Mechanisms of mutation

Question 1

What is a mutation?

Answer 1

A change in DNA sequence that arises (de novo) in an individual or tissue

Question 2

What is a polymorphism?

Answer 2

A germline DNA sequence variation that can be stably inherited

Question 3

What do unstable tautomers do

Answer 3

• Form unstable pairs (T:G, A:C)
  Tautomeric shifts affect base-pairing
• Cause mutations

Question 4

What is a mutatgen?

Answer 4

Agents that cause an increase in the rate of mutation above a spontaneous background

Question 5

What are the mechanisms of mutagens?

Answer 5

• Deamination
• Alkylation
• Depurination
• Hydroxylation/oxidation
• Base analogues
• Intercalating agents
• UV radiation
• Ionising radiation

Question 6

What is deamination?

Answer 6

• Removal of amino group
• C -> U
• Causes mismatches and errors
• Caused by nitrous acid, hypozanthine (A:T -> G:C), xanthine (G:C -> A:T)

Question 7

Describe CpG islands

Answer 7

• Frequency of CG is higher than other regions
• Associated with 5’ end of genes
• Methylation -> repression of expression
• Deamination: C -> T
• Effects of changes variable

Question 8

What are alkylating agents?

Answer 8

• Chemicals that donate alkyl groups to other molecules
• Cause transitions, transversions, frameshifts and chromosome aberrations
• Alkylation can change base-pairing properties
• Can activate errors during repair processes

Question 9

Describe depurination and hydroxylation

Answer 9

• Hydrolysis reactions remove purine rings by cleaving the N-glycosidic bond
• Adenine-containing nucleotide is incorporated across from the depurinated one during replication
• Hydroxylamine hydroxylates the amino group of cytosine: G:C -> A:T transitions

Question 10

Describe base analogues

Answer 10

• Similar structure to DNA bases
• Increase frequency of mispairing when incorporated into DNA
• Not all mutagenic
• 2-aminopurine (adenine analogue)
• 5-bromouracil (thymine analogue)

Question 11

Describe intercalating agents

Answer 11

• Thin, plate-like hydrophobic molecules that insert themselves between adjacent base-pairs
• Mutagenic agents cause insertions during DNA replication
• Loss of agent can result in deletion
• E.g. proflavin, ethidium bromide

Question 12

Describe UV irradiation

Answer 12

• Hydrolysis of a cytosine to a hydrate -> mispairing
• Cross-linking of adjacent thymine -> thymine dimers (block DNA replication)
• Can break covalent bonds between bases
• Cumulative effect