有丝分裂和减数分裂(mitosis_and_meosis)英文版学习资料.ppt

* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * Karyotype Turner’s Female * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * Crossing Over 2.(10) Outline the formation of chiasmata in the process of crossing over. Synapsis- homologous chromosomes pair up and form a tetrad Tetrad- 2 chromosomes joined together (4 chromatids) Crossing over- occurs in tetrads, mixing of chromosomes- Increases genetic diversity Crossing Over /biosci10v/bis10v/media/ch07/crossover.html Meiosis 3. (10) Explain how meiosis results in an effectively infinite genetic variety in gametes through crossing over in prophase I and random orientation in metaphase I. Independent assortment leads to the random aligning of homologous chromosomes Without crossing over there are 223 different types of gametes Crossing over leads to new combinations of alleles With crossing over there are an infinite number of gamete combinations Metaphase I Pairs of homologous chromosomes line up at the equator Anaphase I Homologous chromosomes separate and move to opposite poles Telophase I Homologous chromosomes gather at opposite poles and cytoplasm divides. Meiosis II Meiosis II- end up with 4 haploid cells Prophase II New spindles form around the chromosomes Metaphase II Chromosomes line up at the equator Anaphase II Chromosomes separate, now forming sister chromosomes that move towards opposite poles. Telophase II Chromosomes cluster at the poles and the nuclear envelope begins to form around each set of chromosomes Cytokinesis Cytoplasm divides and results in 4 haploid cells. Spermatogenesis Spermatogenesis- production of sperm 1 spermatid ? 4 sperm cells 1:4 ratio Oogenesis Oogenesis- production of eggs 1 oocyte ? 1 egg + 2 polar bodies 1:1 ratio unequal cytokinesis Mitosis vs. Meiosis Mitosis Meiosis Number of divisions 1 division 2 divisions Number of cells produced 2 4 Chromosome number Diploid (2n) Haploid (1n) Crossing over No Yes-Pr