Unit 4 Genetic mutations & Meiosis

Question 1

Describe what is meant by a gene mutation

Answer 1

• A change in the base sequence of DNA, mutations occur spontaneously during DNA replication
• Resulting in the formation of a new allele
  and base deletion or substitution of a base
  TAC ACA AGT GGC
  Deletion - TAC CA AGT GGC
  Substitution - TAC GCA AGT GGC

Question 2

Describe what can increase the rate of gene mutation.

Answer 2

Mutagenic agents interfere with DNA replication

e.g Radiation( Gamma rays, UV light) OR chemicals

Question 3

Describe the effect of a base deletion & substitution

Answer 3

Substitution
- May have no impact as genetic code degenerate so may end up coding for the same amino acids
- May occur in intron
Deletion
- Deletions result in a frameshift, therefore, changing all subsequent codons - causing incorrect code which can result in different polypeptide chain

Question 4

Describe what is meant by a chromosome mutation.

- Definition of non - disjunction

Answer 4

• Mutations in the number of chromosomes can arise spontaneously by chromosome non-disjunction during meiosis.
  Non-disjunction - when chromosomes or chromatids do not split equally in anaphase

Question 5

Describe what happens in Meiosis I

Answer 5

• Before meiosis DNA replicates so 2 copies of chromosome
• Homologous pairs of chromosomes line up opposite each other at equator of cell
• One of each pair goes to each daughter cell randomly either with maternal or paternal copy

Question 6

Describe what happens in Meiosis II

Answer 6

• Each cell divides again, producing 4 haploid cells from a single diploid parent cell

Question 7

Describe how crossing over produces genetic variation.

Answer 7

• When Homologous pairs of chromosomes associate to form a bivalent
• Chiasma form as chromosomes twist
• Equal lengths of the non sister chromatids alleles are exchanged
• Producing new combinations of alleles

Question 8

Describe how Independent segregation of chromosomes causes variation.

Answer 8

• Homologous pairs of chromosomes line up opposite each other at equator of cell
• It is random which side of equator maternal or paternal chromosomes from each pair lies
• One of each pair goes to each daughter cell randomly either with maternal or paternal copy
• Shuffling of chromosmes leads to new combinations of alleles

Question 9

Describe the different outcomes of meiosis and mitosis

Answer 9

Meosis
- 2 nuclear divisions to produce 4 haploid cells
- Introduces genetic variation
- Cells produced with half number of chromosomes as parent cell
Mitosis
- 1 nuclear divisions to produce 2 diploid cells
- Daughter cells genetically identical
- Cells produced with same number of chromosomes as parent cell

Question 10

Describe the expression to show number of

different combinations of chromosomes following meiosis

Answer 10

2 power n

Question 11

Describe what is meant by a polyploidy chromosome mutation

Answer 11

• When an individual has 3 or more sets of chromosomes instead of 2
• Occurs because during meiosis sister chromatids do not seperate