Meiosis • the process which produces Gametes

Recall:

• Gametes are sex cells

• Gametes are haploid

• Sperm cells are male gametes, Egg

cells are female gametes

• According to the Cell Theory, all new

cells are created from pre-existing cells

Gametophytes • are somatic cells that undergo Meiosis

in order to produce Gametes

• Spermatocytes (Spermatogonia) are

male gametophytes that undergo

Meiosis in order to produce Sperm

Cells

– Spermatocyte cells are diploid

• Oocytes (Oogonium) are female

gametophytes that undergo Meiosis in

order to produce Egg Cells

– Oocyte cells are diploid

Meiosis consists of 2 Stages:

Stage #1: “Meiosis I”

(Also Known As “The Reduction

Stage”)

• The Purpose of Meiosis I is to have

the chromosome number of each

gametophyte reduced from Diploid

(2n) to Haploid (n)

Meiosis I

has 4 Phases:

1. Prophase I

1. Prophase I • this stage is just like Prophase in

Mitosis

• there is an early and late stage

• the Nuclear Membrane dissolves

• Centrioles are positioned

• Spindle Fibres form

2. Metaphase I

2. Metaphase I

Crossover(technically this happens between Prophase I and Metaphase I)

• This stage is very similar to Metaphase in Mitosis

BUT

• The chromosomes DO NOT line up in a row by

their centromeres

• Instead they line up in pairs called TETRADS

• Crossover happens because the chromosomes are

very close to each other (in some cases

overlapping) when they are lined up in pairs

• A piece of one chromatid (containing genes) will

“jump” to a different chromatid

– This creates Variation (the chromatids are

not identical anymore

3. Anaphase I

3. Anaphase I • This stage is very similar to Anaphase

in Mitosis

BUT

• The chromosomes are NOT split at

their centromeres

• The chromosomes pairs are

separated as the spindle fibres shorten

and each individual set of

chromosomes is drawn to opposite

ends of the cell

4.Telophase I

4.Telophase I • This stage is just like Telophase in

Mitosis

• Cytokinesis occurs

• At this point, each cell is now Haploid

Stage #2: “Meiosis II”

• (Also Known as “The Division

Stage”)

• The purpose of Meiosis II is to get 4

sex cells each with a haploid number

of chromosomes

Meiosis II has

4 Phases:

5. Prophase II

• This stage works just like Prophase I

and Prophase in Mitosis

• Keep in mind that the cells are haploid

(n)

6. Metaphase II

• This phase works like Metaphase in

Mitosis

• The Chromosomes are lined up by their

centromeres

• Keep in mind each cell is haploid

7. Anaphase II

• This phase works like Anaphase in

Mitosis

• Spindle fibres shorten pulling the

chromosomes apart at their

centromeres as they are moved to

opposite ends of each cell

8. Telophase II

• This stage works just like Telophase I

and Telophase of Mitosis

BUT

• Each new cell formed is haploid

• All 4 of the new gametes (sex cells)

formed will be different. Why?

• Cytokinesis Occurs and the gametes

fully develop

• The end result of Meiosis is 4 slightly

different haploid gametes