Meiosis is the process where the diploid germ cells of sexually reproducing organisms divide to form haploid gametes. These are the cells, commonly known as eggs and sperm, that join during sexual reproduction to become zygotes -- the new diploid cells that grow into embryos through mitotic cell division. The three ways meiosis contributes to genetic diversity are haploid division, recombination and independent assortment.
Constant Number, Infinite Combinations
Each species has a unique number of chromosomes. In humans, these chromosomes exist in pairs. Each chromosome is made up of many different genes, which are in turn made up of various sequences of DNA base pairs. Meiosis is a special form of cell division where DNA from one special type of diploid cell, called a germ cell, is split up and semi-randomly sorted into four daughter cells, called gametes. Each male germ cell splits into four unique sperm cells (gametes); each female germ cell splits into one unique ovum (gamete) and three inert, non-reproductive polar bodies. Gametes from two individuals join during fertilization to form new zygotes.
One Copy, Four Cells
During the interphase that precedes either mitosis or meiosis, DNA copies itself. However, the parent cell splits only once at the end of mitosis, forming two identical daughter cells with full chromosomal compliments, while meiosis involves two rounds of cell division. Meiosis, therefore, leaves each new cell with only one half of the species' full chromosome number. Each of these cells gets the other half by joining with another gametes. This process of producing haploid cells contributes to genetic diversity by forcing each new individual embryo to take one chromosome from each genetic parent.
Alignment and Invasion
During meiosis, chromosomes recombine, which means that some chromosomes exchange some of their segments. During recombination homologous chromosomes line up next to each other and grow synaptonemal complexes, which stick them together. Intracellular machinery snips the edges of some DNA strands and these snipped edges invade neighboring homologs. Eventually this process sorts itself out so that -- under normal circumstances -- each chromosome has the correct number of genes in the right places, but the combinations vary from the parents' original formulas, thus further increasing genetic diversity.
The final way meiosis contributes to genetic diversity is through independent assortment. During the second cell division after DNA replication, each pair of chromosomes splits up. These lonely homologs migrate to opposite ends of the cell -- called the cellular poles. Which chromosome goes to which pole is completely random. This random sorting, combined with recombination and haploidy, ensure that each zygote gets a totally unique assortment of genes.
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