Meiosis is defined as a form of cellular division by which sex cells, called gametes, are produced. This occurs in male tests and female ovaries in the human body to produce sperm cells and ovum, both needed for sexual reproduction.
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.
Examples of Meiosis
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.
Fruit flies have 4 pairs of chromosomes or 8 chromosomes in regular cells. Before meiosis takes place, each chromosome is replicated, leaving 8 chromosomes and 16 sister chromatids. Meiosis I takes place, and there are 2 cells, each with only 4 chromosomes. Each chromosome is still made of sister chromatids, and some crossing-over may have occurred during metaphase I. Meiosis II now takes place on those two cells. In total, 4 cells are created, again. However, these cells have 4 chromosomes. When two gametes meet to create a new fruit fly, the resulting zygote will have 8 chromosomes of 4 pairs of sister chromosomes, 4 coming from each parent.
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