Question

The mRNA formed from the repeating tetranucleotide UUAC incorporates only three amino acids, but the use of UAUC incorporates four amino acids.

Why?

A) The UAC triplet, produced from the first sequence, does not code any amino acid.
B) The first sequence has a "double U" fragment.
C) The second sequence is more random.
D) The triplets UUA and CUU, produced from the first sequence, code the same amino acid.

Answer 1

D

Step-by-step explanation:

First of all, repeating the first sequence, we will have a mRNA with nucleotides UUACUUACUUAC while we will have nucleotides UAUCUAUCUAUC for the second sequence.

Nucleotide bases on the mRNA are read three at a time i.e. every three nucleotide in an mRNA (codon) specifies an amino acid. A codon is a nucleotide triplet that encode a particular amino acid. There are only four nucleotides (A, C, G and U). However, if these four nucleotides are taken three at a time, there could only be 64 possible combinations. These 64 possible combinations code for only 20 amino acids, this means that there are more possible codons than amino acids. This brought about the DEGENERATE nature of the genetic code i.e. more than one codon can code for a specific amino acid. e.g. Phenylalanine is coded for by UUU and UUC.

In this question, if we read the sequence in three's, each coding for an amino acid, we'll see that for the first sequence:

UUA- leucine

CUU- leucine

ACU- threonine

UAC- tyrosine

The second sequence:

UAU- tyrosine

CUA- leucine

UCU- serine

AUC- isoleucine

We will realize that two codons (UUA and CUU) code for same amino acid (leucine) in the first sequence, hence, a total of 3 different amino acids is incorporated for while 4 different amino acids is incorporated in the second sequence.