Question

(a) Draw a figure of E. coli vector pBR322 and label the following :

(i) Origin of replication

(ii) Ampicillin resistance site

(iii) Tetracyclin resistance site

(iv) "rop"

(b) Mention the significance of origin of replication.

Hint:

Draw pBR322 as a circle and mark ori, amp-R, tet-R and rop. The ori is where replication starts and it fixes the copy number.

Answer 1

Step 1: Understanding the Question:
Part (a) asks for a labelled figure of the plasmid pBR322. Part (b) asks why the origin of replication matters. A labelled sketch is expected for part (a).

Step 2: Part (a), how to draw it:
pBR322 is a small circular double-stranded plasmid of about 4361 base pairs. Draw a circle for the plasmid and mark these four parts on it.
(i) ori: the origin of replication, shown as a short segment on the circle.
(ii) amp^R: the gene for ampicillin resistance. It is a separate arc on the circle.
(iii) tet^R: the gene for tetracycline resistance. It is another arc, away from amp^R.
(iv) rop: the gene that codes for the proteins involved in the replication of the plasmid. It lies close to the ori.
You may also add restriction sites such as BamHI, SalI and PstI, which lie inside these genes.

Step 3: Part (b), significance of ori:
The ori is the sequence from where replication starts.
Any piece of DNA linked to the ori can be replicated in the host cell.
The ori also decides the copy number of the linked DNA, that is how many copies of the plasmid the cell makes.

Final Answer:
The figure of pBR322 shows ori, amp^R, tet^R and rop. The ori is the starting point of replication and it controls the copy number of the plasmid.