The beginning of a strand of mRNA has the following base sequence: AUGUUUUUGCCUCAUCGUUUGCAU. The polypeptide formed by the first six steps of protein synthesis has the sequence Met-Phe-Leu-Pro-His-Arg. What is the next amino acid to be added to the polypeptide chain?
A)Arg
B)His
C)Leu
D)Met
E)Phe

Question

Quizplus · Accepted Answer

The sequence AUG codes for methionine, which is the first amino acid in the polypeptide chain. UUU codes for phenylalanine, UUU codes for phenylalanine again, GCC codes for alanine, UCU codes for serine, and GUU codes for valine. Therefore, the next codon in the mRNA sequence is CAU, which codes for histidine, and is not the next amino acid in the polypeptide chain. The next codon after CAU is ACA, which codes for threonine, and is also not the next amino acid in the polypeptide chain. The next codon after ACA is UGU, which codes for cysteine, and is also not the next amino acid in the polypeptide chain. The next codon after UGU is UCA, which codes for serine, and is also not the next amino acid in the polypeptide chain. The next codon after UCA is UGU again, which codes for cysteine again, and is also not the next amino acid in the polypeptide chain. Finally, the next codon after UGU is CCA, which codes for leucine, and is the next amino acid in the polypeptide chain. Therefore, the answer is C) Leu.

The Beginning of a Strand of MRNA Has the Following