Question

The most important feature in a plasmid to be used as a vector is

• A. origin of replication (ori)
• B. presence of a selectable marker
• C. presence of sites for restriction endonuclease
• D. its size

Answer 1

Answer: (A)

Origin of replication (on), a selectable marker, sites for restriction endonuclease and its size, all are important features required to facilitate cloning into a vector. A good $DNA$ vector should be able to replicate autonomously in the host cell, for which it needs to have an origin of replication site (ori). This is important for the replication of inserted gene. A prokaryotic $DNA$ has a single ori while eukaryotic $DNA$ may have more than one ori. Selectable marker helps in identifying and eliminating non-transformants and selectively permitting the growth of transformants. Cloning sites (recognition sites) are the sites where the $DNA$ is cut by a restriction endonuclease. A vector should be ideally less than $10kb$ in size because large $DNA$ molecules can break down during purification procedure. Option (a) and (c) are absolutely necessary for it to act as vector, while in absence of (b), the efficiency of overall procedure will decrease.