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