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.

• 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