1. What amino acid is bound to a tRNA with the following anticodon: $5^{\prime}$ A U G $3^{\prime}$

The answer is Histidine.

  1. What amino acid is attached to the tRNA with the following anticodon $5^{\prime}$ A G C $3^{\prime}$

The answer is Alanine.

I also referred to this image below but unable to get the answer. Can anyone please explain how to solve this?

enter image description here

up vote 3 down vote accepted

Each tRNA brings a single amino acid and add them to the growing peptide strand. The tRNA has an 'anticodon' which recognizes a codon on the mRNA code. How this occurs: Always visualize.

How to figure it out:

  • You have to find out the complementary bases. With DNA, A is complementary with T, and C is complementary with G. However, we are working now with RNA! It's similar with RNA, with one exception: in RNA, U (uracyl) is used instead of T, so A is complementary with U. C is still complementary with G.

  • Notice that the 5'-3' anticodon is antiparallel to the codon (one is 5'-3', the other is 3'-5'). This means that to find out which amino acid is being added, you have to reverse the code found on the tRNA.

Your questions, let's go step by step:

1) tRNA anticodon: AUG

tRNA complementary strand would be UAC. Reverse to get the 5'-3' direction on the mRNA. The reverse of UAC is CAU, which is His (histidine).

and

2) tRNA anticodon: AGC

tRNA complementary strand would be UCG. Reverse to get the 5'-3' direction on the mRNA. The reverse of UCG is GCU, which is Ala (alanine).

You can also reverse first and complement second. The order does not matter, provided you do both. Always visualize the process, it helps tremendously.

Let's do a final exercise to see if you understand, but in reverse and with some extra thinking. What are the tRNA sequences which bind to the mRNA strand ACUACAGUAUAC? What is the peptide sequence it will produce?

mRNA: ACU ACA GUA UAC

We can use the translation table immediately; it is used to translate mRNA into peptides directly. We find out it produces the peptide: Thr Thr Val Tyr

Reverse mRNA: CAU AUG ACA UCA

Complement it: GUA UAC UGU AGU

(FYI: by convention, we always write mRNA in the 5'-3' direction)

The tRNA anticodons which bring (i) Thr (ii) Thr (iii) Val (iv) Tyr to build the peptide chain are (i) GUA (ii) UAC (iii) UGU and (iv) AGU.

All of these sequences are written in the 5'-3' direction, as per convention. It's easy if you are careful and clear in your working :-)

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