Lecture 14 - Meiosis

Question 1

Haploid gametes number of chromosomes:

Answer 1

23 (n)

Question 2

Diploid gametes number of chromosomes:

Answer 2

46 (2n)

Question 3

Why are gametes produced through meiosis rather than mitosis?

Answer 3

If gametes were produced by mitosis, they would be 2n, and resulting embryos would be 4n.

Need a process of cell division that HALVES no. of chromosomes going into gametes (n), so that diploid number (2n) is retained in zygote.

Question 4

What is the purpose of Meiosis I?

Answer 4

Separate homologous chromosomes to form haploid cells with duplicated chromosomes

Question 5

What is the purpose of Meiosis II?

Answer 5

Separate sister chromatids to form haploid cells with unduplicated chromosomes

Question 6

What are the phases of Meiosis I?

Answer 6

• Prophase I
• Metaphase I
• Anaphase I
• Telophase I and Cytokinesis

Question 7

What are some difference between Mitosis and Meiosis I?

Answer 7

Mitosis
- chromosomes align independently
- no chiasmata
- centromeres on metaphase plate
- chromatids disjoin
- 2n –> 2n

Meiosis I
- homologous chromosomes synapse
- chiasmata
- chiasmata on metaphase plate
- chromosomes disjoin
- 2n –> n

Question 8

What happens during Prophase I?

Answer 8

• Homologous chromosomes align &
  synapse
• Crossing over between non-sister chromatids occurs at chiasmata
• Crossing over results in chromatids now being a mix of pieces from
  each homologous chromosome.

Question 9

What happens during Metaphase I?

Answer 9

• Paired homologous chromosomes move to metaphase plate
• Chiasmata (not kinetochores, as in
  mitosis) line up on metaphase plate

Question 10

What happens during Anaphase I

Answer 10

• Recombined homologous chromosomes separate (disjoin)
• Sister chromatids remain
  attached (unlike mitosis)

Question 11

What happens during Telophase I and Cytokinesis?

Answer 11

• Haploid cells with duplicated
  chromosomes (the pairs
  of sister chromatids) form
• Haploid because only half the genetic information is in each
  new cell

Question 12

What happens during Meiosis II that’s different to Meiosis I?

Answer 12

• Sister chromatids separate during Anaphase II
• Haploid daughter cells are formed

Question 13

If the sperm of a mouse has 3 picograms of DNA, the amount of DNA in the nucleus of a somatic cell about to divide by mitosis would
be:

A) 3 pg.
B) 6 pg.
C) 12 pg.
D) 24 pg.

Answer 13

C) 12 pg. (Chromosomes duplicate before mitosis. 3 x 2 x 2)

Question 14

The structures that move to the spindle poles at anaphase of meiosis I usually differ from those that move to the poles at anaphase II with respect to:

A) chromatid number.
B) gene copy number.
C) the ratio of maternal to paternal genes.
D) all of the above.

Answer 14

D) all of the above.

Question 15

How does sexual reproduction produce variety?

Answer 15

(i) independent assortment of chromosomes
(ii) crossing over and
(iii) random fertilisation of gametes

Question 16

Genetic diversity allows selective responses, for example to:

Answer 16

(i) spatially variable environments
(e.g. climate, ecology)
(ii) changing environments
(e.g. parasites, seasons)
(i) intraspecies competition

Question 17

Drosophila males do not undergo crossing over at meiosis. If Drosophila has a diploid number of 8, how many genetically distinct types of gametes can be produced by a male Drosophila?

A. 4
B. 8
C. 16
D. 32

Answer 17

C. 16

2n (2 x8)

Question 18

Presence of the sexual cycle in a species has the potential to:

A) decrease competition among offspring.
B) allow the species to respond better to parasites.
C) increase evolutionary flexibility.
D) all of the above.

Answer 18

D) all of the above.