A special type of cell division.Only occurs in reproductive

organsReduces the number of

chromosomes in the nucleus -Reduction division

End result: - Gametes in animals - Spores in plants

A.K.A. somatic cells Contain the total number of

chromosomes in an organism Diploid number (2n) Humans have a diploid

number of 46 chromosomes

A.K.A. gametesContain half of the total

number of chromosomes Hap/oid number (n) Humans have a haploid

number of 23

Example: Humans

Diploid =46 Haploid = 23

Female gamete = egg = n = 23 Male gamete = sperm = n = 23

Zygote (fertilized egg) = 2n = 46

meiosismeiosis

mitosis

mitosis

Each diploid cell has two copies of each chromosome

One copy 'donated' by the haploid egg

One copy 'donated' by the haploid sperm

Homologous Chromosomes

Matching chromosomes

Carry information for the same genes

Tetrad

Two homologous chromosomes attached

•Tetrad:- Two homologous chromosomes attached

Divided into two parts: Meiosis I & Meiosis II

DNA replication

Homologous chromosomes form

Homologous chromosomes align at equator and separate into chromosomes

Chromosomes align at equator and separate into sister chromatids

GametogenesisDevelopment of gametes Occurs in organs (gonads)

of reproductive system Ovaries Testicles

Oogenesis-In females - Produce eggs (ovum/ova)

Spermatogenesis.-In males - Produce sperm

1. Random Fertilization (not part of meiosis)- - picking a random mate- - determines which sperm fertilizes which egg

Average number of sperm cells in the ejaculate of a healthy man: 200 and 500 million

- an estimated 400,000 to 450,000 immature eggs residing in each ovary at puberty.

- Any sperm can fertilize any egg.- The resulting zygote could contain anyone of

more than 70 trillion possible combinations of chromosomes.

2. Crossover (Recombination) Totally Random and mistakes can occur

3. Random Line up During Metaphase I

We have pairs of 23 so 223 = 8.4 million combinations

4. Random Seperation During Metaphase II and Anaphase II