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. In the first experiment, 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 synthesized. In a second experiment, the mRNA is translated in the same cell- free system to which you have added SRP. In this case, translation stops after a peptide about 70 amino acids long has been produced. In a third experiment, you use the same cell-free translation system to which you have added both SRP and endoplasmic reticulum vesicles, and you find that the prolactin mRNA now produces a polypeptide that is 199 amino acids long. A) Explain these results. B) Where would you expect to find the polypeptide produced in experiment 3?

Explanation / Answer

First, you have to check orientation. In transcription, the DNA template strand is processed in the 3'->5' direction. Sometimes you be thrown a curve ball and the orientiation will be wrong, and you'll have to reverse the sequence. But the sequence given to us runs in the right direction, so we don't have to reverse it.

2, we model transcription by writing the complementary RNA base below each DNA base. The base pairing rules for going from DNA to RNA are:
C -> G
G -> C
T -> A
A -> U (there is not T in RNA - it uses U instead)

3'-TACGTAGGCTAACGGAGTAAGCTAACT-5' (DNA templated strand)
5'-AUGCAUCCGAUUGCCUCAUUCGAUUGA-3' (mRNA)

Third, we look for a start codon, AUG. The start codon indicates the first amino acid for the polypeptide and sets the triplet reading frame.

We are in luck because the first 3 bases are a start codon. Mentally or in writing, break up wht mRNA (starting with the start codon) into groups of 3 bases. Each one will be a codon.

-AUG CAU CCG AUU GCC UCA UUC GAU UGA-3' (mRNA) Use a genetic code table to lookup each codon and write below it the amino acid it codes for.

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