Transcript Meiosis ppt
Chapter 10 Sexual Reproduction & Genetics Part 1: Meiosis Concepts for Review • Chromatin is the relaxed form of DNA • Chromosome is the coiled form of DNA • Sister chromatids are identical copies of DNA (after S-phase/replication) Key Terms • Homologous chromosomes are chromosomes that make up a pair, one from each parent – Not necessarily identical but… – Are the same length – Have the same genes Key Terms • Diploid refers to complete chromosome sets in a cell (or 2N) – N= the number of chromosomes in a gamete (sex cell) – N chromosomes from the female parent + N chromosomes from the male parent= 2N – For humans, 2N = 46 Key Terms • Haploid refers to single chromosomes in a cell (or N) – One-half of the 2N set (the maternal N or the paternal N) – Gametes (i.e. sperm, egg sex cells) are haploid cells – For humans, N = 23 Bottom Line • diploid means that there are the full number of chromosomes where as haploid means that there are half the number of chromosomes. Key Terms • Meiosis is a type of cell division that reduces the number of chromosomes – Forms gametes (i.e. sex cells) – Occurs in the reproductive structures of organisms that reproduce sexually – Occurs in 2 consecutive cell divisions: meiosis I and meiosis II Before meiosis Interphase occurs…. • Replication of DNA occurs. • Chromosomes Condense Sounds familiar right??? Meiosis I: “Reduction Division” PROPHASE I • Homologous chromosomes pair up • Crossing over occurs • Nuclear envelope breaks down • Spindles form Crossing Over • Chromosomal segments (genetic information) are exchanged between a pair of homologous chromosomes METAPHASE I • Spindle fibers attach to the centromeres • Homologous Chromosomes Line up at the equator ANAPHASE I • Homologous chromosomes separate and move to the opposite poles of the cell TELOPHASE I • The cell divides • The result is two daughter cells. • Each daughter cell is haploid. • The daughter cells are not genetically identical **During Telophase I Cytokinesis occurs at the same time!!** Meiosis II: “Mitotic Division” • Nucleus breaks up • Spindles form and attach to centromere • Sister chromatids randomly align at middle of cell • Sister chromatids separate • Chromosomes move to opposite poles of cell • Cells divide • 4 haploid daughter cells result • Each cell is genetically different Summary of Key Concepts • Chromosomes contain genes (humans have 46 chromosomes or 23 homologous pair) • Homologous chromosomes may contain different forms of the same genes • Meiosis reduces chromosome # by ½ and results in sex cells (gametes) • Crossing-over increasing genetic variation Click here for detailed animation of meiosis which set of chromatids illustrates the result of a single crossover of the homologous chromosomes?