Biotechnology: Principles and Processes — BIOLOGY STD 12 Science — Question

Most commercial cloning vectors have key features that have made their use in molecular biology so widespread.
Control of Expressions: In the case of expression vectors, the main purpose of these vehicles is the controlled expression of a particular gene inside a convenient host organism (e.g. E. coli). Control of expression can be very important; it is usually desirable to insert the target DNA into a site that is under the control of a particular promoter. Some commonly used promoters are T7 promoters, lac promoters and cauliflower mosaic virus's 35s promoter (for plant vectors).
Selectable Marker: To allow for convenient and favorable insertions, most cloning vectors have had nearly all their restriction sites engineered out of them and a synthetic multiple cloning site (MCS) inserted that contains many restriction sites. MCSs allow for insertions of DNA into the vector to be targeted and possibly directed in a chosen orientation. A selectable marker, such as an antibiotic resistance is often carried by the vector to allow the selection of positively transformed cells. All plasmids must carry a functional origin of replication (ori).