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  • 1. Carly Topic 4.2 Summary: Meiosis Meiosis is a reduction division of diploid (somatic) cells into haploid gametes. The number ofdiploid chromosomes can be represented as 2n, and haploid as n. Meiosis is a process of two divisions.First, chromosomes replicate during the S-phase of interphone to make sister chromatids. Then, duringMeiosis I, the homologous pairs are separated, and in Meiosis II the sister chromatids are separated.Homologous pairs are chromosomes with the same size and structures and with the same genes at thesame loci. In meiosis, the pair is made up of one paternal and one maternal chromosome. Whencrossing over occurs in both divisions, the alleles on each corresponding gene are traded to create aunique chromosome; this precipitates genetic variation. The end product takes one somatic cell andreduces it to four haploid cells with half the number of chromosomes as the original. The Process of Meiosis Meiosis begins with interphase, in which chromosomes are replicated and cell processes arecarried out. Homologous chromosomes pair up during prophase I, and crossing over occurs. InMetaphase I these pairs line up at the equator at the cell according to their bivalents, and then inAnaphase I these chromosomes are pulled apart to opposite poles. During telophase I, two nuclei formand cytokinesis splits the cytoplasm. The second round of division begins with prophase II, in which thenuclei break down. Next comes metaphase II, which is essentially the same process as metaphase I, inwhich the chromosomes line up at the equator of the cell. The chromosomes are pulled apart toopposite poles in anaphase II, and the individual haploid cells are created in telophase II. Non-Disjunction Non-Disjunction is an error that can occur during anaphase when the chromosomes are split,and the chromosomes are pulled to opposite poles unevenly. This creates the wrong number ofchromosomes in the gamete. This creates flaws in the genetic makeup of an organism. For example,trisomy 21 is the non-disjunction that causes Down syndrome. Karyotyping Karyotyping is a pre-natal test to test for gender and trisomy disorders. Chromosomes are takenby amniocentesis, and then they are arranged by shape and size, and finally a picture is taken. Gendercan be determined by chromosome pair 1; if the pair is made up of two X chromosomes, then theembryo is female, and if the pair is made up of an X chromosome and a Y chromosome, then theembryo is male.