Overview: Why Meiosis?
When eukaryotic cells divide to form copies in the next generation, mitosis works well to ensure that each daughter cell has the correct number and type of chromosomes. However, when gametes (such as ova and sperm ) combine to form a zygote, each gamete must have half the number of chromosomes in order to form a zygote with the correct number and type. Otherwise, the new zygote would have twice the number of chromosomes it should have.
What Are Homologous Pairs?
Each chromosome in a pair has exactly the same genetic information. For example, the 46 chromosomes in a human cell form 23 homologous pairs. This is so that gametes can form with all the genetic information necessary for reproduction, with half the number of chromosomes, also known as a haploid number.
Steps Involved in Meiosis I
The process involved in forming haploid gametes is a two-step process. The first steps separate the chromosomes from homologous pairs. The sequence is quite similar to mitosis, as DNA has to be synthesized and other structures within the cell have to be readied before meiosis can begin. After it begins, the chromosomes line up next to each other in a tetrad. The tetrad goes through a prophase, metaphase, anaphase, and telophase. However, the end result in Meiosis I is that each daughter cell has half of the chromosomes of the original cell, and they are no longer in pairs.
Steps Involved in Meiosis II
Meiosis II occurs immediately after Meiosis I. There is no period in which chromosomes are synthesized, because all the chromosomes that are needed have already been formed as a result of Meiosis II. It also has four phases that are similar to those in mitosis. During the prophase, the chromosomes are surrounded by new spindle fibers. During the metaphase , the chromosomes line up across the equator of the cell. During the anaphase, chromosomes move to opposite poles of the long cell. Finally, during the telophase and cytokinesis, the daughter cells are totally separate.
Differences between Male and Female Cells
Although the process is very similar in males and females, there are important differences. Females are already born with all the cells to produce mature ova. From fetal development to puberty, those cells remain in Prophase 1. After puberty, they go through rest of the phases of Meiosis I and II, However, the cytoplasm divides unequally, resulting in a smaller cell called a polar body, and a larger cell, that goes through Meiosis II. That cell also divides unequally, so that there is another polar body, and an ovum that contains a great deal of cytoplasm. In males, a sperm cell with the diploid number of chromosomes divides through Meiosis I to form two haploid cells.
Those two cells divide again during Meiosis II to form another two. Sperm do not form polar bodies, as additional cytoplasm is not needed to nourish a zygote.
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