Meiosis

Question 1

Why does independent assortment occur?

Answer 1

When homologous chromosomes are positioned on the equator during meiosis I, there is no control over which chromosome of each pair goes to which side of the cell

Question 2

What is the point of chiasma?

Answer 2

The point at which 2 (non- sister) chromatids of homologous chromosomes are in direct contact. This is where crossing over takes place

Question 3

What is crossing over?

Answer 3

Crossing over leads to the exchange of DNA between homologous chromosomes, resulting in the recombination of alleles, leading to variation

Question 4

Steps of meiosis 1

Answer 4

1. Chromosomes coil up and become visible. They are seen with 2 sister chromatids linked by centromeres
2. Homologous chromosomes pair up so that they are aligned
3. Chiasmata forms between homologous chromosomes, and these allow the crossing over of sections of DNA between homologous chromosomes
4. Nuclear membrane breaks down, so that the microtubules can form spindle fibres which align the chromosomes to the equator.
5. The microtubules of the spindle fibres shorten, and pull on the centromeres so the homologous chromosomes separate to opposite ends of the pole

6 A nuclear membrane forms around chromosomes and cytokinesis separates the 2 cells.

Question 5

What are the stages of meiosis 2?

Answer 5

1. Nuclear membrane breaks down again. Centrosomes again send out microtubules and bind to the centromeres of each sister chromatid. The spindle fibres align the chromosomes across the equators.
2. Microtubules shorten and pull on the centromeres, so the sister chromatids separate to opposite ends of the cell.
3. The new chromosomes group in each end and a nuclear membrane reforms. At the end of Meiosis 2, four genetically identical chromosomes are made.