You perform a series of experiments on the synthesis of the pituitary hormone pr

Question

You perform a series of experiments on the synthesis of the pituitary hormone prolactin, which is a single polypeptide chain 199 amino acids long. The mRNA coding for prolactin is translated in a cell-free protein synthesizing system containing ribosomes, amino acids, tRNAs, aminoacyl-tRNA synthetases, ATP, GTP, and the appropriate initiation, elongation, and termination factors. Under these conditions, a polypeptide chain 227 amino acids long is produced. How might you explain the discrepancy between the normal length of prolactin (199 amino acids) and the length of the polypeptide synthesized in your experiment (227 amino acids)? In the absence of SRP and ER membranes, synthesis proceeds in the wrong direction. In the absence of SRP and ER membranes, introns arent spliced from the mRNA. In the absence of SRP and ER membranes, a prolactin precursor molecule is synthesized In the absence of SRP and ER membranes, stop codons arent recognized.

Explanation / Answer

Answer:

Part A: (c) In the absence of SRP and ER membranes, a prolactin precursor molecule is synthesized.

Explanation:

As a secreted hormone, prolactin contains an N-terminal ER SS that guides it to the rough ER for co-translational transport into the ER lumen. In the absence of SRP and ER membranes, the 28 amino acid ER SS would not be removed by signal peptidase.

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