MEIOSISMEIOSIS

Four haploid Algae Cells

MEIOSISMEIOSIS

“Cell Division which produces Gametes with half the number of chromosomes in the parent’s body.”

In humans, Gametes are haploid cells with 23 homologous chromosomes in each.

Meiosis takes place in the male and female reproductive organs.

Male Gametes – Sperm

Female Gametes - Eggs

MEIOSISMEIOSISMitosis produced TWO new cells, while Meiosis produces FOUR

new cells!

Do you remember Do you remember Mitosis?Mitosis?

MitosisMitosis – “Cell Division” – “Cell Division”

Results in 2 copies of the Results in 2 copies of the Original cell.Original cell.

Why do cells Divide?Why do cells Divide?Grow Old Grow Old Grow too LargeGrow too Large

Compare Mitosis and MeiosisCompare Mitosis and Meiosis

2 DIPLOID

CELLS!

4 HAPLOID CELLS!

Why MEIOSIS?Why MEIOSIS?1. Diploid (2n) organisms must

produce Haploid (n) cells before they can reproduce sexually.

2. Sexual Reproduction guarantees a “mixing” of genes.

3. This mixing of genes helps that species have enough variation to survive changing environmental conditions.

4. MEIOSIS is the process of “mixing” those genes

MEIOSIS - Step 1MEIOSIS - Step 1

The number of The number of chromosomeschromosomes must must be be halvedhalved to form to form gametesgametes..

Diploid Cell (46 chrom.) -----> Diploid Cell (46 chrom.) ----->

4 Haploid Cells (23 chrom.)4 Haploid Cells (23 chrom.)

GAMETES

MEIOSIS - Step 2MEIOSIS - Step 2

Meiosis I: Meiosis I:

DNA DNA replicatesreplicates to form to form ““homologous homologous

chromosomeschromosomes.”.”

46 chrom. --> 92 total chrom.46 chrom. --> 92 total chrom.

After the first cell division, each After the first cell division, each cell has one set of homologous cell has one set of homologous chromosomes which have 2 chromosomes which have 2 copies of identical DNAcopies of identical DNA

http://www.biology.arizona.edu/cell_bio/tutorials/meiosis/page3.html

Prophase I

Telophase I

MEIOSIS - Step 3MEIOSIS - Step 3

Metaphase I: Metaphase I:

““crossing over”crossing over”. .

Genes mix by switching Genes mix by switching chromosomeschromosomes

Source of Genetic Variation, Source of Genetic Variation, which is how organisms which is how organisms evolveevolve

MEIOSIS - Step 4MEIOSIS - Step 4

Anaphase I:Anaphase I: Chromosomes line up but don’t split around the centromeres.

Instead, the entire Instead, the entire chromosome moves chromosome moves apart.apart.

MEIOSIS - Step 5MEIOSIS - Step 5

Telophase I:Telophase I:

Results in two cells, each with 46 chromatids.

Meiosis II begins AFTER Telophase I (the last step in Meiosis I)

MEIOSIS - Step 6MEIOSIS - Step 6

Meiosis II:Meiosis II: the same process as

mitosis, but without DNA replication. It’s called “reduction division.”

2 diploid cells (46 chrom) --> Cell Division

MEIOSIS - Step 7MEIOSIS - Step 7 Results in Results in 4 haploid 4 haploid

cells with 23 cells with 23 chromosomes in chromosomes in each.each.

These cells are also These cells are also called called GametesGametes or or daughter cells.daughter cells.

http://www.biology.arizona.edu/cell_bio/tutorials/meiosis/page3.html

Why Meiosis?Why Meiosis?

1 Diploid (2n) organisms must produce Haploid (n) cells before they can reproduce sexually.

2 Sexual Reproduction guarantees a “mixing” of genes.

3 This mixing of genes helps that species have enough variation to survive changing environmental conditions.

4 MEIOSIS is the process of “mixing” those genes

What is this micrograph showing?

Pollen grains being produced by Meiosis in a Lilly plant