Biology · Biotechnology: Principles and Processes · NEET
A good cloning vector needs three main features: an origin of replication (ori) to copy the DNA inside the host, a selectable marker to pick out transformed cells, and one (single) cloning/recognition site to insert the foreign gene cleanly. Memory hook: think "ORI-SIC" = Ori, Selectable marker, Single Cloning site. NEET repeats these three points, and the biggest trap is that MANY restriction sites are bad, not good.
A good cloning vector carries an origin of replication (ori) to copy DNA and control copy number, a selectable marker to pick transformed cells, and a single cloning site for clean gene insertion. Many recognition sites are undesirable because they cut the vector into several fragments.
Your doubts, answered
Why must a good cloning vector have only ONE (single) restriction site, not many?
To link the foreign DNA, the vector needs very few, preferably a single, recognition site for the restriction enzyme used. If there are two or more sites, the enzyme cuts the vector into several pieces, so you cannot get one clean open vector to insert your gene. NCERT says more than one recognition site 'will generate several fragments, which will complicate the gene cloning.' This is why 'presence of two or more recognition sites' is NOT a desirable feature.
What exactly does the origin of replication (ori) do in a cloning vector?
The ori is the sequence where replication starts. Any DNA linked to it can copy itself inside the host cell. The ori also controls the copy number, that is, how many copies of the linked DNA are made. If you want many copies of your target DNA, you clone it in a vector whose ori supports a high copy number. So ori gives both replication and copy-number control.
What is a selectable marker and why is it needed?
A selectable marker is a gene (usually an antibiotic-resistance gene like ampR or tetR) that helps identify and eliminate non-transformants while allowing only transformed cells to grow. Normal E. coli has no resistance to ampicillin, tetracycline, chloramphenicol or kanamycin, so only cells that took up the vector survive on that antibiotic. Without a marker, you cannot tell which cells received the vector.
Is a selectable marker the same as the cloning site or the ori?
No. The three features do different jobs. Ori = starts replication and sets copy number. Selectable marker = lets you select transformed cells. Cloning site (single recognition site) = the spot where the restriction enzyme cuts to insert foreign DNA. NEET often mixes these in match-the-column questions, so keep the jobs separate.
Why can't a foreign piece of DNA just replicate on its own inside a host?
A stray piece of alien DNA usually cannot multiply in the host's progeny cells because it has no origin of replication. Only when it is linked to a vector (which provides the ori) or integrated into a chromosome can it replicate and be inherited. This is exactly why the vector's ori is essential.
⚠️ The NEET trap ✗ A good cloning vector should have many (two or more) restriction/recognition sites so more enzymes can cut it. ✓ A good cloning vector should have very few, preferably a SINGLE, recognition site. Many sites generate several fragments and complicate cloning, so multiple sites are NOT desirable. 🧠 'Single site = good, many sites = bad.' NTA flips this to trick you (NEET 2022).
Real NEET questions
2022
Which of the following is not a desirable feature of a cloning vector?
A · Presence of origin of replication
B · Presence of a marker gene
C · Presence of single restriction enzyme site
D · Presence of two or more recognition sites ✓
Solution: A good cloning vector should have very few, preferably a single, recognition site for the restriction enzyme. Presence of two or more recognition sites generates several fragments and complicates gene cloning, so it is NOT desirable. Origin of replication, a marker gene, and a single restriction site are all desirable features.
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 also controls the copy number of the linked DNA. A palindromic/recognition site is where restriction enzymes cut, and a selectable marker helps eliminate non-transformants, so only the ori site governs copy number.
2019
A selectable marker is used to:
A · help in eliminating the non-transformants, so that the transformants can be regenerated ✓
B · identify the gene for a desired trait in an alien organism
C · select a suitable vector for transformation in a specific crop
D · mark a gene on a chromosome for isolation using restriction enzyme
Solution: A selectable marker (e.g., an antibiotic-resistance gene) helps identify and eliminate non-transformants while selectively permitting the growth of transformed cells, which can then be regenerated. It does not identify a desired-trait gene, choose a vector, or mark genes for restriction isolation.
Solved Biotechnology: Principles and Processes NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
What are the three main features of a good cloning vector?
Origin of replication (ori), a selectable marker, and a single cloning/recognition site. Ori replicates the DNA and controls copy number, the selectable marker selects transformed cells, and the single site lets foreign DNA be inserted cleanly.
Why is a single restriction site preferred in a cloning vector?
So the restriction enzyme opens the vector at just one point, giving one linear piece to insert the gene. Multiple sites cut the vector into several fragments and complicate cloning.
Which feature controls the copy number of the vector?
The origin of replication (ori). It starts replication and also decides how many copies of the linked DNA are made in the host cell.
Give an example of a selectable marker used in E. coli.
Antibiotic-resistance genes such as ampicillin (ampR), tetracycline (tetR), chloramphenicol or kanamycin resistance. Normal E. coli is sensitive to these, so only transformed cells survive.
Is having many recognition sites good for a cloning vector?
No. This is a common NEET trap. Many recognition sites are NOT desirable because they generate several fragments and complicate cloning. A good vector has very few, preferably one, site.
