9.2 Meiosis and genetic variation

Question 1

What are the two types of cell division

Answer 1

1. Mitosis - Produces two daughter cells with the same number of chromosomes as the parent cell and as each other
2. Meiosis - usually produces 4 daughter cells, each with half the number of chromosomes as the parent cell

Question 2

What must occur to maintain a constant number of chromosomes

Answer 2

The number of chromosomes must be halved at some stage in the life cycle. This halving occurs as a result of meiosis

Question 3

What is the process of meiosis 1

Answer 3

1. The first division, homologous chromosomes pair up and their chromatids wrap around each other. Equivalent portions of these chromatids may be exchanged in a process called ‘Crossing Over’ By the end of this division the homologous pairs have separated, with one chromosome from each pair going into one of the two daughter cells

Question 4

What is the process of meiosis 2

Answer 4

2. The chromatids move apart. At the end of meiosis 2, four cells have usually been formed. In humans, each of these cells contains 23 chromosomes.

Question 5

How does meiosis bring about genetic variation

Answer 5

1. Independent segregation of homologous chromosomes

2. New combinations of maternal and paternal chromosomes by crossing over

Question 6

What is a gene

Answer 6

A length of DNA that codes for a polypeptide

Question 7

What is a locus

Answer 7

The position of a gene on a chromosome or DNA molecule

Question 8

What is an allele

Answer 8

One of the different forms of a particular gene

Question 9

What are homologous chromosomes

Answer 9

A pair of chromosomes, one maternal and one paternal, that have the same gene loci

Question 10

Describe the process of independent segregation of chromosomes and how it brings about genetic variation

Answer 10

During meiosis 1, each chromosome lines up alongside its homologous partner. When homologous pairs arrange themselves in this line they do so at random.

One of each pair will pass to each daughter cell. Which one of the pair goes into the daughter cell, and with which one of any of the other pairs, depends on how the pairs are lined up in the parent cell

Question 11

Describe the process of ‘crossing over’ and how it brings about genetic variation

Answer 11

1. The chromatids of each pair become twisted around eachother
2. During this twisting process tensions are created and portions of the chromatids break off
3. These broken portions might then rejoin with the chromatids of its homologous partner
4. Usually is is the equivalent portions of homologous chromosomes are exchanged
5. In this way new genetic combinations of maternal and paternal alleles are produced