Wednesday, 3 July 2013

Mitosis: Divide and Conquer!

When was the last time you saw 100 trillion organisms all working together for the common good?  When was the last time you looked in the mirror? We don’t often stop to appreciate it, but each one of us represents the organization and cooperation of approximately 100 trillion cells. Each one of them is an organism onto itself, but all of them come together to keep your kidneys filtering and your brain humming like the finely tuned electrical engine that it is. Next time you start to feel insignificant, stop and consider that, in fact, that you are a mountain of biological matter.

What is even more impressive is that at one time, relatively recently in the grand scheme of things, you were only a single cell. Well, put more exactly, you were the combination of a sperm cell from your dad and an egg cell from your mom; but each of those only had half the genetic bounty that you now enjoy so in effect they were half-cells.

It may seem like a lot of work, turning a single cell (what a scientist might call a zygote) into a 100 trillion copies of itself, but the process that did it is happening right now all over your body. The process of normal cell division is called mitosis – there is also a separate process called meiosis that produces the half-cells I mentioned a second ago – and it happens in a few stages that are better explained in the diagram below, rather than me throwing a bunch of jargon at you.

As you can see, it’s a bit of a chore. It involves a lot of coordinated copying and sorting and splitting and a bunch of other tedious work that would get delegated to an intern or co-op student if it were happening in an office. However, despite all the complexity, your cells do this constantly and they do a remarkably good job of it, generally speaking.

Your skin cells get completely replaced once every 35 days on average. The typical blood cell lasts between 5 and 200 days depending which researchers you ask. Overall, it has been suggested that there is not a single molecule in your entire body that was there seven years ago. Yet somehow, you are still you. That is because of devoted and accurate copying of your cells.

But no body is perfect (see what I did there?). Even Babe Ruth struck out once in a while and even Ken Jennings eventually lost on Jeopardy. Likewise, your cells make mistakes every now and then and the results aren’t pleasant. As we learned last week, radioactive energy can wreak havoc on a cell’s ability to copy itself accurately. Chemicals (like those found in cigarettes and processed food), too much time in the sun, or general wear and tear can have the same effect.  The upshot is that the DNA of a particular cell gets confused and starts replicating in overdrive. Things eventually get so out of hand that the clump of cells (now called a tumor) take resources away from other parts of your body, and things begin to shut down. We call that cancer.

Aging is also the result of changes in the way cells divide. As we saw in the diagram near the beginning of this article, cell division hinges on chromosomes. Recent research has shown that chromosomes have pieces of code on their tips that are called telomeres. Each time a cell divides, the telomeres on your chromosomes get a little bit shorter and, as a result, your body functions a little bit less smoothly than it did before. Your muscles get a bit weaker, your skin wrinkles a bit more. The link isn’t a perfect one and a healthy lifestyle can certainly help you postpone some of the consequences, but so far it seems like getting old is just a side effect of cell division.

It really is cool stuff. Your body is an example of nature’s greatest copy machine. Eventually science might even find a way to help your cells out and prolong your life by a couple hundred years. But until then, take it easy on the cigars and Twinkies.


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