Four gametes are made during cell division by meiosis. The gamete cells have half the number of chromosomes as the cell.
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Courtesy of National Center for Biotechnology Information, NIH
Cell Division by Meiosis
Plants, animals and many other species within the domain Eukaryota are able to make more individuals by sexual reproduction. In its simplest form, this means that offspring are made from gametes. Gametes are cells with only half the number of chromosomes as the organism. When two gametes fuse together, the number of chromosomes is correct, with half of those chromosomes (one from each pair) coming from the mother and the other half coming from the father. This set of chromosomes becomes the genetic instructions that describe how the individual will grow and develop.
Forming the gametes requires cells to undergo a special type of cell division called meiosis, which is really two cell divisions happening one after the other. Meiosis will only occur in cells in the male or female sex organs of the plant or animal.
Step 1: In this first phase, called Meiosis I, two daughter nuclei are made from the nucleus of one cell. In a process similar to mitosis, a cell forms two daughter cells that each have a single set of chromosomes, half the total number in the original cell.
Step 2: Each of the two daughter cells is divided into two making four gametes. This step is called Meiosis II but it is really the same process as mitosis.
Gametes only contain one copy of each gene, but the cell they were made from contained two varieties of each gene. The genes in each gamete are chosen randomly from each pair of alleles during meiosis. One variety or the other will be passed on to the offspring.
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