Meiosis

Meiosis Cell division to form the gametes, sperm (male gamete) and egg (female gamete). Characteristic of eukaryotes only: not in prokaryotes. Normal cells are diploid: 2 copies of every gene. Diploidy is useful because 2 copies of every gene means that there a backup copy if one gets mutated. Mutations are very frequent in the cells of large organisms. We wouldn’t survive with just one copy of each gene. Gametes are haploid: 1 copy of every gene Need to choose 1 copy of each gene randomly. Why have sexual reproduction? Shuffling of alleles between parents and offspring leads to new combinations. Bad combinations die without reproducing; good combinations survive and reproduce more offspring.

Overview Start with a diploid cell, with 2 copies of each chromosome, one form each parent. The two copies are called homologues. Chromosomes each with 2 chromatids attached at the centromere. Use 2 cell divisions: Meiosis 1. First separate the homologues Meiosis 2. Then separate the 2 chromatids. The stages of meiosis have the same names as in mitosis: prophase, metaphase, anaphase, telophase. Each of the 2 cell divisions has all of these stages. Meiosis 1 is unusual and needs a bit of study, but meiosis 2 is just like mitosis

Meiosis 1 Two important events in meiosis 1: crossing over in prophase, and the pairing of homologues in metaphase. Crossing over. Homologues break at identical locations, then rejoin opposite partners. This creates new combinations of the alleles on each chromosome. Occurs randomly several times on every chromosome. Results in mixing of the genes you inherited from your parents.

More Meiosis 1 The main event in meiosis 1 is the pairing and separation of the homologues. At metaphase, the pairs of homologous chromosomes line up side by side. This does not happen in mitosis of meiosis 2, but only in meiosis 1. At anaphase of meiosis 1, the pairs of homologues are pulled to opposite poles by the spindle. Note: the centromeres do NOT divide; the chromosomes remain in the 2-chromatid X-shaped state.

Result of Meiosis 1 Go from 1 cell to 2 cells. Each daughter cell contains 1 copy of each chromosome: they are haploid, with the chromosomes still having 2 chromatids each. For humans: start with one cell containing 46 chromosomes (23 pairs) to 2 cells containing 23 chromosomes. As a result of crossing over, each chromosome is the mixture of the original homologues.

Meiosis 2 Meiosis 2 is just like mitosis No replication of DNA between meiosis 1 and meiosis 2. Chromosomes line up individually on the equator of the spindle at metaphase. At anaphase the centromeres divide, splitting the 2 chromatids. The one-chromatid chromosomes are pulled to opposite poles.

More Meiosis 2

Summary of Meiosis 2 cell divisions. Start with 2 copies of each chromosome (homologues), each with 2 chromatids. In meiosis 1, crossing over in prophase mixes alleles between the homologues. In metaphase of meiosis 1, homologues pair up, and in anaphase the homologues are separated into 2 cells. Meiosis 2 is just like mitosis. The centromeres divide in anaphase, giving rise to a total of 4 cells, each with 1 copy of each chromosome, and each chromosome with only 1 chromatid.

Life Cycles Meiosis converts a diploid cell into haploid cells. Fertilization combines the 2 haploid gamete cells (sperm and egg) back into a diploid cell. Eukaryotes alternate between diploid and haploid stages. This is called the life cycle of the organism. In plants, the haploid cells grow into multicellular organisms. Unicellular eukaryotes and many of the more primitive plants (such as mosses and ferns) spend most of their time as haploids. The diploid stage in these organisms is quite short. In higher plants (the conifers and flowering plants), the haploid stage is very short and small. Pollen grains, for instance, have a total of 3 haploid cells in them, and the ovules contain 8 haploid cells. The rest of the plant body is diploid.

Gamete Formation in Animals Differences between male and female gametes. In males, all 4 products of meiosis develop into sperm cells. They lose most of their cytoplasm, remodel their cell shape, and grow a long flagellum (tail). In females, the cell divisions of meiosis are asymmetric: most of the cytoplasm goes into 1 of the 4 meiotic products, which becomes the egg. Eggs also develop large amounts of yolk proteins, which are used to feed the developing embryo. The other 3 meiotic cells are small “polar bodies”, which degenerate.