Lecture 14 - The genetic code

Question 1

How many different combinations of codons are there in nucleotides?

Answer 1

64

Question 2

How many different amino acids are there in nucleotides?

Answer 2

20

Question 3

What is the Nirenberg-Matthaei experiment?

Answer 3

• Bacteria was blended, this created all the means to make a protein apart from a nucleotide sequence
• Added nucleotide sequence of UUUUUUUU
• Set up 20 tubes each with a different amino acid
• Only labelled phenylalanine gave a labelled polypeptide that let the sequences be decoded
• Found out 3 UUUs was enough to code for an amino acid, and it codes for phenylalanine
• 61 of 61 codons coded for amino acids
  3 didn’t and were stop codons

Question 4

What is the meaning of a degenerate code?

Answer 4

It means more than one codon will code for the same amino acid

Question 5

What is a reading frame?

Answer 5

The sequence of codons between a specific start codon to a specific stop codon

Question 6

What is the function of tRNA?

Answer 6

To link a specific amino acid and recognise a codon in mRNA
This ensures a match between the amino acid and the codon

Question 7

What is the structure of tRNA?

Answer 7

Made up of
An anticodon that interacts with a codon
An amino acid acceptor arm where the amino acid binds
Modified nucleotides, especially in the anticodon loop

Question 8

What are aminoacyl-tRNA synthases?

Answer 8

Each tRNA is recognised by one of the 20 amino-acyl-tRNA synthases
Each one is specific for amino acid and tRNA
This means some tRNAs recognise more than one codon

Question 9

How does one tRNA recognise more than one codon?

Answer 9

G-U base pairing
Inosine base pairs with A C and U