Meiosis Biology 1114

Life Cycles are a Diagrammatic Representation of the Events in the Organisms Development and Reproduction

The Problem Mitosis produces two cells with the same number of chromosomes as the parent cell. Mitosis of a diploid cell (2n) produces two diploid daughter cells. If two diploid cells went on to participate in sexual reproduction, their fusion would produce a tetraploid (4n) zygote.

Ploidy Diploid - Two sets of chromosomes - 2n Haploid - 1 set of chromosomes - 1n Polyploid - More than two sets of chromosomes

Homologous Chromosomes carry the same genes Alleles are a type of gene that carry traits

Gametes are Sex Cells Sperm (male) Egg (female)

Gametes each contain one set of genetic information (Haploid) Gametes unite to form a zygote (Diploid) The zygote then divides by mitosis to form an organism

It is essential for organisms that reproduce sexually to half their number of chromosomes, otherwise, with each successive generation, the number of chromosomes would double.

Plant Life Cycles (Alternation of Generations) Sporophyte Phase 1. Diploid Phase 2. Meiosis Occurs Here Gametophyte Phase 1. Haploid Phase 2. Gametes are Produced by Mitosis

Meiosis The process in which haploid gametes (egg and sperm) are formed. Consists of Two Phases Meiosis I Meiosis II

Interphase Identical to Interphase in Mitosis. Start out with a diploid cell.

Prophase I Prophase I is virtually identical to prophase in mitosis. Chromosomes and spindle fibers appear. Nuclear membrane and nucleolus disappears. Homologous chromosomes pair up.

Metaphase I Chromosomes line up in homologous pairs across equator of cell forming tetrads. Spindle fibers attach to chromosomes. Crossing-over may occur.

Anaphase I Spindle fibers contract, separating the homologous pairs of chromosomes.

Telephase I Two daughter cells form each with a single set of chromosomes, half the total number in the original cell. Nuclear membrane and nucleolus may reappear. Cell is now haploid.

Prophase II Nuclear membrane and nucleolus disappear. Centrioles migrate toward poles. A new set of spindle fibers form and chromosomes begin to move toward the equator of the cell.

Metaphase II Chromosomes line up across the equator of the cell.

Anaphase II The spindle fibers shorten, drawing the chromosomes toward each pole of the cell.

Teleophase II Formation of new nuclear membrane. Fading of chromosomes. Cytokinesis occurs resulting in four haploid daughter cells.

Comparison of Mitosis and Meiosis Results in two genetically identical daughter cells. Diploid cell  1 cell division  2 diploid daughter cells Involves 1 single nuclear division after DNA replication. Meiosis Results in four genetically different daughter cells Diploid cell  2 cell divisions  4 haploid daughter cells Involves 2 nuclear divisions with no DNA replication between them.

This Part Reduces Chromosome Number This Part is Simply Mitosis All Over Again to Duplicate Cells

Gender Based Spermatogenesis Meiosis and gamete formation in males. 1 diploid cell  4 mature sperm Oogenesis Meiosis and gamete formation in females. 1 diploid cell  1 mature ovum + 3 polar bodies.