Genetic diversity can arise as a result of mutation or during meiosis

Question 1

What is meiosis?

Answer 1

A type of cell division that produces 4 genetically different haploid daughter cells.

Question 2

What happens before meiosis?

Answer 2

Before meiosis, the DNA unravels and replicates so there are two copies of each chromosome called chromatids. DNA then condenses to form double-armed chromosomes which are formed from two sister chromatids held together by a centromere.

Question 3

Explain the process of meiosis.

Answer 3

1. DNA unravels and replicates to form 2 chromatids. DNA then condenses to form double-armed chromosomes which are formed from the two sister chromatids held together by a centromere.
2. In meiosis 1, the cell divides to result in 2 cells each containing 2 sister chromatids.
3. In meiosis 2, the sister chromatids separate and the cells divide again to form 4 haploid daughter cells each containing 2 chromatids

Question 4

What are the two mechanisms in meiosis that cause variation?

Answer 4

• Independent segregation of homologous chromosomes
• Crossing over between homologous chromosomes.

Question 5

When in meiosis does independent segregation and crossing over take place in?

Answer 5

Meiosis 1 (first round of division)

Question 6

Explain independent segregation, how it causes variation and how it can be calculated mathematically.

Answer 6

In meiosis 1 the homologous pairs of chromosomes line up opposite each other at the equator of the cell. The side the maternal and paternal chromosomes from each homologous pair are at is random. These pairs are then separated so one of each homologous pair ends up in the daughter cell to form a haploid cell.

This creates a large number of possible combinations of chromosomes in the daughter cells produced which can be calculated by 2n where n = the number of homologous pairs of chromosomes.

Question 7

Explain crossing over and how it causes variation.

Answer 7

In meiosis 1, the homologous pairs of chromosomes line up opposite each other at the equator. Parts of the chromatids can be twisted around each other. This puts tension on the chromatids, causing pairs of the chromatid to break. The broken parts of the chromatid recombine with another chromatid, usually on the other chromosome of the pair. This results in new combinations of alleles.

Question 8

Compare mitosis and meiosis.

Answer 8

Mitosis
- One nuclear division
- End results are diploid cells which are two sets of chromosomes
- Creates genetically identical cells

Meiosis
- Two nuclear divisions
- End results are haploid cell (one set of chromosomes)
- Creates genetic variation

Question 9

Explain random fertilisation, how it causes variation and how it can be calculated mathematically.

Answer 9

Random fertilisation is when a random egg and sperm will fuse together which creates a new combination of alleles hence causes variation. It occurs before crossing over and it can be calculated mathematically as (2n)^2

Question 10

What is one event that occurs in division 2 but not in division 1 of meiosis?

Answer 10

Separation of sister chromatids.

Question 11

What are chromosomal mutations and what is process they caused by?

Answer 11

A random change in the number of chromosomes by non-disjunction in meiosis.

Question 12

What is non-disjunction and state the two effects it can have?

Answer 12

When the chromosomes in meiosis 1 or chromatids in meiosis 2 do not split equally during anaphase.
- Changes in whole sets of chromosomes (polyploidy)
- Changes in the number of individual chromosomes.(aneuploidy)

Question 13

State and explain the two effect of non-disjunction.

Answer 13

Polyploidy
- This is a change in a whole set of chromosomes.
- It occurs when organisms have three or more sets of chromosomes rather than 2.
- This mainly occurs in plants.

Question 14

How does the process of polyploidy occur in meiosis 1?

Answer 14

• Each homologous pair is doubled due to DNA replication in interphase.
• Non-disjunction in meiosis 1 means all chromosomes fail to separate equally.
• After meiosis 1, all the chromosomes are in one cell and there are none in the other.
• The two cells divide normally in meiosis 2. The cell containing the chromosomes divides into two diploid cells and the cell containing no chromosomes divides into two cells containing no chromosomes.
• In fertilisation, if the diploid cell fuses with the regular haploid cell, it results in a triploid (polyploidy)

Question 15

How does the process of polyploidy occur in meiosis 2?

Answer 15

• Each homologous pair is doubled due to DNA replication in interphase.
• Normal division in meiosis 1 so all chromosomes separate equally into two cells.
• The non-disjunction in meiosis 2 causes one of the cells to fail to separate equally.
• The failed cell divides into two cells, one with no chromosomes and one containing the chromosomes (diploid).
• The other cell will divide into the two cells equally to produce haploid cells.

Question 16

What are gene mutations and what does it result in?

Answer 16

A change in the DNA base sequence of chromosomes that arise spontaneously during DNA replication. It results in the formation of new alleles.

Question 17

What are the 2 types of gene mutations?

Answer 17

Base deletion
Base substitutions

Question 18

Explain base deletion and why it is harmful.

Answer 18

1 base is lost and all the bases move along in one direction which means every codon is different.

It is harmful because it changes the entire amino acid sequence so the polypeptide produced will be different and may be dysfunctional

Question 19

Explain base substitutions and why it is harmful.

Answer 19

1 base is substituted for another.

Not as harmful because only 1 codon is changed but it can still be harmful if the new codon codes for a different amino acid because it will have a different R group which means different bonds will be formed with other amino acids which means a different tertiary structure

Question 20

Why are not all base substitutions harmful?

Answer 20

The genetic code is degenerate which means the change in the amino acid sequence may still result in the codon coding for the same amino acid

Question 21

What are mutagenic agents?

Answer 21

A factor that increases the rate of gene mutations.

Question 22

What are examples of mutagenic agents?

Answer 22

• Ionising radiation such as X-rays or gamma rays
• UV radiation
• Chemicals that could react with cells to produce mutagenic compounds
• Biological agents such as viruses or bacteria