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Ch. 11.4 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.
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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