Well think about it... We have 23 pairs of chromosomes, half from our mother and from our father. Meiosis is the bodies way of making sex cells or gametes. The easiest example is when we look at a male. The first thing a cell does is to duplicate it DNA, so the cell starts out with 46 pairs of chromosomes. So this cell has a bunch of sister chromatids, and if you know mitosis, goes through prophase, metaphase, and telophase. All this to segregate to two new cells. During metaphase, the process of crossing over occurs (at that point one can see it). Any way, the two new cells each have 23 pairs of chromosomes again, but we can't have that. If this weird sperm cell (remember we're talking about males) was to ever fuse with an egg, one will have 46 pairs of chromosomes, and in this case, more is not better. 100% of the zygotes will abort and never see the light of day.
We humans need 23 pairs of chromosomes, that's it. No more and no less, but don't believe it isn't possible to have, let's say, 24 chromosomes. Consider downs syndrome.
Anyway, back to our two cells. These cells are not like the original parent cell. These cells have different genetics to them, because of crossing over and independent assortment (refer to Mendel for explanation). Imagine a chromsome with two separate strands attached to each other. These strands are call sister chromatids and are relatively identical to each other (at least in size). The cell will run through those different phases I spoke of earlier and split into two new cells, but this time the sister chromatids segregated from each other into a separate germ cell. So now we have four sex cells with 23 chromosomes in them, and not one of them is exactly the same. That's why we look so different from one another and your parents could have twelve kids and none of them will look exactly like you. Even a twin is not really a twin.
I know this is a little overkill, but you need this to understand the concept. Meiosis must split a cell's chromosome number in two because it is going to join with another sex cell (sperm or egg) and fuse to create a lovely zygote. The zygote will grow to create a human being. And for a human being to be a human being, they need 23 pairs of chromosomes. If you need to go through a detailed explanation of the phases, just visit a local textbook. My favorite is Campbell's Biology.
Just think of meiosis as a double mitosis.
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