Presented by-

Devendra Kumar

M.Sc. (Biotechnology)

Enrollment No- 1386/14

*Cell division to form the gametes, sperm (male gamete) and egg (female gamete).*Characteristic of eukaryotes only: not

in prokaryotes.

*Normal cells are diploid: 2 copies of every gene.

*Gametes are haploid: 1 copy of every gene

*Differences between male and female gametes.

*In males, all 4 products of meiosis develop into sperm cells. They lose most of their cytoplasm, remodel their cell shape, and grow a long flagellum (tail).

*In females, the cell divisions of meiosis are asymmetric: most of the cytoplasm goes into 1 of the 4 meiotic products, which becomes the egg. Eggs also develop large amounts of yolk proteins, which are used to feed the developing embryo. The other 3 meiotic cells are small “polar bodies”, which degenerate.

*Start with a diploid cell, with 2 copies of each chromosome, one form each parent. The two copies are called homologues. Chromosomes each with 2 chromatids attached at the centromere.*Use 2 cell divisions:* Meiosis 1. First separate the

homologues* Meiosis 2. Then separate the 2

chromatids.*The stages of meiosis have the

same names as in mitosis: prophase, metaphase, anaphase, telophase.

Two important events in meiosis 1: crossing over in prophase, and the pairing of homologues in metaphase.

Crossing over. Homologues break at identical locations, then rejoin opposite partners. This creates new combinations of the alleles on each chromosome. Occurs randomly several times on every chromosome. Results in mixing of the genes you inherited from your parents.

*The main event in meiosis 1 is the pairing and separation of the homologues.*At metaphase, the pairs of

homologous chromosomes line up side by side. This does not happen in mitosis of meiosis 2, but only in meiosis 1.*At anaphase of meiosis 1,

the pairs of homologues are pulled to opposite poles by the spindle.

Go from 1 cell to 2 cells. Each daughter cell contains 1 copy of each chromosome: they are haploid, with the chromosomes still having 2 chromatids each.

For humans: start with one cell containing 46 chromosomes (23 pairs) to 2 cells containing 23 chromosomes.

As a result of crossing over, each chromosome is the mixture of the original homologues.

Meiosis 2 is just like mitosis

No replication of DNA between meiosis 1 and meiosis 2.

Chromosomes line up individually on the equator of the spindle at metaphase.

At anaphase the centromeres divide, splitting the 2 chromatids.

The one-chromatid chromosomes are pulled to opposite poles.

2 cell divisions.Start with 2 copies of each

chromosome (homologues), each with 2 chromatids.

In meiosis 1, crossing over in prophase mixes alleles between the homologues.

In metaphase of meiosis 1, homologues pair up, and in anaphase the homologues are separated into 2 cells.

Meiosis 2 is just like mitosis. The centromeres divide in anaphase, giving rise to a total of 4 cells, each with 1 copy of each chromosome, and each chromosome with only 1 chromatid.