Introductory Biology

7.1 Meiosis forms haploid cells

Sex is operated to produce genetic variation. Egg cells are rare for girls only produce less than 400 in a life time. However, guys produce thousands of sperm cells every day. Mitosis is for producing two identical daughter cells. Meiosis is for reproducing diversity and result with four haploid nuclei.

Meiosis reduce the cells from a diploid to a haploid. The stages of meiosis are the following: prophase I, metaphase I, anaphase I, telophase I, cytokinesis, prophase II, metaphase II, anaphase II, telophase II, and cytokinesis. In prophase I, the chromosomes become visible/condense and the nuclear envelope breaks down. The process of crossing over occurs, as well, where a part of homologous chromosomes are exchanged producing genetic variation. Crossing over results with new combination of genes and one section of the dad and another section of the moms chromosomes are exchanged. Then follows metaphase I the pairs of homologous chromosomes move to the center of the cell. The whole process in which the homologous chromosomes move to the opposite poles of the cell is anaphase I. Telophase I is the next stage where chromosomes gather at the poles of the cell. Then the cytoplasm divides, which is called cytokinesis.  Prophase II is where a new spindle forms around the chromosomes. The chromosomes line up at the equator once again in metaphase II. During anaphase II, the centromeres divide, the chromatids (chromosomes) move to opposite poles of the cell. Then follows telophase II in which the nuclear envelope forms around each set of chromosomes. Finally the cytoplasm divides in the process of cytokinesis.

Meiosis plays a part in genetic variation (a way to evolution by natural selection) in sexually reproducing areas. Independent assortment randomly divides the homologous chromsomes to opposite poles during anaphase I and can make about 8 million combinations of chromosomes in gametes. Crossing over exchanges parts of homologous chromosomes during prophase I (unlimited possible combinations of genes). Also, 64 trillion possible combinations of genes are produced from random fertilization of gametes in humans. Meiosis is involved in the gamete formation in male/female animals, as well. The males and females behave differently, especially the way they make gametes. Spermatogenesis produces 4 small haploid sperm in males very quickly. Oogenesis produces a single large ovum containing much cytoplasm in females very slowly.



One thought on “7.1 Meiosis forms haploid cells

  1. Yurika,

    Good job with your entry for 7.1 on meiosis. You underestimate guys, however. “guys produce thousands of sperm cells every day” should read, “guys produce hundreds of millions of sperm cells every day.”

    Mr. F.

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