MeiosisMeiosisBiology 1114

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

The ProblemThe 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.

PloidyPloidy

• 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 Plant Life Cycles (Alternation of Generations)(Alternation of Generations)

• Sporophyte Phase

1. Diploid Phase

2. Meiosis Occurs Here

• Gametophyte Phase

1. Haploid Phase

2. Gametes are Produced by Mitosis

MeiosisMeiosis

The process in which haploid gametes (egg and sperm) are formed.

Consists of Two Phases

1. Meiosis I

2. Meiosis II

InterphaseInterphase

Identical to Interphase in Mitosis.

Start out with a diploid cell.

Prophase IProphase 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 IMetaphase I

Chromosomes line up in homologous pairs across equator of cell forming tetrads.

Spindle fibers attach to chromosomes.

Crossing-over may occur.

Anaphase IAnaphase I

Spindle fibers contract, separating the homologous pairs of chromosomes.

Telephase ITelephase 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 IIProphase 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 IIMetaphase II

Chromosomes line up across the equator of the cell.

Anaphase IIAnaphase II

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

Teleophase IITeleophase II

Formation of new nuclear membrane.

Fading of chromosomes.

Cytokinesis occurs resulting in four haploid daughter cells.

Comparison of Mitosis and Comparison of Mitosis and MeiosisMeiosis

• Mitosis

1. Results in two genetically identical daughter cells.

2. Diploid cell 1 cell division 2 diploid daughter cells

3. Involves 1 single nuclear division after DNA replication.

• Meiosis1. Results in four genetically different daughter cells2. Diploid cell 2 cell divisions 4 haploid daughter cells3. 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 BasedGender 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.