Mitosis versus Meiosis

Comparing the daughter cells of mitosis and meiosis, you will find that mitosis ends with two diploid daughter cells, each with a complete set of chromosomes. True, each chromosome is composed of only one chromatid, but the second chromatid is regenerated during the S phase of interphase. Mitosis, then, merely duplicates cells, the two daughter cells essentially clones of the original cell. As such, mitosis occurs during growth and development of multicellular organisms and for repair (replacement) of existing cells.

In contrast, meiosis ends with four haploid daughter cells, each with half the number of chromosomes (one chromosome from every homologous pair). In order for one of these haploid cells to produce a “normal” cell with the full set of chromosomes, it must first combine with a second haploid cell to create a diploid cell. Thus, meiosis produces gametes, that is, eggs and sperm, for sexual reproduction. The fusing of an egg and a sperm, fertilization (or syngamy), gives rise to a diploid cell, the zygote. The single-celled zygote then divides by mitosis to produce a multicellular organism. Note that one copy of each chromosome in the zygote originates from one parent, and the second copy from the other parent. Thus, a pair of homologous chromosomes in the diploid zygote represents both maternal and paternal heritage.

The life cycle of a human illustrates the production of gametes by meiosis and subsequent growth by mitosis.Note that the number of chromosomes in diploid and haploid cells is indicated by 2n and n, respectively. Human cells (except gametes) contain 46 chromosomes (23 homologous pairs). Thus 2n = 46. For human gametes, n = 23. In humans, gametes are produced in the reproductive organs, the ovaries and the testes.

In other organisms, such as plants, meiosis produces spores. Spores are haploid cells that divide by mitosis to become a multicellular haploid structure, the gametophyte. Gametes are produced by the gametophyte by mitosis since the organism is already haploid. The gametes then fuse and produce a diploid cell that grows by mitosis to become the sporophyte. Specialized cells in the sporophyte divide by meiosis to produce haploid spores, which germinate to repeat the life cycle. The fern illustrates this type of reproductive cycle.