Q.Write the dual purpose served by Deoxyribonucleoside triphosphates in polymerisation.

Deoxyribonucleoside triphosphates (dNTPs) are both the substrate (raw material) for the growing DNA chain and the energy source that powers each nucleotide addition during replication.

Concept. In DNA replication, the enzyme DNA-dependent DNA polymerase catalyses polymerisation of deoxyribonucleotides on a template strand. The precursors supplied for this are the deoxyribonucleoside triphosphates — dATP, dGTP, dCTP and dTTP — each a nitrogen base + deoxyribose sugar + three phosphate groups.

Why they serve two purposes. A single molecule of dNTP carries the information (the base to be added) as well as chemical energy (in its phosphoanhydride bonds between the phosphates). This lets the same molecule act as building block and fuel, so no separate energy molecule (like ATP) is needed for chain elongation.

Explanation (the two roles).

1. As a substrate: the innermost (alpha) phosphate of the incoming dNTP joins the 3'-OH end of the growing strand, adding the nucleotide to the new DNA chain. …