In the E. coli cloning vector pBR322, the gene rop codes for:

The correct answer is Option (3) → Proteins for the replication of the plasmid

pBR322 is an E. coli cloning vector. Like any cloning vector it has an origin of replication (ori), selectable markers and cloning sites. Its selectable markers are two antibiotic resistance genes, amp^R and tet^R.

Option (1) Resistance to ampicillin — Incorrect. Resistance to ampicillin comes from the gene amp^R, not from rop.

Option (2) Resistance to tetracycline — Incorrect. Resistance to tetracycline comes from the gene tet^R, the second selectable marker.

Option (3) Proteins for the replication of the plasmid — Correct. rop codes for the proteins involved in the replication of the plasmid. Its place on the plasmid is shown in Figure 9.4.

Option (4) The enzyme β-galactosidase — Incorrect. β-galactosidase belongs to the other selection method, based on blue and white colonies. It is not coded by rop.

Note: rop gets just one line in the whole chapter, and it sits in the title of Figure 9.4. Figure titles may look like small print, but they carry full marks.

NCERT Extract:
Cloning Vectors – “… (i) Origin of replication (ori) : This is a sequence from where replication starts and any piece of DNA when linked to this sequence can be made to replicate within the host cells. This sequence is also responsible for controlling the copy number of the linked DNA. … Normally, the genes encoding resistance to antibiotics such as ampicillin, chloramphenicol, tetracycline or kanamycin, etc., are considered useful selectable markers for E. coli.”

Figure 9.4 E. coli cloning vector pBR322 showing restriction sites (Hind III, EcoR I, BamH I, Sal I, Pvu II, Pst I, Cla I), ori and antibiotic resistance genes (amp^R and tet^R). rop codes for the proteins involved in the replication of the plasmid