Meiosis is a process that reduces the amount of chromosomes in a cell to half the original amount

Why would you want to reduce chromosomes to half? (check with

your neighbor)

Egg = 23

Sperm = 23

Egg + Sperm = 46 in them.

If meiosis reduces the chromosome number

in half, how many chromosomes are in

human sex cells?

Haploid (n)– a half set of chromosomes

Diploid (2n) – a full set of chromosomes

Somatic Cells = ALL body cells (blood, skin, etc) except gametes

Gamete

Skin Cell

Egg

Somatic Cell

Sperm

Sex Cell

Haploid or Diploid?

Homologous Chromosomes – chromosomes that have the same

informationEx. Father/Paternal #11 chromosome

Mother/Maternal #11 chromosome

“Partner” up in Prophase 1

Crossing Over is the exchange of genetic material between homologous chromosomes

Crossing Over acts as a source of

genetic variation within a species

Stages of Meiosis•Interphase—1 time ONLY

– Growth and replication of DNA

• PMATC—2 times

– Prophase, Metaphase, Anaphase, Telophase, Cytokinesis

Diploid Cell—2 copies of each chromosome that replicated during

Interphase

Crossing Over occurred during Prophase 1

Anaphase AWAY

Telophase I/Cytokinesis results in 2 Haploid Cells

Prophase II looks similar to Telophase I, but nuclear envelope is

disappearing again

Anaphase Away—part II

Telophase II and Cytokineses results in 4 HAPLOID sex cells….or gametes….or egg….or

sperm!

What’s different?

Egg (Ovum) is created in the Ovary

Sperm is created in the Testis

Diploid Numbers in other OrganismsName of Organism Diploid Number Image

Adders-tongueOphioglossum reticulatum

1440This fern has the highest known chromosome number of any life form

Field HorsetailEquisetum arvense 216

Red viscacha ratTympanoctomys barrerae

102 Highest number known in mammals, a tetraploid[3].

Hedgehog

90

Diploid Numbers in other OrganismsName of Organism Diploid Number Image

Turkey

80Dogs, Coyotes, Dingos, and Wolves 78

MuleHorse/Donkey Cross 63 *Infertile

Platypus 5210 Sex Chromosomes

Jack Jumper Ant 2 for females, 1 for males

Karyotype – a “map” of chromosomes

2 Main Uses:•Determine sex of the baby

•Screen for Chromosomal Abnormalities

Ex. Down Syndrome

22 pair of Autosomes + 1 pair of sex chromosomes =23 Total pairs

XX = Female

XY = Male

Questions about Meiosis????

Quiz

• What are sex chromosomes called?

• How many chromosomes does a diploid cell have? Haploid?

• What is a karyotype used for?

Quiz

• What is the purpose of crossing over?

• What does meiosis in male gametes create??

• What does meiosis female gametes create?