Meiosis Chapter 10 PDF

Meiosis and Sexual Reproduction Sexual reproduction requires a fertilization event in which two haploid gametes unite to create a diploid cell called a zygote Meiosis is the process by which haploid cells are produced from an original diploid cell  Single DNA replication...

Meiosis and Sexual Reproduction Sexual reproduction requires a fertilization event in which two haploid gametes unite to create a diploid cell called a zygote Meiosis is the process by which haploid cells are produced from an original diploid cell  Single DNA replication followed by two nuclear divisions Meiosis I: Reduction of chromosome number Meiosis II: Division of sister chromatid four haploid daughter cells  Like mitosis, meiosis begins after a cell has progressed through the G1, S, and G2 phases of the cell cycle Key differences in Loading… meiosis: Homologous pairs form a tetrad Crossing over may occur Produces four haploid daughter cells  Tetrad formation and Crossing over Homologous pairs of sister chromatids associate with each other, side by side through synapsis to create a tetrad (bivalent) Crossing over is a physical exchange of genes between non-sister chromatids of the tetrad may occur at the ciasma – arms of the chromosomes tend to separate but remain adhered at a crossover site Increases genetic variation in offspring Loading…  Meiosis vs. Mitosis Mitosis produces two diploid daughter cells that are genetically identical ex: 6 chromosomes in 3Loading… homologous pairs Meiosis produces four haploid daughter cells ex: Each daughter has random mix of 3 single chromosomes  Meiosis helps ensure genetic variation Genetic variability is the main advantage of sexual reproduction Three mechanisms contribute to genetic variation Crossing Over (prophase I) Random alignment (metaphase I) Random fertilization  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. sister chromatids crossing- over chromatid recombinant synapsis between nonsister s daughter of homologues chromatids after chromosomes exchange  Random alignment of homologous chromosomes creates genetic diversity among the haploid cells Maternal and paternal homologs can be aligned along the metaphase plate a variety of ways Possible number of alignments in metaphase I equals 2n (n is the # of chromosomes per set) Humans n=23 so 223 possible random alignments could occur  Variation in Chromosome Number Chromosome variation Between species it is normal for the number of chromosomes to vary Humans - 2 sets of 23 chromosomes (total of 46) Dog - 2 sets of 39 chromosomes (total of 78) Fruit fly - 2 sets of 4 chromosomes (total of 8)  Number of chromosomes may change so that an individual is different from other members of same species (abnormal) Variations in chromosome number can have major effects on organisms Several human diseases Important in evolution of new species  Changes in chromosome number Euploid – the normal number of chromosomes In a diploid (2n) organism, two sets is normal Polyploidy - Organism with more than two complete sets of chromosomes Triploidy (3n); Tetraploidy (4n) common in plants (30-35% of fern and flowering plant species) rare in animals  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. (a) Hyla chrysoscelis a-b: © A. B. Sheldon (b) Hyla versicolor (diploid) (tetraploid) (c) First tetraploid rodent discovered in Chili Aneuploidy - Abnormal number of a particular chromosome Trisomic ex: trisomy 21 (Down syndrome) Normal 2 copies of a chromosome plus a 3rd 2n+1 Monosomic Missing one of normal copies of a chromosome 2n-1  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 1(X 2 3 4 Normal ) fruit fly: Diploid; 2n (2 sets) (a) Normal fruit fly chromosome composition Polyploi d fruit flies: Triploid; 3n (3 sets) Tetraploid; 4n (4 sets) (b) Polyploidy Aneuploi d fruit flies: Trisomy 2 (2n + 1) Monosomy 3 (2n – 1) (c) Aneuploidy Nondisjunction Chromosomes do not separate properly division During meiosis can produce aneuploid gametes  Down Syndrome (Trisomy 21) aneuploid condition three copies of chromosome 21 affects about 1/700 children in US frequency increases with the age of the mother