Ch Meiosis

Meiosis When gametes are formed, there must be a process that separates the two sets of genes so that each gamete ends up with just one set.

Chromosome Number A body cell in an adult fruit fly has 8 chromosomes: 4 came from mom, 4 came from dad. These 2 sets of chromosomes are called homologous, meaning each 4 chromosomes from dad have corresponding chromosomes from mom.

Chromosome Number A cell that contains both sets of homologous chromosomes is said to be diploid. The number of chromosomes in a diploid cell is represented by the symbol 2N. In drosophila, 2N=8. Gametes are haploid, meaning that they only have one set of chromosomes. In drosophila, n=4.

Phases of Meiosis Meiosis is a process of reduction division in which the number of chromosomes per cell is cut in half though the separation of homologous chromosomes in a diploid cell.

Meisosis I In prophase of meiosis I, each chromosome pairs with its homologue to form a tetrad. There are 4 chromatids in a tetrad.

As homologous chromosomes form tetrads, they exchange portions of their chromosomes in a process called crossing-over. Crossing over results in exchange of alleles between homologous chromosomes and makes new combinations of alleles.

Meiosis II During metaphase II of meiosis, chromosomes line up in the center of each cell. In anaphase II, the paired chromatids separate. The four daughter cells produced have the haploid number of chromosomes.

Gamete formation In male animals, the haploid gametes produced by meiosis are called sperm. In female animals, generally only one of the cells produced by meiosis is involved in reproduction. The other three cells produced by the female are called polar bodies and are not involved in reproduction.

Comparing Mitosis and Meiosis Mitosis produces 2 genetically identical diploid cells, while meiosis produces four genetically different haploid cells. Asexual reproduction only involves mitosis.

Meiosis Assignment with Flies Chart showing 1 diploid cell with 2 chromosomes producing 4 haploid gametes. B=black body, E=black eye, b-white body, e=white eye Show prophase 1, metaphase 1, anaphase 1, telophase 1, prophase 2, metaphase 2, anaphase 2, telophase 2. (2 points each) Show how crossover produces recombinant combinations of alleles in prophase 1 (B,e);(b,E) (5 points) Next to the 4 gametes produced by meiosis, glue the offspring fly that matches the appropriate combination of alleles in the gamete. (4 points)

Meiosis Assignment with Flies Example: B,E = black bodied, black eyed fly; b,E = white bodied, black eyed fly

All 8 phases = 16 points Show crossover in metaphase I (5 points) 4 flies glued next to genetically appropriate chromosomes = 4 points “Prettiness” = 5 points Total = 30 points