An mRNA molecule is synthesized that has a random sequence consisting of 25% A, 25% U, and 50% C. Among the amino acids in the polypeptide chains resulting from translation in a laboratory setting in which translation can be initiated anywhere along the mRNA molecule, what is the expected frequency of phenylalanine codons?
A)1/64
B)1/32
C)3/64
D)3/32
E)1/16

Question

Quizplus · Accepted Answer

There are two types of codons that code for phenylalanine: UUU and UUC. The probability of getting UUU is (0.25)^2 = 0.0625, since the probability of getting U at the first position and U at the second position are both 0.25. Similarly, the probability of getting UUC is also (0.25)^2 = 0.0625, since the probability of getting U at the first position and C at the second position are both 0.25. Therefore, the expected frequency of phenylalanine codons is 0.0625 + 0.0625 = 0.125 or 3/24, which is equivalent to 3/64 when simplified.

An MRNA Molecule Is Synthesized That Has a Random Sequence