AP BIOLOGY GENETIC VARIATION CHAPTER 13: MEIOSIS
Meiosis 1
Meiosis 2
Mitosis vs. Meiosis
Mitosis vs. Meiosis Mitosis 1 cell division Meiosis 2 cell divisions daughter cells genetically identical to parent cell and each other produces 2 cells 2n 2n diploid diploid produces cells for growth & repair ; Also produces new organisms in asexual reproduction no crossing over Meiosis 2 cell divisions daughter cells genetically different from parent and each other produces 4 cells 2n 1n diploidhaploid produces gametes crossing over occurs
Replicated chromosome Fig. 13-9a MITOSIS MEIOSIS MEIOSIS I Parent cell Chiasma Chromosome replication Chromosome replication Prophase Prophase I Homologous chromosome pair 2n = 6 Replicated chromosome Metaphase Metaphase I Anaphase Telophase Anaphase I Figure 13.9 A comparison of mitosis and meiosis in diploid cells Telophase I Haploid n = 3 Daughter cells of meiosis I 2n 2n MEIOSIS II Daughter cells of mitosis n n n n Daughter cells of meiosis II
Putting it all together… meiosis fertilization mitosis + development gametes 46 23 46 23 46 46 46 46 46 23 meiosis 46 46 egg 46 46 23 zygote fertilization mitosis & development mitosis sperm
GENETIC VARIATION Sexual reproduction produces genetic variation in three ways: A. independent assortment of chromosomes B. crossing over C. random fertilization metaphase1
Independent assortment of chromosomes - Occurs during metaphase I in meiosis. - Each homologous pair of chromosomes lines up along the metaphase plate independently of other pairs. - When pairs separate during anaphase I, various combinations of chromosomes go to each pole.
Possibility 1 Possibility 2 Two equally probable arrangements of Fig. 13-11-3 Possibility 1 Possibility 2 Two equally probable arrangements of chromosomes at metaphase I Metaphase II Figure 13.11 The independent assortment of homologous chromosomes in meiosis Daughter cells Combination 1 Combination 2 Combination 3 Combination 4
To calculate the number of possible - Gametes from offspring do not have same combination of genes as gametes from parents. Random assortment in humans produces 223 (8,388,608) different combinations in gametes To calculate the number of possible independent assortment combinations of gametes from a parent, calculate 2n where n = haploid number. offspring from Mom from Dad new gametes made by offspring
B. Crossing over Crossing over creates completely new combinations of genes on each chromosome. Creates an infinite variety of gametes.
Recombinant chromosomes Fig. 13-12-5 Prophase I of meiosis Nonsister chromatids held together during synapsis Pair of homologs Chiasma Centromere TEM Anaphase I Figure 13.12 The results of crossing over during meiosis Anaphase II Daughter cells Recombinant chromosomes
C. Random fertilization Sperm + Egg = ? Any 2 parents will produce a zygote with over 70 trillion (223 x 223) possible diploid combinations.
Sexual reproduction creates variability Sexual reproduction allows us to maintain both genetic similarity & differences. Jonas Brothers Baldwin brothers Martin & Charlie Sheen, Emilio Estevez
E. Sperm production Spermatogenesis continuous & prolific process Epididymis Testis germ cell (diploid) Coiled seminiferous tubules primary spermatocyte (diploid) MEIOSIS I secondary spermatocytes (haploid) MEIOSIS II Vas deferens spermatids (haploid) spermatozoa Spermatogenesis continuous & prolific process each ejaculation = 100-600 million sperm Cross-section of seminiferous tubule
F. Egg production Oogenesis eggs in ovaries halted before Anaphase 1 Meiosis 1 completed during maturation Meiosis 2 completed after fertilization 1 egg + 2 polar bodies unequal division Meiosis 1 completed during egg maturation ovulation Meiosis 2 completed triggered by fertilization