The Genetic Code

Question 1

What is meant by ‘degeneracy’ of the genetic code and what are the implications on mutations?

Answer 1

• Multiple codons code for single amino acid
• Codon : 3 nucleotides
• Limits effects of mutations
• Silent mutations: change third nucleotide of codon wont change amino acid produced
• Missense mutations: change first or second nucleotide of codon negatively alter protein function and AA
• Nonsense mutations: stop codon introduced to shorten polypeptide chain
• Frameshift: deletion causing sequence change and alters every AA (unless 3 nucleotides added)

Question 2

What is a mutation?

Answer 2

Permanent change in DNA sequence

Question 3

What is the difference between transition and transversion mutations?

Answer 3

Transition - purine sub by purine
Transversion - purine sub by pyramidine

Question 4

Give 2 examples of diseases that are caused by mutations

Answer 4

• sickle cell anaemia
• cystic fibrosis

Question 5

What type of mutation causes sickle cell anaemia?

Answer 5

Missense

Question 6

How does sickle cell anaemia occur?

Answer 6

• clumped haemoglobin
• glutamine -> valine
• hydrophobic haemoglobin
• conformation change cause block in capillaries

Question 7

Explain the ‘wobble concept’ and discuss the molecular mechanisms that allow it to occur

Answer 7

• single tRNA can bind to multiple codons that differ only in third nucleotide
• Pairing between third base codon and first base anticodon on tRNA
• Weaker bonding with third base codon and first base anticodon
• less tRNA needed to recognise 64 codons
• Wobbling from conformation of tRNA anticodon loop allows flexibility at first base
• Wobble base pairing occurs when ribose-ribose close to nucleotide pair
• Base stacking interactions mean 5’ end is less confined so can pair with different base on 3’ end of a codon