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 from each parent If 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) + egg (female) = zygote n + n = 2n Haploid + haploid = 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 reach poles Haploid 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  Spindles form and move chromosomes to middle  Metaphase II  Chromosomes align  Anaphase II  Sister chromatids split and move to poles  Telophase II  Nuclei form at poles  Cytokinesis occurs  4 daughter cells (haploid)

Comparing Mitosis and Meiosis  Mitosis  For growth, repair, and asexual reproduction  Daughter cells are genetically identical  1 nuclear division = 2 daughter cells same # of chromosomes  Meiosis  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 = 2 n  Humans: 2 23 = 8 million  2 different gametes = 8 million 2 = 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 GenotypeGenderSyndromePhysical Traits XXY, XXYY, XXXYMaleKlinefelter syndrome Sterile, small testicles, breast enlargement XYYMaleJacob’s syndrome Normal male traits, previously thought ‘super- males’, karyotype ID only XOFemaleTurner syndrome sex organs don't mature, sterile, short stature XXXfemaleTrisomy 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