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Transcript
Chapter 11 (combine with ch. 10) II. Homologous chromosomes A. Two chromosomes of each pair in a 2N cell=homologous chromosomes 1. Each pair of homologous chromosomes has genes for the same traits (one from ma/one from pa) III. Meiosis (2N to N) A. Production (cell division) of N cells =sex cells/gametes 1. Male gametes=sperm (testes) a. spermatogenesis-N 2. Female gametes=eggs (ovaries) a. oogenesis-N 3. Sperm (N) fertilizes an egg (N)=zygote (2N) N + N = 2N B. Sexual reproduction C. Meiosis consists of 2 separate divisions: meiosis I (reductional) & meiosis II (equational). 1. Meiosis I begins with one diploid (2N) cell. 2. Meiosis II is simply mitotic process. 3. End of meiosis II = 4 haploid(N) daughter cells Diploid & Haploid cells Chromosome Numbers of Common Organisms Organism Body Cell (2n) Gamete (n) 4 Fruit fly 8 Garden pea 14 7 10 Corn 20 12 Tomato 24 Leopard Frog 26 13 Apple 34 17 Human 46 23 Chimpanzee 24 48 Dog 78 39 Adder’s tongue fern 1260 630 D. Phases 1. Interphase I a. cell replicates its chromosomes • 2 identical sister chromatids held together by a centromere Interphase 2N=4 2. Prophase I a. homologous chromosomes coil up (thicken) & nuclear memb disappears b. spindle forms c. tetrad = h.c. line up with each other gene by gene d. h.c. actually break & exchange genetic material-crossing over * genetic recombination (leads to variation) * new combi of genes Homologous chromosomes Sister chromatids Tetrad Nonsister chromatids Crossing over in tetrad Gametes 3. Metaphase I a. centromere of each chromosome becomes attached to a spindle fiber b. equator = h.c line up in the middle Metaphase I 4. Anaphase I a. h.c. separate & move to opposite ends of cell (poles) b. cell elongates 5. Telophase I a. nuclear memb reappears b. spindle broken c. cytokinesis d. 2 haploid daughter cells Telophase I e. each daughter cell has ½ the genetic information of original cell (parent) 6. Interkinesis a. no DNA replication 2nd Division similar to Mitosis 7. Prophase II a. nuc memb disappears b. centriole replicates 8. Metaphase II a. chromosomes pulled to center of cell b. line up randomly at equator Metaphase II 9. Anaphase II a. centromere of each chromosome splits b. sister chromatids separate & move to opposite poles 10. Telophase II a. nuclear memb reappears b. spindles breakdown c. cytokinesis *cell furrowing (animal) d. 4 haploid cells-gametes *not identical E. Genetic recombination 1.Genetic recombination=reassortment of chromosomes & genetic information, either by crossing over or by independent assortment of h.c. a. Prophase I b. major source of variation among orgs 1. Polyploidy a. orgs with more than the usual number of chromosome sets • polyploidy is rare in animals & almost always causes death of zygote IV. Differences between mitosis & meiosis? •DY