The big thing that happens is that eggs or sperm are made. The word “gamete” includes both egg and sperm cells. People have 23 pair of chromosomes in almost every cell in their bodies (but not their gametes). The chromosomes come in pairs because you got 1 chromosome #1 from your mom and one chromosome #1 from your dad. It’s like that for every chromosome. If a sperm cell with 23 pair of chromosomes got together with an egg cell with 23 pair of chromosomes, the baby would have 4 of each chromosome, and that just won’t work. Even if it did, the next generation would have 8 of each chromosome, and it would double every time.

So meiosis separates the two chromosomes from each pair, giving each gamete only one copy of each chromosome. That way when the egg and sperm get together the baby has only 2 of each chromosome.

The benefit of sexual reproduction, instead of producing identical clones, is variation. Each egg is different from every other egg, and the same is true for sperm. This happens for 2 reasons:

1. A gamete randomly gets either the copy from mom or the copy from dad independently for each chromosome. This means that a gamete might get chromosome #1 from mom, but it’s pure chance whether it gets chromosome #2 from dad or mom.

2. During one phase of meiosis (prophase), the chromosomes swap parts. Before the chromosomes are split up, each one makes a copy of itself, so for a while there are 4 copies of each chromosome in the cell that will divide to make gametes. These four can twist together and exchange pieces so that each one is a sort of patchwork of pieces of the chromosome from mom and the one from dad. Even if two cells get a chromosome #1 from dad, for example, one might have a bit from mom. The cell divides twice, so the cells at the end only have 1 copy of each chromosome.

At the end of meiosis in males, 1 cell will have made 4 sperm cells. Each one will have one copy of each chromosome. Each one will be different from all of the other sperm made by the same male. At the end of meiosis in females, there will only be one egg because at each division, one cell hogs most of the cytoplasm (the liquid and organelles in the cell). It will have one copy of each chromosome. Each one will be different from all of the other eggs made by the same female.

Non-human animals and plants have different numbers of chromosomes, but if they make egg and sperm cells, they do meiosis.

Why do you think an egg has to have a lot of cytoplasm and a sperm cell doesn’t?

If you are interested in questions like this, you may want to study cell biology or genetics.

Meiosis is a type of cell division that results in the production of gametes, or sex cells, of multi-cellular organisms. Unlike typical somatic (body) cells, gametes are haploid, which means they have one copy of the full genome as opposed to two. Meiosis occurs in two phases, Meiosis I and Meiosis II.

In Meiosis I, we begin with a diploid cell (has two copies of the full genome). The first phase of Meiosis I is called Prophase I. It is during Prophase I that DNA is exchanged between homologous chromosomes in a process called "crossing over" or "genetic recombination," which is important for maintaining diversity in the genetic pool from one generation of individuals to the next. The nuclear envelope also dissolves in this phase, centrioles move to the poles of the cell, and microtubules/spindles come from the centrioles and attach to the centromeres of the chromosomes. After Prophase I comes Metaphase I. In Metaphase I, homologous pairs of chromosomes align along the metaphase plate. The next phase is Anaphase I, where the chromosomes move apart two the two sides of the elongating cell, along the spindle fibers. Subsequently in Telophase I, the microtubules disappear and the cell divides into two cells, each with one set of chromosomes. Each chromosome has a pair of "sister chromatids."

From there, Meiosis II begins. The first phase of Meiosis II is Prophase II, where again the nuclear envelope and nucleoli disappear, and the centrioles move to opposite poles of each of the two new daughter cells. Metaphase II comes next, and similarly to Metaphase I, the sister chromatids line up along the center of the cells. In Anaphase II, the sister chromatids move apart toward opposite ends of the cell, and in Telophase II, the cells divide and nuclear envelope/nucleolus reappear. At the end of Meiosis II, there are four cells, each with a haploid set of chromosomes.

The short of it is that the cell first replicates the DNA (thus making itself temporarily tetraploid), and then divides twice to make four haploid daughter cells - in animals, these are gametes, and in plants, they're spores. I don't know what they are in fungi but it's different yet.

There are a number of phases of meiosis involving the movements and rearrangements of chromosomes to ensure that the resulting daughter cells each have one copy of every gene, as well as to ensure that the possible outcomes are as varied as possible. These include crossing over (in prophase I, pairs of chromosomes from each parent exchange genetic material), and independent assortment (in anaphase I, whether a given chromosome from one parent goes to a given daughter cell has no effect on which parent's copy of a different chromosome goes to the same daughter cell). Otherwise, the two cell divisions are similar to those of mitosis, including the same four phases (prophase, metaphase, anaphase, telophase, cytokinesis).

There are two types of cell divisions our cells undergo: mitosis and meiosis. Mitosis is regular cell division in which the copied cells, or daughter cells, are exactly alike with the same DNA. Meiosis is cell division for gametes, or reproductive cells (sperm and egg). We have 23 pairs of chromosomes (which contain our DNA), so 46 in total. In pictures, the two pairs look like criss-crossed sticks. In mitosis, the number of chromosomes is preserved and we end up with 46 chromosomes in the daughter cells. In meiosis, the number is halved and we end up with 23 total in each cell. The reason is because in a regular cell, 23 chromosomes come from the mother and the other 23 come from the father. So you need meiosis to divi up the chromosomes so they can add later during reproduction.

The phases is meiosis and mitosis are the same, except meiosis undergoes 2 divisions. The order for mitosis is: interphase, prophase, prometaphase, metaphase 1, anaphase 1, telophase 1, interphase 2, metaphase 2, anaphase 2, telophase 2, cytokinesis.

​Interphase: the cell has 46 chromosomes and it starts to compact the chromosomes into chromatin (densely packed DNA).
Prophase: chromosomes double to 92 chromosomes so they can crossover. Crossing over occurs only in meiosis where the two different sets chromosomes combine to make a new mixture (it's hard to explain it, you should look at a picture). Crossing over is the reason for genetic variety.
Prometaphase: the nucleus of the cell divides are the microtubules (fibers) attache to the centromere, or center of the chromosomes.
Metaphase 1: all chromosomes line up in the middle of the cell.
Anaphase 1: chromosomes pull apart
Telophase 1: cell division begins
Interphase 2: 2 cells are formed with 46 chromosomes each (this time they don't double up to cross over)

To visualize and completely understand the process of meiosis (and mitosis) you should really look at some pictures, either in your text book or online.