tRNA & The Genetic Code

Question 1

By convention which way is a triplet read?

Answer 1

5’ to 3’

Question 2

Which amino acid does the start codon code for?

Answer 2

Methionine

Question 3

What is the sequence of the start codon?

Answer 3

AUG

Question 4

What is an open reading frame?

Answer 4

A sequence which begins with a start codon and ends in a stop codon with no stop codons inbetween

Question 5

What are synonyms?

Answer 5

Codons that specify the same amino acid, they tend to have the same first 2 nucleotides and vary at the 3rd

Question 6

What is the effect of mutations in the 3rd position of a codon?

Answer 6

It usually has a much smaller effect than the other 2 positions

Question 7

What is the effect of a mutation of the 1st position of a codon?

Answer 7

Gives similar amino acids

Question 8

Generally what type of amino acid does a U or C in the 2nd position give?

Answer 8

A hydrophobic amino acid

Question 9

Generally what type of amino acid does an A or G in the 2nd position give?

Answer 9

Mostly polar

Question 10

Are there as many tRNAs as there are anticodons?

Answer 10

No

Question 11

Does tRNA only contain conventional bases?

Answer 11

No, after RNA synthesis some bases can be exchanged

Question 12

What enzyme is used to attach the amino acid to the correct tRNA?

Answer 12

Amino acyl tRNA synthetases

Question 13

Why is attachment of amino acids to tRNA by amino acyl tRNA synthetases crucial?

Answer 13

• The ribosome does not check the amino acid so the correct one must be attached
• Peptide bond formation between free aa is energetically unfavourable so they must be activated

Question 14

How does amino acyl tRNA synthetase ensure the correct amino acid is attached?

Answer 14

The enzyme must recognise the correct shape of both the tRNA and amino acid

Question 15

How do synthetases recognise tRNA?

Answer 15

• Acceptor stem interactions
• Anticodon loop

Question 16

How do synthetases proofread?

Answer 16

• After attachment in the synthesis site the amino acid is forced into a second binding pocket called the editing site
• Only incorrect amino acids will fit into the editing site where they are hydrolysed from tRNA

Question 17

How is a tRNA activated?

Answer 17

• Sythestase binds to the aa and transfers adenine onto the amino acid
• Energy from ATP is stored in the phosphodiester bond
• 3’OH group on tRNA acts as a nuclephile to attack the phosphodiester bond
• Energy from ATP is now stored in the bond between tRNA and aa