Meiosis (Ch 11.4)

Background Chromosome = package form of DNA in a cell Genes = genetically inherited traits Genes normally have two alleles per gene (there are some exceptions like ABO blood type) Genes are carried on chromosomes from the parents to the offspring that ride on gametes (sex cells: sperm or egg) Genes can be found in specific locations on the chromosomes

Chromosome Number Homologous Chromosomes: each set of chromosomes that come from the parents Example: Fruit Flies → 8 chromosomes (4 from dad and 4 from mom) Diploid: a cell that contains both sets of homologous chromosomes from the parents (2N) N = the single set of chromosomes coming from one parent 2N = the complete set of homologous chromosomes from mom and dad Haploid: a single set of chromosomes (N) Example: Fruit Flies → 2N = 8 chromosomes (4 from mom and 4 from dad) N = 4 chromosomes coming from each parent

Phases of Meiosis Meiosis: the process during sexual reproduction where chromosomes are split in half through the separation of homologous chromosomes The two divisions creates the four haploid cells → one haploid cell from mom (23 chromosomes = 11 pairs → egg) will join with a haploid cell from dad (23 chromosomes= 11 pairs → sperm) in the offspring (46 chromosomes = 23 pairs) 2 divisions → Meiosis I / Meiosis II

Meiosis I Before Meiosis, the cell goes through interphase (G1 / S / G2) Prophase I Each replicated chromosome pairs with the corresponding homologous chromosome (tetrad) Each tetrad contains 4 chromatids Crossing-over: the chromatids cross over one another -- then the chromatids (contain alleles) are exchanged Spindle fibers form and attach to the tetrads

Meiosis I (Cont.) Metaphase I Spindle fibers pull the homologous chromosomes to line up across the center Anaphase I Spindle fibers pull the homologous chromosome pairs towards opposite ends of the cell Telophase I The nuclear membrane forms, spindle fibers disappear, and the cells begin to pinch off Cytokinesis One cell becomes two cells

Meiosis I

Meiosis II The two cells now undergo a second cell division to get from 2N → N Prophase II Chromosomes become visible -- chromosomes do not need to form tetrads because they already crossed over in Prophase I Spindle fibers form from centrioles Metaphase II Spindle fibers pull the chromosomes to line up at the center of the cell

Meiosis II Anaphase II Spindle fibers pull the sister chromatids apart Telophase II The nuclear envelope begins to form, spindle fibers disappear, and the two cells begin to pinch off Cytokinesis The two cells pinch off completely to form four haploid cells (N)

Meiosis II

Gametes → Zygotes Meiosis II = 4 haploid cell (N) = gametes male gametes = sperm (23 chromosomes  11 pairs) female gametes = eggs (23 chromosomes  11 pairs) When the female gamete and the male gamete come together, fertilization begins to form the zygote Zygote → mitosis → forms a new organism

Mitosis vs. Meiosis Mitosis Both Meiosis asexual reproduction similar chromosomes in the daughter cells 2N parent cell → 2N daughter cells (8 chromosomes) 2 daughter cells = identical 1 division Interphase sexual reproduction homologous chromosomes move to separate daughter cells = genetic variation 2N parent cells → N daughter cells (4 chromosomes) 4 daughter cells = genetically different 2 divisions

Gene Linkage Mendel → independent assortment: genes located on different chromosomes separate independently of each other Morgan Drosophilia = fruit flies Each chromosome is a group of linked genes Mendel’s principle of independent assortment holds true Gene linkage: alleles of different genes tend to be inherited together when the genes are located on the same chromosome