Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A): A piece of DNA that needs to be propagated during recombinant DNA procedures requires a vector.
Reason (R): DNA replication does not initiate randomly at any place in DNA but starts from a definite region termed the origin of replication.
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 cannot start copying DNA at just any point. Replication begins only at a special sequence called the origin of replication (ori). A DNA molecule without such a site cannot be copied in a host cell.
Assertion (A): A piece of DNA that needs to be propagated during recombinant DNA procedures requires a vector. — True. A foreign gene or DNA fragment is joined to a vector (such as a plasmid) before it is put into a host, so that many copies of it can be made.
Reason (R): DNA replication does not initiate randomly at any place in DNA but starts from a definite region termed the origin of replication. — True. In E. coli there is a definite region of DNA where replication starts.
Why R explains A? A says a DNA piece needs a vector to be multiplied. R says replication can start only at an origin of replication. A bare piece of foreign DNA usually has no such origin, so it cannot be copied in the host. The vector provides the origin, and the joined piece is copied along with it. So R is the correct explanation of A.
NCERT Extract:
The Machinery and the Enzymes – “… The DNA polymerases on their own cannot initiate the process of replication. Also the replication does not initiate randomly at any place in DNA. There is a definite region in E. coli DNA where the replication originates. Such regions are termed as origin of replication. It is because of the requirement of the origin of replication that a piece of DNA if needed to be propagated during recombinant DNA procedures, requires a vector. The vectors provide the origin of replication.”