Transcript Meiosis
MEIOSIS Chapter 8 Chromosome Pairs Human somatic (body) cells consist of 23 homologous chromosome pairs Identical length, centromere position, and loci Alleles are different versions First 22 are autosomes Pair 23 are sex chromosomes Few homologous parts XX = female and XY = male Gametes (eggs or sperm) consist of 23 unpaired chromosomes Karyotypes Size ordered chart of all chromosomes in a cell Size Banding patterns Centromere location Visualizes chromosomal makeup Matched up homologs Cell Types n = number of chromosomes in a single set Diploid cell has 2 homologous sets of chromosomes One If from each parent synthesis has occurred, each appears as sister chromatids Total number of chromosomes is the diploid number (2n) Humans have 23 sets so diplod nubmer is (2 * 23 = 46) Haploid cell has 1 set of chromosomes Called gametes Total number of chromosomes is the haploid number (n) 22 autosomes and 1 sex chromosome (X or Y) Egg always X and sperm is X or Y Fertilization Sperm (male) + n Haploid + + egg (female) n haploid = zygote = 2n = diploid Meiosis makes gametes Only in reproductive areas Reduce chromosome number by half Haploid cells keep number of chromosomes from doubling with each generation Mitotic phase ensures that chromosome number is maintained Meiosis (Overview) Creates haploid gametes (n) in diploid (2n) organisms 2 phases Meiosis I (homologs separate) Meiosis II (sister chromatids separate) Produces 4 daughter cells with ½ number of chromosomes Only time homologs associated with each other Meiosis I Prophase I 90% of meiotic time Synapsis is when the chromosomes pair up 2 homologous duplicated chromatids = tetrad Crossing over exchanges genetic info Similar to other processes in mitotic prophase Metaphase I Tetrad arranged in middle of cell Spindle microtubules attach to the centromere of 1 chromosome (2 sister chromatids) Meiosis I (cont.) Anaphase I Chromosomes move to respective poles, splitting the tetrad Telophase I Chromosomes Haploid reach poles sets at each Cytokinesis Occurs with telophase 1 Creates 2 haploid daughter cells Interphase may or may not occur No duplication occurs regardless Meiosis II Like mitosis except it starts with haploid cells Prophase II Metaphase II Sister chromatids split and move to poles Telophase II Chromosomes align Anaphase II Spindles form and move chromosomes to middle Nuclei form at poles Cytokinesis occurs 4 daughter cells (haploid) Comparing Mitosis and Meiosis Mitosis Meiosis For growth, repair, and asexual reproduction Daughter cells are genetically identical 1 nuclear division = 2 daughter cells same # of chromosomes Produces gametes for sexual reproduction Haploid daughter cells with a half of each homologous pair 2 nuclear division = 4 haploid cells Meiosis I has all unique stages Both Chromosomes duplicate only once Meiosis II and mitosis events Sources of Genetic Variability Independent orientation at metaphase I (meiosis I) Random fertilization Crossing over in prophase I Mutation is the ultimate source Independent Orientation and Random Fertilization Genetic diversity from reshuffling of genes during sexual reproduction Occurs in metaphase I 1 chromosome from mom and 1 from dad Tetrads randomly line up on either side About equal probability for all cases Total arrangements = 2n Humans: 223 = 8 million 2 different gametes = 8 million2 = 64 trillion Crossing Over Exchange corresponding segments between homologs Occur at chiasma Results in genetic recombination Varies from original chromosome Happens average of 1 – 3 times in human meiosis Errors in Meiosis Abnormal chromosome numbers or size Nondisjunction, pairs of chromosomes don’t separate Meiosis I = 4 abnormal cells Meiosis II = 2 abnormal and 2 normal Unknown what causes Body generally miscarries when this occurs Often leads to disorders or syndromes Down Syndrome Trisomy 21 Most common chromosome abnormality and most serious birth defect in US Incidence rate increases with age of mother Generally don’t reproduce Sterile or chances of passing to offspring Sex Chromosome Disorders Sex Chromosome Abnormalities Genotype Gender Syndrome Physical Traits XXY, XXYY, XXXY Male Klinefelter syndrome Sterile, small testicles, breast enlargement XYY Male Jacob’s syndrome Normal male traits, previously thought ‘supermales’, karyotype ID only XO Female Turner syndrome sex organs don't mature, sterile, short stature XXX female Trisomy X Karyotype only identifier Structural Abnormalities Duplication Huntington’s disease Deletion Cri du chat: developmental & neurological problems Inversion Less likely to be harmful to affected Increase miscarriage Translocation Not between homologs Can cause cancers