Cell Division

Meiosis

By: K. Ingemunson

Review Mitosis

• Nuclear division involving replication and division of chromosomes.

• New cell contains complete set of chromosomes. (Two copies of genetic info)

• Results in DIPLOID cells (chromosomes occur in pairs)

Fertilization

• BODY CELL: full number of chromosomes (two copies = diploid)

• SEX CELL: half the number of chromosomes (one copy = haploid)

• FERTILIZATION: we get genes from each of our parents

Diploid Cells

• Chromosomes after replication (ready for mitosis)

• Chromosomes in early interphase (actually chromatin)

Both are Diploid

Homologous Pairs

Diploid Cells cont.

• All cells in the body except sex cells are diploid.

Sex Cells

• Germ Cells

• Only contain one copy of genetic information

• Have one chromosome of each pair or half that of diploid cells.

• Haploid (N) state.

# of Cells

Total Chromosomes

in a diploid cell

# of homologous

pairs in a diploid cell

Total chromosomes

in haploid cell

HUMAN 46CAT 36

23 23

1818

Why is Haploid Important?

• If diploid cells are fertilized…

+

Chromosome will keep doubling!!

2N + 2N 4N

Why is Haploid Important?

• If haploid cells are fertilized…

+

Like all cells of the body!!

N + N 2N

Sexual Reproduction

• Reproductive process in which haploid sex cells (gametes) fuse to produce a diploid fertilized egg (zygote)

+

MEIOSIS

• Cell division that reduces the number of chromosomes in half

• Occurs in sex cells (germ cells)

• Two Divisions– MEIOSIS I– MEIOSIS II

Meiosis I

Meiosis II

Prophase I

• Chromosomes form• Nuclear membrane and

nucleolus disappear• Synapsis – homologous

chromosomes pair up• Crossing Over –

exchange of DNA creates variability

Metaphase I

• Homologous chromosome line up at the equator

Anaphase/Telophase I

ANAPHASE I:

• Homologous chromosomes separate and move to opposite poles

• Cell becoming Haploid

TELOPHASE I:

• Cell membrane pinches in

Interphase????

Prophase II

• Cells are haploid

• Cells get ready to divide again.

Metaphase II

• Chromosomes align at the equator

• Looks like mitosis (except cells are haploid)

Anaphase/Telophase II

ANAPHASE II:

• Sister chromatids move to opposite poles

TELOPHASE II:• Cell membrane pinches in

• Nuclear membrane reappears.

Resulting Sex Cells

• Four HAPLOID sex cells are produced.

MEIOSIS