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Genetic recombination occurs in a rather simple and interesting way. The main method for genetic recombination is a process called crossing over. In addition to crossing over, genetic recombination can occur just by the way chromosomes line up at the equator. I'll go over each of these ideas here. Crossing Over
Note that although in the beginning of prophase I the sister chromatids of each chromosome within the tetrad are identical, by the end of metaphase I (shown as anaphase I begins in this animation as the tetrad begins to split), the sister chromatids are now different. In fact, every single chromatid within the tetrad is now unique. By the end of meiosis II, each of these chromatids will be in a separate gamete-- so you should now be able to understand how come each gamete has a different genetic composition from the others. The exact way that the chromatids cross over within any one tetrad is completely random. And the number of times that the chromatids cross over is also random... although it averages about three crossing overs that occur for any one tetrad during human cell meiosis. If you consider that there are thousands of genes on any one chromosome, and that the chiasmata can be at any point between any of the genes, and that there can be multiple crossing overs on any one tetrad, you should be able to see that there is almost an infinite number of ways that our chromatids could end up at the end of metaphase I. And consider on top of what I have already mentioned that there are 23 tetrads each undergoing this during meiosis (not just one). So each time one of our cells undergoes meiosis, it will produce 4 daughter cells in a unique way, and none of those will be the same. It is extremely unlikely, therefore, for any two sperm or any two eggs to ever be even somewhat identical. You will see that this is made even less likely by the orientation of the tetrads and chromosomes during metaphase I and II. Orientation during Metaphase I (and II)This method for getting genetic recombination is a little bit less important than crossing over, yet it is still involved. The tetrads line up during metaphase I, and then pull apart during
anaphase I. As they line up, one of the two homologous chromosomes is closer to one
pole, and its homologue is closer to the other pole. In option 1, the blue chromosome is on the left, while in option 2, the green chromosome is on the left. The smaller tetrad below it remains oriented so that the orange chromosome is on the left. Therefore, if the tetrads were oriented on the equator as they are in option 1, the blue and orange chromosomes would go into a daughter cell together (as shown at the bottom). But if the tetrads were oriented on the equator as they are in option 2, the green and orange chromosomes would go into a daughter cell together. You see, just by changing the orientation of one tetrad, the possibilities for the daughter cells at the bottom (2 from option 1 and 2 from option 2) are entirely different. It may not seem like a big deal if one color goes one way and the other color goes the other way. However, if you consider that one of each homologous pair is what you got from your mother, and the other was from your father, it might make more sense... What are the chances that all of the chromosomes you got from your mother would all go toward one pole (and all from your father to the other)? Do you see that there is almost no chance for that to happen? I drew the tetrads here after having crossed over just to show you that crossing over yields even more options for the daughter cells. And really prevents us from making gametes identical to what we received from either our mother or our father. Instead, we end up making gametes that are unique blends of the genetic information we received from our parents-- so that we give genetic information from both our mother and our father into the gametes to create our next generation. All that I described so far was the effect of the way the tetrads
orient during metaphase I. In addition, the In one case (option 1), both chromatids that have picked up pieces of their homologue from crossing over are going into the same daughter cell. In the other case (option 2), they are going into different daughter cells. |
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© 2006 STCC Foundation Press, content by Dawn A. Tamarkin, Ph.D. Last changed: January 21, 2007 |
While there are tetrads, the
chromatids within them shift around a bit. This can lead to chromatids crossing over
one another. Such a cross-over point is called a chiasma, and many of them are
called chiasmata. At such chiasmata, the bits of crossed-over chromatids can swap
with one another. I have tried to show this in my little animation here. For
another figure of crossing over, go peek at this 
In this drawing, I have the tetrads aligned so that the poles are to the left
and right, while the equator for each option runs from top to bottom.
way the chromosome line up during metaphase II can also add to variability
in the daughter cells. You see, the chromosomes in meiosis II do not have identical
sister chromatids. So, in this drawing, I have tried to show how the way the
chromosomes line up during metaphase II can affect the genetic composition of the daughter
cells. Do you see that when the blue chromosome is lined up differently (option 1
versus option 2), the daughter cells that are generated are different?
