Question

When enzymes are elongating (building) a newly synthesized DNA strand in PCR, new nucleotides are being added to the_of the growing strand. A 5 phosphate group B. A nitrogenous base C. 3' hydroxyl of deoxyribose D. 2 H of deoxyribose

Answer 1

Ans c) 3^' hydroxyl of deoxyribose.

During elongation phase of DNA replication, DNA polymerase adds new nucleotides to the 3^' OH of the growing strand with the replication progressing in 5^' to 3^' direction. The nucleotide has a phosphate group, a ribose/ deoxyribose sugar, and a nitrogenous base. The phosphate group is attached to the 5^' position of the deoxyribose sugar while its 3^' position is occupied by the hydroxyl group. The motive behind the incorporation of new nucleotide always at 3^' OH end of growing DNA chain is the nucleophilic approach used by 3^' OH attacking the phosphorous atom of incoming dNTPs and simultaneously a phosphodiester bond is formed with the release of inorganic pyrophosphate. Mg ion, in turn, helps in minimizing the interaction between the O and the H, leaving a nucleophile 3^'O^- . Since 3^' hydroxyl group is a better nucleophile as compared to others this explains the addition of new nucleotide here. One more reaction that is essential for DNA replication, the pyrophosphate hydrolysis is easily accomplished in this way.