On to MEIOSIS. This is where things start to get messy. Like a teenage boy showed up and threw a week’s worth of socks all over his room… but I digress.
So in the case of meiosis, the body is interested in making eggs (and sperm) that are different from each other. Our end product will NOT be identical.
Let’s pause here for emphasis.
Remember MITOSIS? That’s when we wanted identical. We keep those cells in our bodies and use them. Randomness and change is NOT what we want for growth and repair. But when it’s time to make children (sock puppet babies), we want to roll the dice. We want randomness. Genetically speaking. Those of us who have chosen to reproduce already know that life chaos is a common outcome of such a choice and not necessarily what we thought we were signing up for.
OK, back to Meiosis. This time, we need to take a cell, duplicate its DNA, then divide it twice, reducing it to a haploid state. Haploid, you say?
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).
- Chromosomes duplicate (same as in Mitosis)
- 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.
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.
CELL DIVISION ROUND 1!
- Tetrads line up on the spindle’s metaphase plate —>
- 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)
- 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.
- 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.)
- 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).
So, while we were watching Jill’s cells divide down to a haploid state, Joe’s cells were doing the same thing. Now, Jill produced eggs, and Joe produced sperm.
(As another side note, I’ll mention that it would be wrong to say Jill produced 4 eggs. She only made one. During each round of cell division to produce an egg, one cell gets all the goodies in the cytoplasm and grows much larger. The DNA in the tiny cells are jettisoned. BUT we don’t know which ones were tossed away, and we’re looking at how we get genetic variation… so for argument’s sake, we’re going to pretend we get 4 eggs. In the case of sperm, it’s the cytoplasm that’s jettisoned, so you really do get four functional sperm at the end.)
Ready to intoduce the third and final level of genetic variation? Well, toss those socks in the drier and let them get busy. 🙂
Look at the eggs and look at the sperm. During sex, one of those sperm will beat out the other to fertilize an egg. Do you see how the sock (DNA) makeup of the fertilized egg would differ based on which egg combines with which sperm? This is how we get another bit of randomness, the final throw of the dice, which we refer to as random fertilization (genetic varation #3).
So there are THREE sources of genetic variation. (And I will tell you as a former biology professor that this WILL be on the test. It will be on EVERY biology teacher’s test. Every. Single. Time.)
1) Crossing over in the tetrad state
2) Independent assortment of chromosomes (when mom’s and dad’s duplicated chromosomes separate in metaphase I )
3) Random fertilization of eggs by sperm
Leaving out crossing over, that’s 16 different gamates Jill can produce. Combine that with Joe’s 16 possible gamates and… math: 16 x 16… That’s 256 possible different children – and I’m only accounting for two of the three sources of genetic variation.
(A human cell contains 23 pairs of homologous chromosomes. I’ll do the math for you: that’s 8.5 million possible gamates from independent assortment alone. Combine that with random fertilization, and you have 72 trillion possible different outcomes. That doesn’t even account for crossing over.)
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