What is origin of replication needed for?

Correct answer: For replication of the vector

The direct answer is A: the origin of replication is needed for replication of the vector. In genetic engineering, a vector such as a plasmid carries useful foreign DNA into a host cell. The origin of replication, written as ori, is a particular DNA sequence where copying of the vector begins. Step by step: the vector enters the host; the host's copying machinery recognises ori; the vector DNA is copied; daughter cells can receive vector copies. Thus ori helps maintain and multiply the vector, although its exact copy number depends on the vector and host. Option A is correct because it states this function. Option B is wrong because protein digestion is done by digestive enzymes such as proteases, not ori. Option C is wrong because cell colour is not the basic function of an origin of replication; colour may be used in some markers, but ori itself starts DNA copying. Option D is wrong because water absorption is a physiological process unrelated to vector replication. Memory cue: ori means the starting point for copying DNA.