Mitosis and Meiosis

Mitosis and meiosis are two similar processes that lead to duplication of the genome and multiplication of cells. Mitosis occurs when a somatic cell (non-sex cells, such as epidermal cells, muscle cells, etc.) divides into two new cells. This happens because our cells need to regenerate because cells have a certain lifetime. Meiosis occurs when a germ cells (such as what leads to eggs or sperm) divides to four new cells that with 1 copy each of the genome that can contribute to a fertilized egg (zygote) and lead to a new organism.

First, each human being has two copies of their genome in all of their somatic cells. These copies may actually be two slightly different forms of a gene, called alleles. During the one cycle of mitosis, these copies are replicated so that the original cell can split into two “new” cells, each with each of the two alleles for each gene.

Our germ cells are still haploid before the process of meiosis. During the two cycles of meiosis, the original germ cell makes a single duplication of the genome (so now there are four alleles of each gene, with two identical pairs). Instead of only dividing once like in mitosis, there are two different division events in meiosis. After genome duplication, the original cell divides into two daughter cells with two copies each of the genome. This is the process of the fist meiotic cycle, Meiosis I. During the second cycle, the genome doesn’t duplicate. Instead the two new daughter cells split themselves into two more new daughter cells each. Now there are four daughter cells, each with one copy of the genome. These cells become gametes, such as sperm and egg cells.

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