Genes 1

Question 1

Definition of a mutation

Answer 1

Change in DNA sequence of rearrangement of genetic material

Question 2

Types of mutations

Answer 2

Small scale changes

• Addition/Deletion
• Substitution

Large scale changes

• Chromosome rearrangements
• Transposable element insertions (insertionally inactivate target genes)

Question 3

How can mutations arise?

Answer 3

Misrepair of DNA damage
e.g. from ionising radiation / free radicals / mutagenic agents

Spontaneous events
e.g Error in DNA replication
Movement of transposable elements

Question 4

Types of DNA damage

Answer 4

Base damage:

• mismatch
• base modification
• abasic site (no base)
• bulky adduct
• interstrand crosslink (base pairs with other in different frame)

Phosphate backbone damage:

• single strand break
• double strand break

Question 5

Characteristics of a double strand break

Answer 5

• Disrupt molecular continuity
• Difficult to resolve/repair
• Can be genotoxic + cytotoxic
• Associated with several human diseases
• Can lead to small insertions/deletions at break site if DNA strands are ligated back together

Question 6

Mechanism of alkylating agents on DNA + examples

Answer 6

• Addition of alkyl group onto base
• Induces mispairing at replication
• Causes base substitution
  e. g. EMS, MNNG

Question 7

Mechanism of intercalating agents on DNA + examples

Answer 7

• Intercalate between base-pairs of dsDNA
• Add or delete single nucleotide during replication
• Induce frameshift mutations
  e. g. Ethidium Bromide, Acridine Orange

Question 8

Describe transposable elements

Answer 8

DNA sequences that can move from 1 part of the genome to another

• Specific DNA sequences
• Supernumerary (many copies)
• Always contained within other DNA molecule, never in free form
• Transpose as a discrete unit to random sites
• Insertionally inactivate target gene
• Ubiquitous (found everywhere)

Question 9

2 forms of replication fork slippage?

Answer 9

Template strand loops out -> one nucleotide is omitted in new strand

Newly synthesised strand loops out -> one extra nucleotide is added in new strand

Question 10

Examples of dominant disease causing mutations

Answer 10

• Dwarfism
• Achondroplasia
• Huntingtons

Question 11

What is the Ames test

Answer 11

Good primary screening for mutagens

Mutant strains of Salmonella tymphimurium are generated that have a mutation in the
gene encoding an enzyme used to synthesize histidine. These his- strains can not
grow on agar plates that have not been supplemented with extra histidine

Test on agar with and without his

Assess if agent is Agar plates with no extra histidine
mutagenic by testing if it
promotes mutations in
his- cells to revert them
back to his+

Question 12

Reversion (mutation) frequency

Answer 12

number of revertant colonies (each arise from a single original cell)
per total number of cells plated

example: if 100 colonies grow when 1,000,000 cells are plated
Mutation/reversion frequency = 100 revertants per 1,000,000 cells
= 1 revertant per 10000 cells
= 1 X 10-4

Question 13

Summarise the DNA damage response signalling cascade

Answer 13

(various post translational modifications)

Sensor proteins -> Transducer proteins -> Mediator proteins/effector proteins ->

Cell cycle checkpoints (temporal arrest until DNA damage is repaired)
Senescence (permanent arrest, can no longer grow/devide)
Apoptosis

DNA repair

-> Cancer supression