Which feature allows an expression vector to be replicated in host cells?

Which feature allows an expression vector to be replicated in host cells?

• A. A promoter that drives transcription
• B. A cloning site to insert the gene of interest
• C. A selectable marker for identifying hosts
• D. An origin of replication to enable replication in the host

Answer: (D)

The key thing this question tests is what lets a circular DNA vector be copied inside the host cells. That happens because the vector has a sequence called the origin of replication, which is recognized by the host’s DNA replication machinery to start copying the plasmid. With a functional origin, the plasmid is replicated along with the host genome and maintained through cell divisions, so you get multiple copies and stable expression of the gene of interest. The other features serve different roles: a promoter drives transcription of the inserted gene, enabling expression; a cloning site provides a place to insert the gene; a selectable marker helps identify which cells carry the vector. None of these by themselves enable the plasmid to be replicated in the host, which is why the origin of replication is the correct choice.

The key thing this question tests is what lets a circular DNA vector be copied inside the host cells. That happens because the vector has a sequence called the origin of replication, which is recognized by the host’s DNA replication machinery to start copying the plasmid. With a functional origin, the plasmid is replicated along with the host genome and maintained through cell divisions, so you get multiple copies and stable expression of the gene of interest.

The other features serve different roles: a promoter drives transcription of the inserted gene, enabling expression; a cloning site provides a place to insert the gene; a selectable marker helps identify which cells carry the vector. None of these by themselves enable the plasmid to be replicated in the host, which is why the origin of replication is the correct choice.