Question

 Section 7.2 | Meiosis

 1. Explain the purpose of meiosis in sexually reproducing organisms and compare to the purpose of mitosis

 2. Explain difference between meiosis I compared to meiosis II.

 3. Describe the five phases of meiosis I: prophase I, Prometaphase I, Metaphase 1, Anaphase I, Telophase I

Answer 1

1. The purpose of meiosis is to convert a diploid cell in to haploid. Meiosis actually forms gametes. Gametes need to be made haploid , in order to have a diploid zygote. Two gametes fuse to form a zygote. That is why reducing the chromosomal number to half is essential. So meiosis converts a diploid cell in to haploid. Another important purpose of meiosis is to bring variation in populations. This is possible because of the crossing over in the Prophase I of meiosis.

On the other hand mitosis helps in keeping the chromosomal number constant in the cells that it produces. This is essential because, mitosis leads to growth , repair etc by producing body cells. All the body cells need to have same number of chromosomes. So it is mitosis which keeps the chromosomal number constant. Mitosis does not bring variations in the daughter cells as it makes carbon copies of the cells. So mitosis is for producing cells which are exactly similar to parent cells including the number of chromosomes.

2. Meiosis I is reductional division. It reduces the chromosomal number to half or converts a diploid cell in to haploid.

MeiosisI Prophase is of very long duration . It involves many changes which are not observed in meiosis II. There is synapsis , which is pairing of homologous chromosomes. Crossing over is exchange of chromatid bits in homologous chromosomes which happens in prophase I of meiosis.

MeiosisI Metaphase has homologous chromosomes arranged on the equator. So here, we have paired chromosomes on the equator.

Meiosis I anaphase has homologous chromosomes separating from one another to reach the poles.

In meiosis II prophase is of shorter duration. It does not involve synapsis and crossing over.

Meiosis II metaphase has only one chromosome attached to spindle fibres. So non homologous chromosomes are arranged on the equator and they are unpaired.

Meiosis II anaphase has sister chromatids separating from one another to form daughter chromosomes. There is splitting of each chromosome to form daughter chromosomes.

Meiosis I makes the cell haploid. Meiosis is almost like mitosis and makes exact copy of the daughter cells produced by meiosis I

3.

Prophase I:

Chromatin starts to condense . Because of condensation, chromosomes become visible. Homologous chromosomes start pairing to form bivalents. Bivalents enter in to tetrad stage which means, each chromosome now develops two chromatids and together 4 chromatids are in homologous chromosome. So it is called tetrad stage. Now non-sister chromatids exchange the bits to bring a process called crossing over. After crossing over, the paired chromosomes repel and try to separate. But near the region of cross over, they can't separate. This region of contact near the crossing over is called chiasmata. As the chromosomes condense further, the chismata appears to be slipping to poles, which is called terminalisation. Nuclear membrane and nucleolus disappear. Centrioles separate and spindles form between the centrioles.

Pro-Metaphase I:

The cell at this stage has no nuclear membrane and nucleolus. Centrioles are in the opposite poles. Chromosomes are floating in the cytoplasm.

Metaphase I:

Homologous chromosomes are arranged on the equator. Each pair is attached to one spindle fibre.

Anapahse I:

Spindle fibres pull the homologous chromosomes apart and drag them to poles (centrioles). So in this phase homologous chromosomes are separated and pulled to poles.

Telopahse I:

Chromosomes become thin and long chromatin. Nuclear membrane and nucleolus reappear. Spindle fibres disappear. Two nuclei are formed with reduced number of chromosomes.

This completes the nuclear division (karyokinesis ). It is followed by division of cytoplasm (cytokinesis ).

This divides the cell completely in to two daughter cells after meiosis I. These cells now under go meiosis II to complete meiosis division.