16/10/2010 § 1 Comment
So, why sex? One possible answer could be: Meiosis. Because of meiosis, eukaryotic cells have the ability to switch around genes and alleles and create a source of genetic diversity within species. It as a process that is an exact reverse of mitosis. Whereas mitosis begins with one cell and ends up with two, meiosis requires two cells to make one new one. In the midst of this kind of production, a few specific details are switched to create not an identical cell from either of the first two ones but a combination of the two cells.
All eukaryotic cells go through both haploid and diploid phases in their life cycle. When a cell moves from a haploid phase to a diploid phase, they are doing mitosis, but when a diploidy cell moves to a haploidy status, then the cell is doing meiosis.
The first out of eight stages in meiosis is Prophase I. In prophase one, the chromosomes condense and are replicated. They also cross-over and exchange a piece of each other’s DNA. In this phase, homologous chromosomes also find each other and pair up. Spindle fibers and centrosomes line up during Prophase I at opposite ends of the cell and start directing the fibers towards each homologous pair. Below is a crude illustration of the chromosomes condensing and replicating during Prophase I.
In Metaphase I, the second stage out of eight, the fibers place the chromosomes lined up at the equator of the cell, positioned with their respective homolog partner. The way the homologous partners line up at the equator though, is completely up to fate and chance. Mostly chance. Maybe the maternally derived chromosome gets to be on one side of the pole and the paternally derived chromosome has to be on the other side; it’s all up to chance. This is called independent assortment and is a large part in creating genetic diversity during meiosis. The third stage of Anaphase I consists of the fibers pulling one homolog from each pair to opposite sides of the cell. From there, Telophase I and cytokinesis pulls the cell apart and produces two daughter cells, where each cell can relax, and start again.
Prophase II is when the spindle apparatus reforms in each daughter cell. In Metaphase II, similarly to Metaphase I, the spindle fibers take ahold of each chromosome’s centromere. Anaphase II is when the spindle fibers in the cell each segregate the sister chromatids from their pairs. (Each sister chromatid gets a centromere, though.) Now, in both sister cells, each chromosome (now made up of only one chromatid), move to opposite poles of their cell. In Telophase II, cytokinesis happens again, pinching into the cells and dividing them into another two cells—each. The product is four cells.
Whilst all this happened, the genetic information on all four of the cells can be different because of the cross-over way back earlier during Prophase I. Also, during both metaphases, independent assortment occurred when the chromosomes lined up at the equators in certain positions by chance. All of the organizing and arranging is neat and efficient but the details are all by chance, which makes each daughter cell unique afterwards.
This class, however confusing the topic was and as much as it was difficult to follow and keep up, explained the basics of meiosis, which occurs during sex. The process takes two parenting cells to make one new and unique baby cell. Meiosis is so unlike mitosis because of the newness of the product. Meiosis continuously creates new cells and because meiosis is the process that produces specialized cells like gametes—the reproductive cells of an organism, like the sperm cells or egg cell—and spores, the gametes can fuse together to form new offspring.
Now. Why sex? Meiosis, once again, is the source of genetic diversity in all eukaryotic cells. The diversity and changes in every cell creates variety in the species so say when the environment has to change, the species has a numerous variety of cells in their species so that natural selection won’t kill off all of them. Genetic diversity and variation is essential to the evolution of cells. The cross-over during Prophase I and the independent assortment during both Metaphases in the Meiosis process are therefore essential to genetic diversity and variety, too.
(At least, I think that’s what we were trying to get at last class.)