MeiosisChapter 11-4

Chromosome Number• All individuals receive one set of

chromosomes from the male parent and one set from the female parent.– In humans, 23 come from mom and 23

come from dad.

Chromosome Number• Each chromosome from the female

parent pairs up with a chromosome from the male parent. – These are called homologous

chromosomes because they contain similar, but not identical, genetic information.

Chromosome Number• A cell that contains both sets of

homologous chromosomes is said to be diploid (2n).–In humans, the diploid number is 46

(2n = 46).

–All somatic (body) cells are diploid.• Ex. Skin cells, liver cells, nerve cells, lung cells, muscle cells, etc.

Chromosome Number• Cells that only contain one set of

homologous chromosomes are said to be haploid (1n).– In humans, the haploid number is 23

(1n = 23)

– All sex cells (gametes) are haploid.• Ex. Egg and sperm cells

Meiosis• Meiosis – a type of cell division which

produces haploid (1n) gamete cells.– Cuts the number of chromosomes from the

diploid parent cell in half (2n 1n).

Phases of Meiosis• Meiosis I – first division

• Meiosis II – second division

Meiosis IProphase I

• Homologous chromosomes pair up with each other, forming a tetrad

• Crossing over may occur at this time.

Crossing Over• Crossing over – the exchange of genetic

material between homologous chromosomes.– Adds to genetic variation in offspring

Meiosis IMetaphase I

• The tetrads line up at the center of the cell.

Meiosis IAnaphase I

• The homologous chromosomes separate and move to opposite sides of the cell.

Meiosis ITelophase I & Cytokinesis

• A new nucleus begins to form around each set of chromosomes and cytokinesis occurs.

• Results in two haploid (1n) daughter cells.

Meiosis I Overview

Meiosis IIProphase II

• New spindle forms in each haploid cell from meiosis I.

Meiosis IIMetaphase II

• The chromosomes line up at the center of the cell.

Meiosis IIAnaphase II

• The sister chromatids separate and move to opposite ends of the cell.

Meiosis IITelophase II & Cytokinesis

• A new nucleus forms at each end of each cell and cytokinesis occurs.

• Results in four haploid (1n) daughter cells.

Meiosis II Overview

Meiosis Overview

Gamete Formation• In male animals, the gametes produced by

meiosis are called sperm cells.

Gamete Formation• In female animals, the gametes produced by

meiosis are called egg cells. – Only one of the four cells produced by meiosis in

females becomes a mature egg cell.

Fertilization• Fertilization – the process of a sperm joining

an egg.– When a haploid (1n) sperm joins a haploid (1n)

egg, a diploid (2n) zygote cell forms.

– The zygote divides through mitosis to form a growing embryo.

Comparing Mitosis and Meiosis

Mitosis MeiosisParent cell Diploid (2n) Diploid (2n)

# of divisions 1 2

# of daughter cells 2 4

Type of daughter cells

Diploid (2n) somatic cells

Haploid (1n) gametes

Genetic Information Identical Unique