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Nucleic Acids and Protein Synthesis: Question 9

Syllabus 6.2

Multiple choice AS 1 mark

An mRNA codon being read at a ribosome has the sequence 5'-G U C-3'. Which of the following is the anticodon, written 5' to 3', of the tRNA molecule that binds to this codon?

Choose an answer to check it, then compare with the worked solution below.
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Worked solution

Working through the options

A tRNA anticodon binds to an mRNA codon by complementary base pairing, and the two molecules are antiparallel to one another. Just as the two strands of DNA are antiparallel.

Start by pairing each base of the codon with its complementary RNA base (A with U, G with C), keeping the bases in the same left-to-right order as the codon:

Codon (5’→3’): G U C Complementary bases (same order): C A G

Because the codon and anticodon are antiparallel, this row of complementary bases (C A G) is aligned in the 3’ to 5’ direction relative to the anticodon, since it sits directly opposite the codon’s 5’ to 3’ direction. To read the anticodon in its own, conventional 5’ to 3’ direction, this row must be reversed:

Anticodon (5’→3’): G A C

Checking each option:

  • A. 5’-GAC-3’ is exactly the reversed-complement of the codon, correctly accounting for the antiparallel arrangement. Correct.
  • B (5’-CAG-3’ is the complementary bases in the same order as the codon, without reversing for antiparallel orientation) a common mistake, but the wrong polarity. Incorrect.
  • C. 5’-GUC-3’ is identical to the codon itself, not its complement. Incorrect.
  • D (5’-CTG-3’ uses thymine (T), but tRNA is RNA and would contain uracil (U) instead) this looks like a DNA strand, not an anticodon. Incorrect.

Final answers

  • A. The anticodon, written 5’ to 3’, is GAC.