Biology · Biotechnology: Principles and Processes · NEET
The origin of replication, written as ori, is a specific DNA sequence from where replication (copying of DNA) starts. In a cloning vector, ori does two jobs: it lets any DNA joined to it multiply inside the host cell, and it controls the copy number (how many copies of that DNA are made). Memory hook: "ori = start button + copy counter." No ori, no multiplication, so a vector must have one.
Ori is one fixed sequence on the plasmid (red dot). It does two NEET-important jobs: it starts replication so linked DNA multiplies, and it controls the copy number of that DNA.
Your doubts, answered
Is the origin of replication (ori) the same as the replication fork?
No. The ori is a fixed DNA sequence that acts like a start point where replication begins. The replication fork is the moving Y-shaped opening in the DNA helix where the two strands are being separated and copied. Replication starts AT the ori and then the fork moves along the DNA. NEET can mix these up, so remember: ori = fixed starting site, fork = moving copying point.
How does ori control the copy number of a plasmid?
Copy number means how many copies of the linked DNA are made inside one host cell. The ori sequence itself decides this. Some ori sequences allow only a few copies, while high-copy-number ori sequences allow many copies. NCERT says clearly: if you want to recover MANY copies of your target DNA, clone it in a vector whose ori supports a high copy number. This is a favourite NEET line.
Why does an alien piece of DNA need ori to survive in a host?
A random piece of DNA put into a host bacterium cannot copy itself, so it is lost as the cell divides. Only DNA that is part of a chromosome or joined to an ori can replicate. When alien DNA is linked to a vector carrying ori, the host machinery treats it like its own and copies it every time the cell divides. So ori gives the alien DNA the ability to multiply.
Is ori a gene?
No. The ori is a non-coding DNA sequence, not a gene. It does not code for a protein. It is simply a recognition sequence that tells the cell's replication enzymes where to start copying. In pBR322, note that a separate gene called rop codes for proteins involved in plasmid replication, but ori itself is only a sequence, not a protein-coding gene.
How is ori different from the cloning site (recognition site)?
The ori is where replication starts and it controls copy number. The cloning site (also called recognition site) is where a restriction enzyme cuts, so the foreign DNA can be inserted there. Different jobs: ori = copying, cloning site = cutting and inserting. A good vector has ori PLUS very few (preferably single) recognition sites.
⚠️ The NEET trap ✗ The selectable marker or the recognition site controls the copy number of the DNA in a vector. ✓ The origin of replication (ori) controls copy number. The selectable marker only helps identify transformants, and the recognition site is only where the restriction enzyme cuts. 🧠 Copy number belongs to ori ONLY. NEET 2020 asked exactly this and the answer was Ori site, not marker or recognition site.
Real NEET questions
NEET 2020
The sequence that controls the copy number of the linked DNA in the vector is termed
A · Palindromic sequence
B · Recognition site
C · Selectable marker
D · Ori site ✓
Solution: The origin of replication (ori) is the sequence from where replication starts and it is also responsible for controlling the copy number of the linked DNA in a vector. A palindromic sequence or recognition site is where restriction enzymes cut, and a selectable marker only helps eliminate non-transformants. NCERT Ch 9: 'This sequence is also responsible for controlling the copy number of the linked DNA.'
NEET 2024
The following diagram showing restriction sites in E. coli cloning vector pBR322. Find the role of 'X' and 'Y' genes:
A · Gene 'X' controls the copy number of the linked DNA and 'Y' codes for a protein involved in replication of the plasmid ✓
B · Gene 'X' is for protein involved in replication and 'Y' for resistance to antibiotics
C · Gene 'X' is responsible for recognition sites and 'Y' for antibiotic resistance
D · Gene 'X' is for antibiotic resistance and 'Y' for protein involved in replication of the plasmid
Solution: In pBR322, ori (X) is the sequence that controls the copy number of the linked DNA, while rop (Y) codes for proteins involved in replication of the plasmid. Antibiotic resistance is handled by the separate ampR and tetR genes, so options naming resistance for X or Y are wrong.
ReNEET 2026
Match List-I with List-II: A. Transformation B. Cloning site C. Selection D. Ori ; I. Restriction enzyme II. Transfer DNA to host bacteria III. Replication IV. Antibiotic
A · A-II, B-I, C-IV, D-III ✓
B · A-I, B-II, C-IV, D-III
C · A-III, B-IV, C-II, D-I
D · A-IV, B-I, C-III, D-II
Solution: Transformation = transfer of DNA into host bacteria (II). Cloning site = acted on by restriction enzyme (I). Selection = uses antibiotic-resistance markers (IV). Ori = controls replication of the plasmid (III). So A-II, B-I, C-IV, D-III.
Solved Biotechnology: Principles and Processes NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Ori stands for origin of replication. It is the DNA sequence from where replication (DNA copying) begins.
What are the two main functions of ori in a cloning vector?
First, it starts replication so any DNA joined to it can multiply inside the host cell. Second, it controls the copy number of that linked DNA.
Which vector feature decides high or low copy number?
The ori decides it. To get many copies of your target DNA, you clone it in a vector whose ori supports a high copy number.
Is ori a desirable feature of a good cloning vector?
Yes. Presence of an origin of replication is one of the required features of a good cloning vector, along with a selectable marker and a single recognition site.
Does ori code for a protein?
No. Ori is a non-coding sequence that only marks where replication starts. In pBR322 a separate gene, rop, codes for replication proteins.
