The advantage of sexual reproduction is that it generates genetic diversity, which makes a population of mating organisms better able to survive environmental pressures. Meiosis is the process of producing gametes, which are sperm cells and egg cells. Gametes have only half the number of chromosomes that normal cells have, because a sperm and an egg fuse to form a cell that has the full number of chromosomes. Genetic diversity arises due to the shuffling of chromosomes during meiosis.
Process of Meiosis
A man produces sperm and a woman produces eggs because their reproductive cells undergo meiosis. Meiosis starts with one cell that has the full number of chromosomes specific to each organism -- human cells have 46 chromosomes. It ends with four cells, called gametes, that each have half the full number of chromosomes. Meiosis is a multi-step process in which a cell makes a copy of each strand of DNA, called a chromosome, and then divides twice. Each time it divides, it cuts its DNA content in half. In humans, a cell goes from having 46 strands of DNA, and then 96 after each is copied. The first division of meiosis cuts 96 in half into 46. The second division cuts 46 into 23, which is the number of chromosomes in a sperm or an egg.
At the beginning of meiosis, the chromosomes condense from long strands into short, thick finger-like structures. In humans, condensed chromosomes look like an X. Half of the 46 chromosomes in a human cell came from the mother, while the other 23 are similar but came from the father -- they form 23 pairs, like 23 pairs of non-identical twins. Chromosomes that form a pair are called homologous chromosomes. During the early part of meiosis, the homologous chromosomes pair up with their non-identical twins and exchange regions of DNA. This process is called crossing over, and results in a shuffling of DNA regions between two homologous chromosomes. Chromosomes are purposely broken and rejoined in new combinations.
Meiosis not only shuffles regions of DNA between homologous chromosomes, it shuffles whole chromosomes among the four gametes that result at the end. The distribution of chromosomes among four gametes is called random segregation. If the process of “crossing over” is like tearing blue cards and red cards apart, and then taping the pieces together to get striped cards, then “random segregation” is combining a red deck and a blue deck, shuffling them, and then randomly dividing them into four decks. Random segregation produces four decks of cards that contain different combinations of blue and red cards.
The third way that meiosis generates genetic diversity is through the separation of homologous chromosomes into the gametes. As described above, homologous chromosomes are like pairs of non-identical twins. One chromosome of the pair came from mom, the other from dad. Each homologous chromosome can contain the same genes, or slightly different versions of the same gene -- which is why they are like non-identical twins and not identical twins. Independent assortment describes the process in which the two homologous chromosomes of a pair must go into separate gametes. This ensures that each gamete can have only one of two homologous chromosomes, meaning each can have only one version of a gene, though the original cell might have had two slightly different versions of a gene.