A researcher has labeled the 5\' end of a DNA primer with 32P radioisotopes. The

Question

A researcher has labeled the 5' end of a DNA primer with 32P radioisotopes. The primer can hybridize to a complementary sequence near the 3' end of a long, spliced mRNA molecule composed of three exons. The mRNA molecule can interact with two RNA-binding proteins: one of them (protein 1) binds to the first exon and the other (protein 2) binds to the second exon. The researcher uses reverse transcriptase to extend the primer in the presence of the mRNA and either of these proteins, then separates the cDNA products by gel electrophoresis, and finally visualizes the labeled DNA bands by autoradiography. The results are presented schematically in the following drawing. Assuming that the reverse transcriptase falls off the template when confronted by bound proteins, which lane (A or B) in the gel corresponds to the reaction in the presence of protein 1? Write down A or B as your answer

Explanation / Answer

Lane A corresponds to the problem as the 3' end when extended till 5'end and falls off when protein 2 is encountered. thus length corresponds to length of exon1 and exon2. Important thing to be noted is that the cDNA copy formed covers exon 1 and exon 2. B should correspond to the full length of cDNA if no proteins are present for reverse transcriptase fall off.

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