Mutation and DNA repair

Question 1

What does it mean by the genetic code is “degenerate”?

Answer 1

• There are 64 codons but only 20 amino acids.

- The genetic code is degenerate, which means that 18 of the amino acids are coded for by more than 1 codon.

Question 2

What are examples of stop codons?

Answer 2

UAA, UAG, UGA

Question 3

What is the start codon?

Answer 3

AUG

Question 4

What can happen if the genetic code goes wrong?

Answer 4

Mutations can give rise to:
- an altered protein structure (e.g. because of changes to amino acids)
- altered protein levels (e.g. due to transcription failure etc)
Mutations give rise to over 3000 genetic disorders including cystic fibrosis and sickle cell anaemia.

Question 5

Define “mutation”

Answer 5

A mutation is a permanent alteration to genetic structure. There are two main groups of mutations:

• Gross mutations: effects more than 1 nucleotide
• Point mutations: effects only 1 nucleotide

Question 6

What is a base substitution and what can it lead to?

Answer 6

Base substitutions are the simplest types of point mutations. A single nucleotide is replaced by another nucleotide during replication.
This can lead to:
Silent mutations (no change in amino acid)
Missense mutations (change in amino acid)
Nonsense mutation (stop codon formed)

Question 7

What is the effect of base substitutions and base deletions?

Answer 7

Base substitutions/base deletions cause a “Frame shift”. This alters what amino acids are formed, and if a stop codon is translated then can even alter the length of the protein.
If the mutation occurs earlier on in the sequence then the frame shift is more severe.

Question 8

What is cystic fibrosis?

Answer 8

Cystic fibrosis is an autosomal recessive disease.
it effects 1:25 people so it’s one of the most common inherited diseases.
It causes a viscous mucus to be secreted from the exocrine glands (lungs, pancreas). This then blocks ducts and causes fibrous cysts to form - causing breathing and digestive problems.

Question 9

What is mutagenesis?

Answer 9

Mutagenesis is the process in which mutations are formed.
Spontaneous = occurs because of irregular replication
Induced = occurs because of external agents e.g. x rays or gamma rays

Question 10

What is an example of spontaneous mutagenesis?

Answer 10

Tautomeric shifts:
Tautomeric shifts make one nucleotide look like another. This is a spontaneous change in the chemical structure of a base.
A tautomeric shift is a transition mutation as it makes a purine look like another purine, or a pyrimidine look like another pyrimidine.

Question 11

What are some ways cells defend against mutation?

Answer 11

During replication = proof reading of anti parallel strands

Post replication = excision