The genetic code

Question 1

What is a codon

Answer 1

3 nucleotides have a particular genetic meaning
= DNA is non overlapping

Question 2

Describe in vitro translation I (the first experiments to attempt to understand genetic code)

Answer 2

• Cell extracts from bacteria
• Extracts then ‘programmed’ to make protein using artificial RNAs
• Poly(U) RNA (no start codon) was shown to direct the incorporation of phenylalanine into polypeptide
• The codon UUU was shown to encode phenylalanine
• Later experiments tested other homopolymers and heteropolymer RNAs

Question 3

Genetic code is degenerate, what does this mean?

Answer 3

Codons can code for multiple amino acids

Question 4

Describe non-Watson-Crick base pairing

Answer 4

• Not all organisms have 61 diff tRNAs
• tRNAs can base-pair with more than 1 codon due to ‘wobble’
• Many tRNAs have a modified nucleotide at the 1st position of the anticodon
• G in the 1st position can recognise codons ending with C or U
  Example: Inosine base-pairs with A, C or U (but not G)

Question 5

Describe tRNA structure

Answer 5

• Cloverleaf secondary structure
• 5’ and 3’ ends are drawn together
• Very rich in nucleotides that have undergone post-transcriptional modification
• The amino acid is attached to the 3’ hydroxyl group of the 3’ terminal A nucleotide
• Modification of the 1st position of the anticodon allows ‘wobble’
• Coaxial stacking = folded into an L shape (tertiary struc)

Question 6

What is the secondary structure of tRNAs

Answer 6

Cloverleaf

Question 7

Describe the process of charging tRNAs

Answer 7

→ the process of adding specific amino acids (critical for translation)
→ carried out by aminoacyl-tRNA synthetases (important for gene expression)
- A single aminoacyl tRNA synthetase charges all isoacceptor tRNAs
- Reaction requires ATP