Question

Below is the sequence from the 3′ end of an mRNA. 5′-CCGUUACCAGGCCUCAUUAUUGGUAACGGAAAAAAAAAAAAAA-3′ If you were told that this sequence contains the stop codon for the protein encoded by this mRNA, what is the anticodon on the tRNA in the P site of the ribosome when release factor binds to the A site?

A) 5'-CCA-3'
B) 5'-CCG-3'
C) 5'-UGG-3'
D) 5'-UUA-3'

Answer 1

The correct anticodon on the tRNA in the P site when the release factor binds is 5'-CCG-3', which corresponds to the mRNA codon 'CCA', adjacent to the stop codon 'UAA'. Therefore, the correct answer is B.

Step-by-step explanation:

The student's question about the anticodon on the tRNA in the P site of the ribosome, when the release factor binds to the A site, refers to the termination of translation in protein synthesis. Looking at the provided mRNA sequence, we identify the stop codon which in this case is 'UAA' found at the 3' end before the poly(A) tail. Since 'UAA' is the stop codon, we will not be looking for an anticodon that pairs with this sequence because tRNAs do not have anticodons for stop codons. Instead, release factors bind to the stop codon to signal termination.

However, based on the question, if we were to look at the last tRNA bound at the P site, we need to find the codon for this tRNA. That would be the codon adjacent to the stop codon. Reading the mRNA sequence from 3' to 5', before the 'UAA' stop codon, we have the codon 'CCA'. The anticodon for 'CCA' would be 'GGU' because during translation anticodons are antiparallel and complementary to the mRNA codons. Therefore, the correct anticodon on the tRNA in the P site when the release factor binds would be 5'-CCG-3' (Option B), as tRNA sequences are usually given from the 5' to 3' end.

Answer 2

D

Step-by-step explanation:

The mRNA sequence given is in the 5' to 3' direction, the anticodon on the tRNA is in the opposite direction to the mRNA so would be in the 3' to 5' direction.

First we have to find the stop codon in the mRNA which is UAA, UAG or UGA.

5′-CCGUUACCAGGCCUCAUUAUUGGUAACGGAAAAAAAAAAAAAA-3′.

Now that we have found the stop codon, we can determine what the anticodon is by using the complementary sequence to the stop codon. This will be 3'-AUU-5', because A is complementary to U and G is complementary to C. The answers are given in the 5' to 3' direction, therefore we turn it around so that it is 5'-UUA-3' and we have our answer.