Assertion (A): Origin of replication is an essential part of a vector…

The correct answer is Option (1) → Both Assertion (A) and Reason (R) are correct and R is the correct explanation of A.

Assertion (A): Origin of replication is an essential part of a vector. — True. A is about what a vector must have. The origin of replication (ori) is the first of the features needed for cloning into a vector.

Reason (R): Ori is responsible for initiating replication. — True. R is about what ori does. It is the sequence from which replication starts, and it also controls the copy number of the linked DNA.

Why R explains A? DNA polymerase cannot start replication at any random place; it needs an origin. A piece of foreign DNA has no such origin of its own in the host, so it can be copied only when it is linked to a vector that carries ori. Because ori is what starts replication, a vector without it could not multiply the inserted DNA. So R gives the reason for A, and R is the correct explanation of A.

NCERT Extract:
Cloning Vectors – “… The following are the features that are required to facilitate cloning into a vector. (i) Origin of replication (ori) : This is a sequence from where replication starts and any piece of DNA when linked to this sequence can be made to replicate within the host cells. This sequence is also responsible for controlling the copy number of the linked DNA.”
The Machinery and the Enzymes – “… 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.”