Meiosis

Question 1

What does meiosis involve?

Answer 1

2 nuclear divisions and creates 4 haploid daughter cells from a single diploid parent cell.

Question 2

What are the 2 mechanisms involved in meiosis that increase genetic variation?

Answer 2

Crossing over and independent segregation.

Question 3

When do crossing over and independent segregation occur?

Answer 3

The first division of meiosis so meiosis 1.

Question 4

What is a diploid?

Answer 4

Has 2 complete sets of chromosomes represented as 2n.

Question 5

What is a haploid?

Answer 5

Has a single set of unpaired chromosomes represented as n.

Question 6

Describe how a cell divides by meiosis.

Answer 6

1. Meiosis I (first nuclear division) separates homologous chromosomes. Chromosomes arrange into homologous pairs. Crossing over between homologous chromosomes. Independent segregation of homologous chromosomes
2. Meiosis II (second nuclear division) separates chromatids.

Question 7

What is the outcome of meiosis?

Answer 7

4 genetically varied daughter cells.

Question 8

Why is the number of chromosomes halved during meiosis?

Answer 8

Homologous chromosomes are separated during meiosis I (first division).

Question 9

Explain how crossing over creates genetic variation.

Answer 9

Homologous pairs of chromosomes associate.
Chiasmata form.
Alleles are exchanged between chromosomes.
Creating new combinations of (maternal & paternal) alleles on chromosomes.

Question 10

Explain how independent segregation creates genetic variation.

Answer 10

Homologous pairs randomly align at equator → so random which chromosome from each pair
goes into each daughter cell.
Creating different combinations of maternal & paternal chromosomes.

Question 11

Other than mutation and meiosis, explain how genetic variation within a species is increased.

Answer 11

Random fertilisation
Creating new allele combinations

Question 12

Explain the different outcomes of mitosis and meiosis.

Answer 12

• Mitosis produces 2 daughter cells, whereas meiosis produces 4 daughter cells. As there is 1 division in mitosis, whereas 2 divisions in meiosis.
• Mitosis maintains the chromosome number whereas meiosis halves the chromosome number. As the homologous chromosomes separate in meiosis but not mitosis
• Mitosis produces genetically identical daughter cells, whereas meiosis produces genetically varied daughter cells. As crossing over and independent segregation happen in meiosis but not mitosis.

Question 13

Explain the importance of meiosis.

Answer 13

Two divisions create haploid gametes.
So the diploid number is restored at fertilisation.
Independent segregation and crossing over creates genetic variation

Question 14

How can you recognise where meiosis and mitosis occur in a life cycle?

Answer 14

Mitosis occurs between stages where chromosome number is maintained
- Eg. diploid (2n) → diploid (2n) OR haploid (n) → haploid (n)
Meiosis occurs between stages where chromosome number halves
- Eg. diploid (2n) → haploid (n)

Question 15

Describe how mutations in the number of chromosomes arise.

Answer 15

Spontaneously by chromosome non-disjunction during meiosis
- Homologous chromosomes (meiosis I) or sister chromatids (meiosis II) fail to separate during meiosis
- So some gametes have an extra copy (n+1) of a particular chromosome and others have none (n-1)

Question 16

Suggest how the number of possible combinations of chromosomes in
daughter cells following meiosis can be calculated.

Answer 16

2^n where n = number of pairs of homologous chromosomes (half the diploid number)

Question 17

Suggest how the number of possible combinations of chromosomes
following random fertilisation of two gametes can be calculated.

Answer 17

(2^n)^2 where n = number of pairs of homologous chromosomes (half the diploid number)