Section 11.4 Meiosis

A) Stages of Meiosis Goal: 1. Final product will be 4 haploid (half the number of chromosomes) cells 2. Make gamete cells (egg and sperm cells)

Meiosis: 2 divisions Result: one copy of each chromosome in a gamete. Sister chromatids separate Haploid Diploid Meiosis I (reduction division) Meiosis II (equational Homologs separate Result: one copy of each chromosome in a gamete.

A) Stages of Meiosis Interphase- G1-growth S phase-replicate DNA (after this step the cell will have 96 chromosomes) G2-growth

A) Stages of Meiosis 1) Meiosis I a) Prophase (same as Mitosis w/ extra) 2. Crossing over: exchange pieces of chromosome between mother and father chromosomes a) genetic recombination: mixing up genes

The two chromosomes connected make a tetrad.

a) Prophase I Chromosomes with the same characteristics pair. Crossing over occurs. Chromosomes condense. Spindle forms. Nuclear membrane disappears.

Recombination (crossing over) Occurs in prophase of meiosis I Generates diversity A a A a B b B b C C c c Letters denote genes Case denotes alleles Creates chromosomes with new combinations of alleles for genes A to C.

b) Metaphase I 1. Chromosome pairs align along the equator of the cell

c) Anaphase II Chromosome pairs separate and move to opposite poles. Independent assortment: random separation of chromosome pairs = genetic recombination

Telophase I Nuclear membrane reassemble. Spindle fibers disappear. Cytokinesis divides cell into two. Result: 2 Genetically different cells with 46 chromosomes

Meiosis I :reduction division Prophase I 96 Chromosomes Metaphase I Anaphase I Telophase I 46 (Diploid) Nucleus Spindle fibers Nuclear envelope

Prophase II Nuclear membrane fragments. Spindle fibers form.

Metaphase II Chromosomes align along equator of cell.

Anaphase II Chromatids separate and move to opposite poles. This increases genetic diversity again.

Telophase II Nuclear membrane assembles. Spindle fibers disappears. Cytokinesis divides cell into two. Result: 4 genetically different cells with 23 chromosomes, Haploid Cells

A) Stages of Meiosis Results of Meiosis: 4 Haploid cells are created Contains half the # of chromosomes as original cell Genetic variation will increase chance of survival

Mitosis Meiosis Number of divisions 1 2 Number of daughter cells 4 Genetically identical? Yes No Chromosome # Same as parent Half of parent When Throughout life At sexual maturity Role Growth and repair Sexual reproduction

B) Formation of Gametes Animal cell: meiosis occurs in reproductive organs a) Female: Ovary Male: Testis

Sperm and Egg

Oogonium (diploid) Mitosis Primary oocyte Meiosis I Secondary (haploid) Meiosis II (if fertilization occurs) First polar body may divide Polar bodies die Ovum (egg) Second polar body a A X Mature egg

C) Asexual and Sexual Reproduction Asexual Reproduction: reproduction of offspring from 1 parent a) No genetic diversity Sexual Reproduction: reproduction of offspring by a sex cell from each parent a) Results with genetic diversity