Thursday, 10 January 2008

What's the difference between mitosis and meiosis?

This an amazingly common question, most probably because of its ubiquitousness as an exam topic.

In my opinion, the vast majority of the difficulties can be resolved simply by understanding the point of the two exercises:

  • In mitosis, the cells are simply trying to divide to form two identical copies. For instance, a particular lymphocyte (say the right one to battle the common cold I just picked up) will divide to make numerous replicas of itself. To give another example, recall that a cell has an optimum size (see here for why) - and so if the tissue needs to expand, the cells usually meet this requirement by dividing to make more of themselves, rather than by each cell getting bigger. So you know already what the result will be: two identical cells, each with the full complement of chromosomes (46, or 23 pairs).
  • In meiosis, the body is trying to make germ (sex) cells - spermatozoa or ova. Simply understanding this fact allows you to foresee several consequences. Firstly, meiosis only happens in the testes (if you're a man) or the ovaries (if you're a women). Secondly, although you've obviously gotta start with a normal (46 chromosome) cell, the result must be cells containing half the normal chromosomes (i.e. 23). Why? Because one of your germ cells is going to have to fuse with one of your mate's germ cells in order for conception to happen. The only way to make a normal (46 chromosome) cell when this happens is if each germ cell has half the correct amount, so that the two can be put together to form the normal total.

With that in mind, try this fabulous interactive demonstration of the differences between mitosis and meiosis. The demonstration requires Flash.

That ought to help you understand it. But for the sort of table that your examiner might want (hopefully without the diagrams!), try this helpful web page.

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