Question

The piece of RNA below includes the region that codes for the binding site for the initiator tRNA needed in translation. 5′-GUUUCCCGUAUACAUGCGUGCCGGGGGC-3′ Which amino acid will be on the tRNA that is the first to bind to the A site of the ribosome
A) methionineB) arginineC) cysteineD) valine

Answer 1

Answer: B) Arginine

Explanation: Having the sequence (mRNA):

5′-GUUUCCCGUAUACAUGCGUGCCGGGGGC-3, we can go through the following procedure:

1. Identifying the binding site for the initiator tRNA, which is the so-called "start codon" AUG: 5′-GUUUCCCGUAUACAUGCGUGCCGGGGGC-3′

2. Look for the next codon after AUG, which is CGU:

5′-GUUUCCCGUAUACAUGCGUGCCGGGGGC-3

3. Look in a codon table in which amino acid corresponds to CGU. Then you will find that amino acid is Arginine.

The reason you should follow the above procedure is that during translation, ribosomes are in charge of creating a new peptide bond from mRNA using different tRNA. A tRNA is a structure with a codon attached to it and an amino acid. If a tRNA has a UAC codon attached to it, then this tRNA is going to scan the mRNA looking for the corresponding AUG (the so-called start codon). This is why the tRNA with the UAC codon attached to it is called "initiator tRNA". When initiator tRNA finds the start codon, the large subunit of the ribosome would place this tRNA plus mRNA in its so-called P site (Polypeptide site). On the right of the P site is the A site (Amino acid site) which will be waiting for the tRNA which attached codon corresponds to the next codon after the start codon AUG in the mRNA. For this case, that codon which A site is waiting for is CGU, which is attached to a tRNA containing the corresponding amino acid which is Arginine.