Chapter 10 Study Guide

Chapter 10 Study Guide - Study Guide 10 UNIT III: - Meiosis...

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Study Guide 10 UNIT III: - Meiosis (and Genetic Variation) - Mendel & the Gene (linkage and crossing over) - How Do Genes Work ( DNA = genes; the genetic code) - DNA Synthesis. .. (Replication) - Transcription and Translation - Control of Gene Expression in Bacteria - Control of Gene Expression in Eukaryotes A. Sexual Reproduction - Mechanisms for Achieving Genetic Diversity 1. A reminder: eukaryotic chromosomes come in pairs ( Fig. 12.1 ). All somatic cells of an organism are diploid , containing a full complement of paired homologous chromosomes ( Table 12.1 ). Karyotyping (Box 12.1) reveals that each of our cells contains 46 (23 pairs of homologous) chromosomes ( Fig. 12.6 ). Male sperm and female egg cells (the germ cells) each contain only one each of the pairs, and are called haploid cells. Meiosis ( Fig12.2, 12.4 ) describes the events involving chromosomes during a pair of cell divisions in which diploid cells give rise to haploid sex cells. Meiosis is the first step in forming egg and sperm cells that will later join during fertilization. 2. Mitosis and meiosis differ in several ways, notably in that the latter is really two divisions, and in that mitosis maintains the same number of chromosomes in parent and progeny cells. Put another way, mitosis generates 2N (diploid) cells while meiosis generates N (haploid) cells, where N = the number of non-homologous chromosomes (see below, or Fig. 12.5 ): 3. The difference between meiosis and gametogenesis. In humans and other animals, meiosis creates haploid cells which must mature by gametogenesis (eggs get larger as they store nutrients while sperm get smaller and grow a flagellum). In some organisms, haploid cells may undergo several rounds of mitosis, in some cases forming an entire haploid organism, before some cells of this haploid organism become gametes (see Fig. 29.21 , alternation of generations ).
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4. Homologous chromosomes and genetic diversity a) Like chromosomes, genes come in pairs called alleles . Alleles of the same gene can code for the same, or for different versions of the same characteristic (e.g., eye color, hair color, etc.). Thanks to the reduction divisions of meiosis and the re-pairing of alleles from different sources during fertilization, each new generation arising from sexual reproduction differs in the overall combination of alleles among its siblings and from the parental generation: The same thing with 2 chromosomes (N=2): (also, see Figs. 12.7, 12.9 )
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b) Pieces of chromosomes can exchange places, increasing the potential genetic diversity of sexual reproduction ( Figs. 12.7 12.9, 12.11 ). Such genetic recombination occurs between homologous chromatids during meiosis. In the microscope, the formation of chiasmata during synapsis in prophase I of meiosis usually indicates that the homologous chromatids are undergoing recombination (i.e., crossing over ; Figs. 12.7; 12.9 ): 5. Bacteria can mate by conjugation , exchanging portions of their DNA and
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Chapter 10 Study Guide - Study Guide 10 UNIT III: - Meiosis...

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