A reduction division is required to reduce the number of chromosomes to half so sexual reproduction can occur. 2. This reduction division is known as meiosis. D. The Sexual Life Cycle 1 . Adult cells are diploid, and gametes are haploid. 2. Sexual reproduction Joins haploid gametes to produce a new diploid individual. 9. 2 The sexual Life cycle (P. 173; Figs. 9. 3, 9. 4) A. Somatic Tissues 1 . In the sexual life cycle, there is an alternation of diploid and haploid generations. 2.
B. Meiosis I 1 . Meiosis is similar to mitosis, except that it involves two divisions, meiosis I and meiosis II, and the resulting cells are haploid, rather than diploid like those produced by mitosis. 2. Also, a phenomenon called crossing over occurs during proposes I of meiosis I when pieces of monoester chromatics exchange places to promote new genetic combinations in the offspring. 3. Prior to meiosis l, during enterprise, the DNA replicates and the cell readies itself for cell division. 4.
Meiosis I consists of four stages: proposes I occurs as the chromosomes condense, homologous pair, and crossing over occurs; metastases I involves formation of the spindle apparatus and alignment of chromosome pairs along the center of the cell; in anapest l, homologous chromosomes are separated and pulled toward the poles; and during telephones l, the chromosomes gather at each pole and prepare for the second division. 5. During meiosis l, the alignment of homologous pairs along the center of he cell is random, with different combinations of parental chromosomes possible for each daughter cell. This process is known as independent assortment.
C. Meiosis II 1 . Meiosis II, which also has four stages, follows after meiosis l, and the result is the separation tot the sister chromatics to tort tour haploid gametes. D The Important Role of Crossing Over 1 . Because of crossing over, the sister chromatics of each chromosome cannot be pulled apart during meiosis l. Comparing Meiosis and Mitosis (p. 178) 9. 4 How Meiosis Differs From Mitosis (p. 178; Figs. 9. 8, 9. 9) A. Meiosis has two unique features: synapses and reduction division. B. Synapses 1 . The process of pairing throughout the length of the homologous chromosomes is called synapses. . Synapses holds the homologous chromosomes close enough to allow crossing over to occur. C. Reduction Division 1 . Since DNA only replicates once, before meiosis l, the two divisions of meiosis result in halving the chromosome number in the daughter cells (gametes). 9. 5 Evolutionary Consequences of Sex (p. 180; Fig. 9. 10) A. Sexual reproduction has the capacity to generate new genetic combinations. B. Independent Assortment 1 . The random alignment of homologous during meiosis I results in an astounding umber of possible kinds of gametes that can be produced.
C. Crossing Over 1 . Crossing over allows for combinations of genes that may never have existed previously. D. Random Fertilization 1 . Since the new zygote is the product of two gametes, each with new variation within them, fertilization adds even more genetic diversity. E. Importance of Generating Diversity 1 . The evolutionary process is revolutionary in that the pace of evolutionary change is quickened by genetic recombination. 2. Sexual reproduction adds genetic versatility to the offspring, a trait favored in the vertebrates.