Question

Choose the correct answer from the alternatives given :
A DNA template plus primer with the structure
3 P - TG C GAATTAG C GACAT - P5
5 P - ATCGGTACGACGCTTAAC OH 3
(where P = a phosphate group) is placed in an in vitro DNA synthesis system containing $M{g}^{2+}$, an excess of the four deoxyribonucleoside triphosphates, etc. and a mutant form of E. Coli DNA polymerase I that lacks 5 $\to$ 3 exonuclease activity. The 5 $\to$ 3 polymerase and. 3 $\to$ 5 exonuclease activities of this aberrant enzyme are identical to those of normal E. Coli DNA polymerase I. It simply has no 5 $\to$ 3 exonuclease activity. What will be the structure of the final product?

• A. 3P-TGCGAATTAGCGACAT- P 5
  5 P -ATCG GTAC GACG CTTAATCG CTGTA-OH 3
• B. 3 P -T6CGAATTGGCGACAT- P 5
  5 P -ATCG GTACGAC GCTTAAC C G CTGTA-OH 3
• C. 3HOTGCGAATTAGCGACAT P 5
  5-ATCG GTAC GACG CTTAATCG CTGTA - P 3
• D. 3 P TGCGAATTAGCGACAT P 5
  5 P -ACGCTTAATCGCTGTA-OH3

Answer 1

The correct option is A 3P-TGCGAATTAGCGACAT- P 5

5 P -ATCG GTAC GACG CTTAATCG CTGTA-OH 3
The DNA synthesis will not occur on the left end because all DNA polymerases require a free 3 OH terminus, but here the 3 terminus of potential primer strand is blocked with a phosphate group.