Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A): In DNA replication, deoxyribonucleoside triphosphates act as substrates and also provide energy for the polymerisation reaction.
Reason (R): The two terminal phosphates in a deoxynucleoside triphosphate are high-energy phosphates, same as in the case of ATP.
In the light of the above statements, choose the most appropriate answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Both (A) and (R) are true and (R) is the correct explanation of (A).
DNA polymerase builds a new DNA strand by joining deoxynucleotides one by one. Joining them needs energy, and replication of a whole genome is a costly process. The raw materials are deoxyribonucleoside triphosphates (dNTPs), such as dATP and dGTP, each carrying three phosphate groups.
Assertion (A): In DNA replication, deoxyribonucleoside triphosphates act as substrates and also provide energy for the polymerisation reaction. — True. The dNTPs serve a dual purpose. Their nucleotide part is added to the growing strand, and the extra phosphates released pay for the joining.
Reason (R): The two terminal phosphates in a deoxynucleoside triphosphate are high-energy phosphates, same as in the case of ATP. — True. Like ATP, a dNTP carries two high-energy phosphate bonds at its end.
Why R explains A? A says that dNTPs supply energy as well as material. R tells us where that energy comes from: the two terminal phosphates are high-energy phosphates. When a dNTP is joined to the chain, these two phosphates are split off and their energy drives the reaction. So R is the correct explanation of A.
NCERT Extract:
The Machinery and the Enzymes – “… Not only do these polymerases have to be fast, but they also have to catalyse the reaction with high degree of accuracy. Any mistake during replication would result into mutations. Furthermore, energetically replication is a very expensive process. Deoxyribonucleoside triphosphates serve dual purposes. In addition to acting as substrates, they provide energy for polymerisation reaction (the two terminal phosphates in a deoxynucleoside triphosphates are high-energy phosphates, same as in case of ATP).”