IMAT 2011 Q53 [Base Sequence]

The base sequence of a section of DNA is shown below.

C A T G C A C A T C G T G C C C A A

The maximum number of different amino acids this section codes for is:

A. 4

B. 5

C. 6

D. 9

E. 18

The answer is B, there are 5 different amino acids that the given sequence codes for.

We know that in the process of transcription-translation-replication that are involved with amino acids and proteins, that codons are made up of 3 nucleotides.

The question is asking for the maximum number of different amino acids:

  1. Count the number of nucleotides, and divide that by 3, since that is the minimum number of nucleotides that make up a codon. In this case, there are 18 total nucleotides, giving a total of 6 codons.
  2. However, what’s being asked are the maximum number of different amino acids, which means that you should not consider any repeating codons when you count.
  3. If you notice, the sequence C A T happens twice. We know that the genetic code is unambiguous, which means that each codon specifies one amino acid only. Thus, it is not possible that the sequence C A T specifies more than 1 amino acid.

Therefore, there is a maximum of 5 different amino acids that the given sequence codes for given that there are only 5 unique codons (CAT, GCA, CGT, GCC, CAA)

Reminders:

  • Remember to always read the sequence given! Do not just count! and read the question very carefully to discern what it’s asking for.