Meiosis

Question 1

Define Meiosis.

Answer 1

Meiosis is the nuclear division of cells in which the parent cell divides to form 4 genetically different daughter nuclei.

Question 2

What is “Ploidy level”?

Answer 2

The number of sets of chromosomes in the nucleus of the cell.
- Diploid: 2 sets of chromosomes, 1 set derived from each parent
- Haploid: 1 set of chromosomes. (Gametes formed at end of meiosis are haploid)

Question 3

What are homologous chromosomes?

Answer 3

A pair of chromosomes which have the same size, and have centromeres in the same position. They also have the same genes arranged in the identical sequence of gene loci.

Question 4

What are all the stages of meiosis?

Answer 4

Meiosis has two stages:
- Meiosis I –> Prophase I, Metaphase I, Anaphase I, Telophase I
- Meiosis II –> Prophase II, Metaphase II, Anaphase II, Telophase II

Question 5

Describe what happens in Prophase I.

Answer 5

• Chromatin condenses into chromosomes
• Nuclear envelope disintegrates
• Spindle fibres are attached to the centromeres of the chromosomes via the kinetochore.
• Homologous chromosomes pair up (called SYNAPSIS)
• Chiasmata are formed between non-sister chromatids of a pair of homologous chromosomes.
• Crossing over occurs between non-sister chromatids of a pair of homologous chromosomes.

Question 6

Describe what happens in Metaphase I.

Answer 6

• Pairs of homologous chromosomes (known as bivalents) are arranged along the equator
• Direction of each chromosome in each bivalent is independent to that of other bivalents (the process of Independent assortment)

Question 7

Describe what happens in Anaphase I.

Answer 7

• Homologous chromosomes separate and move towards opposite poles.
• Sister chromatids remain attached to the centromere and move as a single unit towards the same pole.

Question 8

Describe what happens in Telophase I.

Answer 8

• Homologous chromosomes reach opposite poles.
• Spindle fibres disintegrate.
• 2 daughter nuclei with only 1 set of chromosomes are formed.

Question 9

Describe what happens in Meiosis II.

Answer 9

The process of meiosis II is the same as that of mitosis, except the number of chromosomes will be halved, and the sister chromatids are no longer identical (if crossing over took place in prophase I).

Question 10

What is the significance of meiosis?

Answer 10

• Prevents the doubling of chromosomes in sexual reproduction.
• Generates genetic variation

Question 11

What are some abnormal events that can occur during meiosis?

Answer 11

• Structural aberration
• Numerical aberration (non-disjunction)