Q27.A cloning vector may not contain
(A) Origin of replication
(B) Multiple cloning site
(C) Promoter
(D) Selectable marker
The correct answer is (C) Promoter. A basic cloning vector requires ori, MCS, and selectable markers for replication, insertion, and selection, but a promoter is specific to expression vectors, not essential for all cloning vectors.
Cloning vectors simply propagate DNA; promoters drive transcription only when protein expression is needed.
Option Analysis
Origin of replication (A): Essential (ori) sequence enabling autonomous replication within the host cell using its machinery. Without it, the vector cannot be maintained.
Multiple cloning site (B): Essential polylinker region with unique restriction enzyme sites for precise foreign DNA insertion via sticky-end ligation.
Promoter (C): Regulatory sequence for transcription initiation; required in expression vectors but absent in basic cloning vectors focused on DNA propagation.
Selectable marker (D): Essential gene (e.g., antibiotic resistance) allowing identification of host cells successfully transformed with the recombinant vector.
A promoter is not a required component of every cloning vector, distinguishing basic cloning from specialized expression vectors. This nuance trips up molecular biology students in exams.
Essential Features
Origin of replication enables propagation. Multiple cloning site facilitates inserts. Selectable markers screen transformants. Promoters appear only for protein production.
