Meiosis: Reduction divisional
What is Meiosis?
Meiosis is the process in eukaryotic, sexually-reproducing animals that reduces the number of chromosomes in a cell before reproduction. Many organisms package these cells into gametes, such as egg and sperm. The gametes can then meet, during reproduction, and fuse to create a new zygote. Because the number of alleles was reduced during meiosis, the combination of two gametes will yield a zygote with the same number of alleles as the parents. In diploid organisms, this is two copies of each gene.
Function of Meiosis:
Meiosis is necessary for many sexually-reproducing animals to ensure the same number of chromosomes in the offspring as in the parents. The act of fertilization includes two cells fusing together to become a new zygote. If the number of alleles of each gene is not reduced to 1 in the gametes that produce the zygote, there will be 4 copies of each gene in the offspring. In many animals, this would lead to many developmental defects.
In other organisms, polyploidy is common and they can exist with many copies of the same gene. However, if the organism cannot survive if they are polyploidy, meiosis must occur before reproduction. Meiosis occurs in two distinct divisions, with different phases in each.
Human meiosis occurs in the sex organs. Male testis produce sperm and female ovaries produce eggs. Before these gametes are made, however, the DNA must be reduced. Humans have 23 distinct chromosomes, existing in homologous pairs between maternal and paternal DNA, meaning 46 chromosomes. Before meiosis, the DNA in the cell is replicated, producing 46 chromosomes in 92 sister chromatids. Each pair of sister chromatids has a corresponding (either maternal or paternal) set of sister chromosomes. These pairs are known as homologous chromosomes. During meiosis I, these homologous chromosomes line up and divide. This leaves 23 chromosomes in each cell, each chromosome consisting of sister chromatids. These chromatids may no longer be identical, as crossing-over may have occurred during metaphase I of meiosis I. Finally, meiosis II takes place, and the sister chromatids are separated into individual cells. This leaves 4 cells, each with 23 chromosomes, or 4 haploid cells.
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