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Monday, November 27, 2006

I'm thankful for Cell Division

Cell Division doesn't get a lot of time on CNN. Perhaps it should. It's one of the many things that make life possible. We take it too much for granted. Without cell division, we wouldn't be here. How so? (I recently posted on the question of why we have cells in the first place.) We are made of many cells, all coming from one single one, the fertilized egg. We have become this closely linked community of cells by cell division.

Living things can be grouped into two groups, based on their cell type. Prokaryotes have no nucleus or chromosomes; eukaryotes have both. We are eukaryotes. Prokaryotes have mechanisms of cell division, but I'll stick with eukaryotes in this post.

During their science education, students are taught about mitosis and meiosis. These two related processes are ways of controlling the duplication and division of chromosomes. Chromosomes carry almost all of our genes, our heredity.

First, mitosis. Mitosis copies each chromosome as a cell divides. Therefore, when this happens, each of the two daughter cells get the same genetic material as the parent cell. (There are occasional mistakes in the copying process, but they are extremely rare.) That means that, for example, I have the same genes for eye color in each eye, and the same genes for skin color all over my body. (There are, of course, much more important genes than these, but we can see what these do directly.) I have, in fact, the same genes in almost every cell that were in the fertilized egg that gave rise to me, over six decades ago.

Since most of an organism's genes are important, and since some of them are expressed in every cell of that organism, mitosis is a critical process. We depend on it absolutely.

So what is meiosis? Meiosis is a process that does two critical things. It can be thought of as a variation on mitosis. What are these two things? One of them is to cut the number of chromosomes in half. The second is to introduce variety. Unlike mitosis, the daughter cells resulting from meiosis are not identical.

Why is cutting the chromosome number in half so important? Because two cells, a sperm and an egg, fuse to make a zygote, a fertilized egg. If they had the same chromosome number as the organisms that produced them, the chromosome number would double every generation, and reproduction would be impossible.

What about the variety? I received 23 chromosomes from my father, and 23 from my mother, in the sperm and the egg that went to form me, back when I was a fertilized egg, 23 pairs in all. (Human chromosomes are of 22 types, plus another pair that determine the the sex of the new organism.) As a male, I have had a group of cells, in my testes, where meiosis is happening all the time. I'm constantly producing new sperm. Each of these, because of the variety produced by meiosis, may be unique. Let's imagine that we have one of the little creatures by the tail, and can check on its chromosomes. Let's see. Chromosome 1 came from my dad. Chromosome 2 did, too. But chromosomes 3, 4, and 5 came from my mother, and 6 from my father. Over eight million possible combinations (2 to the 23rd power) are possible. And, it's more that that, much more than that. If I could really look at the chromosomes in a sperm, chromosome 2 might be seen to be mostly from my father, but with a little group of genes which came to me from my mother's chromosome 2. It seems possible that I never have, and never will, produce two sperm which are exactly alike. Meiosis also occurs in the production of the egg. The details are different, but the result is the same -- variety. I would expect that, of the hundreds of eggs my wife had or produced during her time of fertility, no two were alike. So, we have two daughters, alike in some ways, but certainly not identical. All because of meiosis. See "Why is there sex?"

This variety is not just good because it produces organisms, including humans, with various abilities and talents. It is good because it makes it possible for natural selection to work. The variety that means that, under some circumstances, some organisms are more fit than others, hence will have more offspring, is generated by accidents to the genetic material during these two processes of cell division, and by the reshuffling of the chromosomes that occurs during meiosis.

I believe, but cannot prove, that God planned these two processes, and made them happen in living things. I'm glad He did.

By the way, I'm also thankful for the Wikipedia, and for Rebecca, who suggesting that November be a month of blogger gratitude emphasis. I previously posted musings about my gratitude for Carbon atoms, and for the electromagnetic spectrum. I also thank you, my readers.

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