Question

A portion of an mRNA attached to a ribosome reads: 5 GACCAUUUUUGA 3

If a tRNA with a histidine amino acid attached is in the P site of the ribosome, a tRNA with which amino acid attached will enter the A site?
tyrosine
a) phenylalanine
b) proline
c) serine
d) aspartic acid

Answer 1

The correct answer is a) a) phenylalanine

Step-by-step explanation:

Every triplet codon in the mRNA codes for an amino acid. These amino acids are attached to the acceptor arm of a tRNA. Each tRNA carries a specific amino acid according to the sequence of anticodon it has which is complementary to the triplet codon present on mRNA.

A new tRNA enters through A site of a ribosome and P site is occupied by a previous tRNA. Therefore a new tRNA binds to the next three triplet codon of mRNA when it moves towards the P site.

CAU is the codon that codes for histidine(5 GACCAUUUUUGA 3) amino acid and in the question P site have tRNA with histidine as attached amino acid so the next three codons are UUU(5 GACCAUUUUUGA 3) which codes for phenylalanine so a tRNA that have attached phenylalanine will enter through A site of the ribosome.

Answer 2

a) phenylalanine

Step-by-step explanation:

A triplet codon results into a particular amino acid. mRNA is read in 5' to 3' direction during translation. tRNA carrying an amino acid enters the A site first. Then a peptide bond is formed between the new and old amino acid. The polypeptide along with the tRNA moves to P site while the now empty old tRNA moves from P to E site and exits. Hence a tRNA is first in A site followed by P site and then finally E site.

Here, a tRNA with histidine amino acid is present in the P site. From the given sequence, only CAU codon can result into histidine. So if CAU region is present in P site that means that UUU will be present in A site. UUU codes for phenylalanine amino acid so the tRNA entering the A site will carry phenylalanine.