Life SciencesHuman Reproduction- Meiosis

By Ipeleng.R.Makgaka

What is meiosis?

• Meiosis like Mitosis is cell division• However meiosis is the process that converts

diploid nuclei to a haploid nuclei.• In other words meiosis takes diploid body cell

which has 46 chromosomes and converts it into a haploid sex cell which has 23 chromosomes.

Where does meiosis take place?

• Meiosis takes place in the sex organs, producing gametes.

• Gametes are sex cells: sperm or eggs.• Gametes have to be haploid so that once

fertilization takes the haploid sperm can fuse with haploid egg to form diploid zygote.

• Therefore a zygote has a diploid number of chromosomes, one set from each parent

A Life cycle

Meiosis consists of...

• Meiosis consists of two stages namely, Meiosis I and Meiosis II

• Meiosis I consists of five phases namely, InterphaseI, ProphaseI, MetaphaseI, AnaphaseI, TelophaseI.

• Meiosis II consists of four phases namely, ProphaseII, MetaphaseII, AnaphaseII, TelophaseII.

Meiosis I- Interphase I

• The cell builds up energy in order for DNA Replication to take place.

• DNA is replicated to make sure that chromosomes are duplicated.

• The cell does not change structurally.

Meiosis I- Prophase I• The chromosomes coil and

become individual chromosomes.

• The nucleolus and Nuclear envelope begin to disappear.

• Homologous chromosomes come together and exchange genetic information through the process of crossing over.

• This will ensure genetic variation.• The Centrioli moves towards

opposite poles with spindle fibres between them.

Meiosis I- Metaphase I

• The Centrioli has reached the poles.

• The homologous pairs align at the cell equator.

• The two chromosomes attach to one spindle fibre by means of the centromere.

Meiosis I- Anaphase I

• The Spindle Fibres begin to contract.

• This causes the duplicated chromosomes to move towards opposite poles

Meiosis I- Telophase I

• The duplicated chromosomes have reached the opposite poles.

• A nuclear envelope and nucleolus begin to re-form around chromosomes.

• Cell devides forming 2 separate haploid cells

Meiosis II- Prophase II

• Chromosomes coil and become compact if they uncoiled during Telophase I.

• The nuclear envelope and nucleolus begin to disappear.

• The Centrioli move to opposite poles, forming spindle fibres in between them.

Meiosis II- Metaphase II

• The individual duplicated chromosome align along the equator.

• One chromosome per spindle fibre are attached by means of the centromere.

• The Centrioli has reached the opposite poles.

Meiosis II- Anaphase II

• The spindle fibres contract causing duplicated chromosome to split in half.

• These daughter chromosomes begin to move towards opposite poles

Meiosis II- Telophase II

• Daughter chromosome reach the opposite poles.

• The two cells invaginate and form four daughter haploid cells

• They uncoil and form a chromatin network.

• Nuclear envelope and nucleolus form around chromatin network again.

Difference between meiosis I and meiosis II

Meiosis I Meiosis II

Begins with Interphase= DNA Replication Begins with Prophase= No DNA replication

Begins with Diploid number of chromosomes

Begins with haploid number of chromosomes.

Prophase consists of crossing over Prophase consists of no crossing over.