Recall Jill’s karyotype (the one on the left). She has two copies of each autosomal chromosome, one from mom & one from dad, for a total of 8 chromosomes (4 pair). We call that being diploid (two copies of each chromosome). We can’t simply force together two regular cells and create a new individual (a baby sock puppet), or that baby would end up with 16 chromosomes (8 pair).

THAT, dear readers, is incompatible with life.

And so we take our starting cell, duplicate the DNA, then divide TWICE in order to arrive at a haploid state.

Haploid = half. We want the egg to have 4 sock chromosomes and the sperm to have 4 sock chromosomes. That way, when they meet and join together, the resulting sock puppet baby will have 8 chromosomes (4 pair – with one copy of each from both mom and dad). So we’re going to go from diploid (pairs) to haploid (half=one of each autosome).

Ready?

INTERPHASE 1

• Chromosomes duplicate (same as in Mitosis)

PROPHASE 1

• DNA condenses into chromosomes [Imagine someone using yarn (chromatin) to knit (condense the chromatin) into socks] to appear as identical sister chromatids joined together at the centromere
• Sister chromatids join up with the sister chromatids of their matching autosomal chromosome (homologous pairs) to form a tetrad
• Mitotic spindle forms

Ok, already we’ve reached our first key point. Look closely –>. This is a tetrad. In mitosis, sister chromatids don’t associate with each other (they go line up and separate independently). This is a KEY difference between mitosis and meiosis.

So, why do this? Well, this is the first time we ‘roll the dice’, the first time we introduce genetic variation. (Genetic Variation Step #1)  How does this happen?

While in these tetrads, something really unique happens. Tetrads can and do form chiasmata.

What?

I know, another vocabulary word. But this step is important. Let me show you.

While stuck to each other as tetrads, an element of variation is added to our DNA. Homologous chromosomes (the same-sized chromosomes of mom and dad) can exchange chunks of DNA. We call this crossing over.

What you see here is me, using a pair of scissors and some extra socks, to illustrate what our socks would look like after a chiasmata formed and DNA was swapped. This happens in EVERY tetrad, effectively recombining the chromosome you got from mom and the chromosome you got from dad into something completely new.

THIS is a key reason that no matter how many children you have, none of them will ever be exactly the same. Now, you’ll have to excuse me from doing this with all our socks. It’s a bit more sewing that I’m prepared to do to illustrate meiosis. I will, however, do a little digital sewing for you later on (starting in Anaphase 2) to illustrate this DNA swapping.

From here on out, things look a lot like mitosis, with a few key differences.

 

 

METAPHASE 1

• Tetrads line up on the spindle’s metaphase plate —>

ANAPHASE 1

• Homologous chromosomes separate —>
• Sister chromatids remain attached to each other

Did you catch that last point? Sister chromatids remain attached.

TELOPHASE 1 and CYTOKINESIS

• The cell divides in two.
• Note the immediate difference from mitosis: the two cells are already different from each other.

Look at the two sides.

Do you see what happened? Because the sister chromatids stuck together, two different cells have formed. One cell has all of dad’s #2 chromosome, the other has all of mom’s #2 (etc.). They are NOT identical anymore.

I could have decided to move the green strip sock pair to the left and send the pink striped on to the right. (Maybe the shrimp pair also went right and the octopus left. Imagine the different possible combindations).

This is a second level of randomness. We call this independent assortment. This is the introduction of another layer of genetic variation. (Genetic Variation Step #2)

 

PROPHASE 2

• Another round of cell division starts (this time there is no DNA replication).
• Not that two separate (non-identical) cells are dividing at the same time.

METAPHASE 2

• Sister chromatids line up on the metaphase plate —>

Now, don’t get tricked here. It looks like mitosis, but we’re not producing identical cells, not at all. Not only did we create two different cells by letting the sister chromatids stay together while independently assorting (#2), but recall that we did the whole crossing over thing back at the beginning (#1).

So, looking below into Anaphase 2, I’ve digitally sewn in the DNA swapping because I want you to remember that we are producing four completely different cells. Look below (Anaphase 2) where I’ve drawn in the little black arrows. Back when the chromosomes were in the form of tetrads, crossing over happened. A green-striped swapped part of its DNA with a pink-striped sock. And the shrimp and octopus socks did a little swapping too. (REMEMBER: swapping only happened between homologous chromosomes.)

 

ANAPHASE 2

• Sister chromatids separate from each other

(A side note. If a pair of sister chromosomes were to fail to separate, one cell would be missing a chromosome, while the other had an extra copy. For example, in the case of Down syndrome, two copies of chromosome #21 end up in the same cell. When fertilization occurs, dad’s chromosome #21 means the resulting child will have three copies of #21. )

 

TELOPHASE 2 and CYTOKINESIS

The two cells we had at the beginning of the second round of cell division now result in four haploid cells.

There are two very important points here:

1) Each cell only contains HALF the normal amount of DNA.

2) Because of tetrad swapping (genetic variation #1), none of these cells look like the other (e.g. pink/green strip and octopus/shrimp swapping).