar.
You may also have heard meiosis pronounced as mee-osis. We've been guilty of that.
But technically if you look up the pronunciation, "my-osis" is the correct one, so we'll stick with that.
Meiosis is a process that contributes to the variety so
we're going to jump right into it. You might remember that humans have 46 chromosomes. That’s
how many all the body cells in your body have. But there are some cells in your body that don’t have
46 chromosomes. Human sperm cells and human egg cells actually only have 23 chromosomes. Why?
Well think about it. If a human sperm cell has 23 chromosomes and a human egg cell has 23 chromosomes, when they
come together that makes 46. That will now allow a new developing fertilized egg to develop
into a human. Meiosis is all about sperm and egg cells; or if you want to get really fancy, gametes.
Gametes is definitely the fancier word. Meiosis is what we call a reduction division because you have a starting cell
that has 46 chromosomes in humans and your ending cells only have 23 chromosomes. So it's reduced, It's a reduction division.
And in its production of sperm and egg cells, the cells are all different from each other. In males (remember males make sperm)
all the sperm cells are going to be different when they are produced. They're not identical clone sperm cells.
This whole process is really different from mitosis.
Mitosis is really great with repairing damage or worn out cells. Mitosis is really great for growth because mitosis
makes body cells. You know mitosis has to make identical cells
like when you had a cut on your skin, you want to make identical skin cells to replace them.
That's what mitosis was all about.
Meiosis is not about making identical cells
The cells that meiosis makes are not identical and they are going to be sperm and egg cells.
Now before we get into the stages of meiosis, we have to remember
what comes before meiosis. This actually also happened in mitosis. It’s that stage known
as interphase. If you remember interphase, it’s when the cell is growing, it’s replicating
its DNA, it’s carrying out cell processes. Just like mitosis, interphase happens one
time. So it happens before meiosis is going to start. And you might remember from mitosis that little phrase 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 will still use those terms so that's great,
but because meiosis is actually a reduction division,
you're going to divide twice.
You’re going to do PMAT twice. So we’re going to go through it so
it will make more sense.
Here's the good news though about PMAT happening twice.
In meiosis, the stages have numbers after them. So there is no prophasei in meiosis--there is prohase 1 and prophase 2.
And there's no metaphase in meiosis---there is a metaphase 1 and a metaphase 2.
So there's a number and that really helps.
So let's start with the very first step- prophase I.
One thing I like to remember about prophase is that the word “pro,” if you’re
talking about the Greek root, it actually means “before." so 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. Now I know
crossing over probably sounds like something very different, but this is a really neat
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.
It makes for recombinant chromosomes which can contribute to the variety that we were
mentioning that siblings, even if they have they same parents, can be different. Well this is the process that contributes.
Next we move into metaphase I. I always like to remember the M for middle. In metaphase I, the chromosomes
are now going to line up in the middle. It’s a little different from how it is in mitosis though
because they’re still going to be in pairs so they're not going to be in a
single file line. Next anaphase I, think A for away
or A for apart because this time, thechromosomes are going to move apart. And then we end with telophase
I, where we end up with two cells. So this time, you can think of the t for two, at least for now,
at the end of meiosis I, because you will end up with two cells. However, we're going to end up with more than that in meiosis II.
So the very first step in meiosis II is prophase II. We're not doing interphase again. Interphase only happens once. And prophase II is not going to be as eventful as it was
in prophase I because they are not going to have those homologous pairs. And they're also are not going
to be doing any crossing-over. That's done.
You have your chromosomes and the spindles starting to form but it 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 so it is very similar to how it looks in mitosis. They are not in pairs like they were before. Anaphase II, again-remember A for away
This time, though, it’s actually
the chromatids that are going to be pulled apart. Chromatids are getting pulled apart to opposite
sides of the cell. And in telophase II, remember we had two cells that were going to divide,
So now if they both divide, we’re going to end with four cells. Now keep in mind that meiosis is designed in males
to produce sperm cells and in females to produce egg cells. And because of independent assortment
and crossing over, you’re going to have variety. So for example, in males,
the four sperms cells that are produced each time, they are all different from each other. And they're going to be different
from the original starting cell.
because that original starting cell had 46 chromosomes and in humans, those ending sperm cells only have 23 chromosomes. So they are
not identical to the original and they are not identical to each other, that contributes
to variety. Also, just something to think about- scientists are often looking
into the process of meiosis because sometimes those chromosomes don’t separate correctly.
It’s called nondisjunction when maybe a cell receives too many or too few chromosomes
because they do not separate correctly. This can contribute to some genetic disorders, which is something scientists
are often studying. And when we get to genetic disorders, we’ll talk about that relationship
with meiosis. Well that’s it for the Amoeba Sisters, and we remind you to stay curious.
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