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.
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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.
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.
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.
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.