Chromosomes and Cell Division

The internet (along with textbooks and teachers) demonstrates an appalling unwillingness to explain simply how many chromosomes there are at any given point in a cell - or for that matter, what chromosomes are! (A pair of squiggly lines? Just one?) For all you Google searchers looking for what I couldn't find, here's the answer in plain terms. Chromosomes are what you find illustrated as a squiggly line. A somatic (body) cell has 23 kinds of them, and 2 copies of each - a total of 46. Before mitosis or meiosis, each squiggly line replicates, producing 46 of the structures we generally associate with the term "chromosome": 2 squiggly lines joined in the middle. In mitosis, these 46 replicated chromosomes are broken back down into their unreplicated predecessors and split amongst two cells.

(In explaining meiosis we'll ignore crossing over, which is explained adequately in many other places.) In Meiosis I, the 46 replicated chromosomes are not split as they are in mitosis, but rather divied up. That is to say, instead of each daughter cell having one copy of both homologous chromosomes, the daughter cell has a replicated set of only one. Even after Meiosis I, then, the daughter cells are already haploid, containing only 23 chromosomes, rather than the normal 46. In Meiosis II, the replicated chromosomes in each daughter cell are split (as in Mitosis) and divided amongst 4 final daughter cells, giving a final product of 4 cells with only one copy apiece of the 23 human chromosomes. These are gamete cells, which combine to form a brand new cell with a full, double set of these 23 chromosomes - half from one parent, and half from the other. The entire process is drawn out below. Enjoy.

And to any teachers or (especially) textbook writers out there: spend the extra four minutes/1 page and draw the whole thing out. It makes everything make sense. Fight the conventional idiocy - if not for our sake, then for that of the poor, defamed chromosomes you've made us resent.