Suppose that a scientist performed an mRNA mixed copolymer experiment to decode

Question

Suppose that a scientist performed an mRNA mixed copolymer experiment to decode the nonstandard genetic code of a recently discovered species of archaea. The mixture used to synthesize the mRNA contained four parts uracil (U) and one part cytosine (C) nucleotides. The polypeptide translated from this mRNA contained only phenylalanine, serine, leucine, and proline. From previous experiments, the scientist already knew that the organism's genetic code is a triplet code, like the genetic code of humans. Furthermore, the scientist knew that all codons starting with UU code for phenylalanine, UC code for serine, CC code for proline, and CU code for leucine. Using this information, calculate the percentage of amino acids in the polypeptide expected to be composed of proline. A scientist has obtained a sequence of chimpanzee (Pan troglodytes) DNA, which is believed to encode a chemokine receptor gene. The scientist is examining the sequence to identify potential open reading frames (ORFs), which contain both a start and stop codon. Using the partial sequence of chimpanzee DNA below, identify the total number of ORFs.

Explanation / Answer

Probability of a U = 4/5 (four parts, total 4+1=5 parts)

Probability of a C = 1/5 (one part)

Probability of UU = 4/5 x 4/5 = 0.64 (phenylalanine 64%)

Probability of UC = 4/5 x 1/5 = 0.16 (Serine 16%)

Probability of CU = 1/5 x 4/5 = 0.16 (Leucine 16%)

Probability of CC = 1/5 x 1/5 = 0.04 (Proline 4%)

Thus, the percentage of amino acids in the polypeptide expected to be composed of proline is 4%.

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