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Chapter 9

Biology - Chapter 9 notes (Meiosis).docx

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Biology 1201A
Richard Gardiner

Biology: Exploring the Diversity of Life Chapter 9: Meiosis (PAGES 181-182, 191-203) - Sexual vs. Asexual reproduction o Sexual Reproduction  During Fertilization, the nuclei of an egg and a sperm cell fuse, producing a cell called a zygotes  Production of offspring through the union of male and female gametes (A mature haploid)  Each new individual gets ½ genetic information from one parent & ½ from other parent  Offspring are genetically different from both parents  Sexual reproduction produces much greater variation:  Evolution requires pre-existing genetic variation  Sexual reproduction provides much of the pre-existing variation o Asexual Reproduction  Individual inherits all its genes from one parent  Offspring are genetically identical to parent 1. One celled organisms divide 2. Self-Pollinating flowers (dandelions) 3. Hydra budding  Genetic variation comes only from mutations  Greatly reduced ability to evolve - Homologous Chromosomes ((homo = same; logos = information)) o Are of the same size o Contain the same gene loci (gene location) o May contain different alleles (alternate forms of a gene) o A diploid nucleus contains two sets of homologs (2n)  One set from each parent  1 chromosome in the pair – Paternal Chromosome; derived from the male parent  Other chromosome – Maternal Chromosome; derived from female parent - Sexual Cycle o Cells that are destined to divide by meisosis are called Meiocytes o Somatic (body) cells have 23 pairs of chromosomes  Homologous chromosomes = two chromosomes of a given matched pair o Each chromosome (long linear strands of DNA) has many genes  Each gene occupies a specific “locus” on a specific chromosome  Homologous chromosomes have homologous genes at homologous loci (but the two genes at the homologous loci need not be identical) o Sex Chromosomes  One pair of homologous chromosomes is different  Human females have a true homologous pair XX 1201A  Human males XY  X and Y are different, Y has 13 known genes, X has 179 known genes  X and Y are called sex chromosomes  Autosomes = all other non-sex chromosomes - Phases of Meiosis o Two successive nuclear divisions occur, Meiosis I (reduction) and Meiosis II (division) o Two successive nuclear divisions occur,  Meiosis I (reduction) and Meiosis II (division)  Meiosis produces 4 haploid cells o Meiosis I reduces the ploidy level from 2n to n (reduction) o Sameness – chromosomes are replicated and partitioned to ensure that the cells produced have the same number of chromosomes and the same DNA sequence  Mitosis produces 2 diploid cells * o Meiosis II divides the remaining set of chromosomes in a mitosis-like process (division) o Difference –halved chromosome number and recombined (The formation of new combinations of genes, as by crossing-over between homologous chromosomes) chromosomal DNA sequence  Meiosis I  Division in meiosis I occurs: o During the preceding interphase the chromosomes are replicated to form sister chromatids. – These are genetically identical and joined at the centromere. o Prophase  In prophase I, the chromosomes condense and homologous chromosomes pair up to form tetrads.  In a process called synapsis, special proteins attach 2 homologous chromosomes pairs tightly together.  At several sites the chromatids of homologous chromosomes are crossed (chiasmata) and segments of the chromosomes are traded.(Recombination – mixes alleles into new combinations and contributes to genetic variability) o Prometaphase I  Nuclear envelope breaks down and spindles move into former nuclear area  Kinetochore microtubules connect to chromosomes, microtubules from one pole attach to both sister kinetochores of one duplicated chromosome, etc. o Metaphase I  Movements of the spindle microtubules (ones not connected to chromosomes) align tetrads on the metaphase plate o Anaphase I  The spindle microtubules separate the two chromosomes of each homologous
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