Meiosis The Diploid Cell Cell with 2 sets of chromosomes Contains chromosomes for each homologous pair Somatic Cells = Body Cells: Diploid All human cells except sex cells are diploid Fertilization restores the diploid number 2n, where n is the number of chromosomes an individual has Humans: n = 23, 2n = 46 The Haploid Cell Cell containing one of each kind of chromosome Ex. sperm and egg cells (gametes) 1 set of chromosomes Half the number of chromosomes of diploid n Sperm + Egg = Zygote Haploid = one of each kind Diploid = two of each kind 2n = diploid n = haploid + or Mitosis vs. Meiosis Mitosis: t = two: Diploid 2n=46 Meiosis: o = one: Haploid n = 23 Meiosis Gametes are produced in specialized body cells Sperm and Egg Cells 2 divisions: Meiosis I and Meiosis II Meiosis occurs in sex cells, not body cells 4 daughter cells Each cell has half the chromosomes of the parent Meiosis I Interphase occurs: the cell grows and DNA replicates Meiosis I begins Original cell produces two new cells Like Mitosis Prophase I DNA Coils tightly into chromosomes Spindle fibers appear Each chromosome lines up next to the homologue Synapis occurs: pairing of homologous chromosomes Tetrad: Each pair of homologous chromosomes Crossing Over Crossing Over: why we do not look exactly like our parents. Crossing Over: why we do not look exactly like our parents. Portions of the chromatid breaks off and attaches to adjacent chromatids on the homologous chromosome Permits the exchange of genetic material between maternal and paternal chromosomes Occurs during Prophase Genetic Recombination Crossing over produces a new mixture of genetic material Causes of Variation Chromosomes are assorted randomly Crossing over may occur Cells do not have identical genetic info as each other or the parent + more chance of survival and evolution - mistakes more likely Metaphase I Tetrads line up randomly along the mid-line Spindle fibers attach to centromeres Anaphase I Homologous chromosomes move to the opposite poles Random separation or Independent Assortment results in separation of maternal and paternal chromosomes. Telophase I Chromosomes reach opposite ends of cell Cytokinesis begins Cell is now haploid Meiosis II Occurs in each cell formed in Meiosis I Interphase does not occur again Prophase II Spindle fibers form and move the chromosomes to the mid-line of the dividing cell Metaphase II Chromosomes move to the mid- line of the dividing cell facing opposite poles of the dividing cell Anaphase II Chromatids separate and move to opposite poles of the cell Telophase II Nuclear membrane forms around the chromosomes in each of 4 new cells Cytokinesis II Cytoplasm divides Cell membrane closes off End Result: Four new cells that contain half of the original cells number of chromosomes Creates sex cells = gametes What exactly is a gamete? Formed by meiosis Haploid reproductive cells In humans, meiosis occurs in the testes and ovaries Meiosis Animation Why do we need haploid gametes? SPERM EGG Female gameteMale gamete 23 Fertilization restores the diploid number 2n 1n 1n 2n fertilization meiosis Mitosis and cell growth Mitosis vs. Meiosis Mitosis vs. Meiosis Animation Nondisjunction Failure of homologous chromosomes to separate properly during meiosis. Both chromosomes of a homologous pair move to the same pole of the cell. 1 gamete has an extra chromosome Or 1 gamete is missing a chromosome Nondisjunction Animation Chromosome Pop Quiz! Chromosome: Rod-shaped coiled and compact DNA Colored body Found in the nucleus Looks like X Humans have 23 pairs of chromosomes Female = XX, Male = XY (23 rd pair) Trisomy: Extra Chromosomes Trisomy 13: Pataus Syndrome Trisomy 21: Downs Syndrome XXX: Trisomy X XYY: Jacobs Syndrome XXY: Klinefelters Syndrome Monosomy: Missing Chromosomes Most do not survive Turner XO : develop as girls, but infertile OY: lethal