11/22/2010 Meiosis Why Meiosis? Bellringer- (write on page 28) • Each type of organism has a specific number of chromosomes (humans have 46 chromosomes) • Because a each parent passes along a chromosome, there has to be some way for offspring to have the same number of chromosomes as their parents and not double the number. Why do we need diversity? What type of reproduction do humans use? Why Meiosis? • Gametes that are produced are sperm cells for males and egg cells for females • Sexual reproduction: when haploid egg and sperm cells join together to form a diploid zygote • In humans, each sperm and egg cell has 23 chromosomes (which is half of 46, the normal number of chromosomes) or Sperm (23) + Egg (23) = Zygote (46) Before Meiosis • Homologous chromosomes are attached to each other Why Meiosis? • For the most part, healthy human zygotes cannot have more than 46 chromosomes • Once formed, zygotes undergo mitosis to grow and develop (asexual reproduction) Before Meiosis • Tetrad: 2 homologous chromosomes attached together (= 4 sister chromatids) – Homologous chromosomes: 2 of the same type of chromosome • Chromosomes copy themselves so that they can be passed along to the daughter cells 1 11/22/2010 Genetic Variation Genetic variation: rearrangement of alleles that can produce completely new combinations (essential for evolution to occur) Genetic Variation • Genetic Recombination: the reassortment of chromosomes and genetic material, either by crossing over or independent assortment, which increases genetic variety Meiosis I—2 new cells are formed Prophase I: • Tetrads condense(appear) • Crossing over occurs between chromosomes • Nuclear membrane disappears Metaphase I: • Tetrads line up in the middle of the cell Genetic Variation • Crossing Over- When the homologous chromosomes pair so tightly that a piece of the chromatid breaks off. The piece changes place with a piece from the other homologous tetrad. This scrambles the traits around = genetic variety. Genetic Variation • Both crossing over and independent assortment are completely RANDOM…so this is why humans can sometimes look completely different from other family members Meiosis I Anaphase I: • Tetrads split and each set of sister chromatids moves to opposite ends of the cell. Telophase I: • Cytokinesis (division of the cytoplasm) results in 2 new cells, each with homologous chromosomes. 2 11/22/2010 Meiosis II 4 new cells formed- *Like Mitosis Prophase II: • Centrioles on the chromosomes form spindle fibers. Metaphase II: Chromosomes (sister chromatids) line up randomly in the middle. Mistakes in Meiosis • Nondisjunction: when homologous chromosomes don’t separate from each other during meiosis • So homologous chromosomes move together into a new gamete giving that gamete too many chromosomes, while the other new gamete is missing the chromosome Meiosis II Anaphase II: • Sister chromatids separate and move to opposite ends of the cell Telophase II/Cytokinesis: • New nuclei are formed and four daughter cells are produced, each containing ONE chromosome from each homologous pair Mistakes in Meiosis • Organisms with extra chromosomes can survive • Organisms with too few chromosomes usually do not survive 3 11/22/2010 Mistakes in Meiosis Conditions that Result from Extra Chromosomes • Polyploidy: when organisms have more than the normal number of chromosome sets Examples of Nondisjunction Turner’s Syndrome – instead of 46 chromosomes, they might have double or triple that number – Can be lethal for animals but not plants (plants just usually get bigger) Genotype (X=Female sex chromosome, Y= male sex chromosome) XO Trisomy X (metafemales) XXX Klinefelter’s syndrome XXY Jacob’s syndrome XYY Down syndrome TRISOMY 21 (3 copies) Patau’s syndrome TRISOMY 13 Lethal condition in males OY Examples of Nondisjunction • Turner’s Syndrome • Edward’s Syndrome – Genotype: Females-X – Trisomy 18 – About 95% die before birth – Of liveborn infants, only 50% live to 2 months, and only 5–10% will survive their first year of life • Down’s Syndrome Comparing Mitosis and Meiosis Mitosis DNA replicates in interphase Meiosis DNA replicates in interphase ONE division of chromosomes Separate sister chromatids TWO divisions of chromosomes Separate homologous chromosomes (meiosis I) Separate sister chromatids (meiosis II) Makes 2 identical DIPLOID cells Makes 4 HAPLOID cells Chromosome # is identical to parent cell Chromosome # is HALF of the parent cell Used for replacing old cells, growth, and repair Used to make gametes 4