pBR322 NCERT Diagram — Labelled Plasmid Map, Restriction Sites & NEET Points

This is the diagram you came for: the complete, labelled map of pBR322, the first artificial cloning vector and the one NCERT uses to teach gene cloning. Below the diagram, you'll find every feature explained, the full restriction-site table, and the one concept NEET tests most — insertional inactivation. Everything an NCERT student needs, in one place.

Save or screenshot this diagram. Below, each feature is explained in NCERT terms, followed by the restriction-site table and the insertional-inactivation concept NEET tests every year.

What Is pBR322? (The One-Paragraph NCERT Answer)

pBR322 is the first artificial cloning vector ever constructed, built in 1977 and named after its creators Francisco Bolivar and Raymond Rodriguez (the "p" stands for plasmid, "322" is the lab designation). It's a small, circular, double-stranded DNA molecule of 4361 base pairs. NCERT uses it as the textbook example of a cloning vector because it has every feature a good vector needs: an origin of replication, two selectable marker genes, and multiple unique restriction sites for inserting foreign DNA.

The 4 Key Features of pBR322

🎯 Every Feature in the Diagram — Explained
ori	Origin of Replication	The sequence where DNA replication begins. It controls copy number — how many copies of the plasmid exist per cell. Any foreign DNA linked to the ori replicates inside the host.
ampR	Ampicillin resistance gene	A selectable marker. Cells carrying the plasmid survive on ampicillin-containing medium; cells without it die. Contains the PstI, PvuI, and ScaI restriction sites.
tetR	Tetracycline resistance gene	The second selectable marker. Contains the BamHI and SalI restriction sites — the key sites for insertional inactivation (explained below).
rop	Replication of plasmid	Codes for proteins involved in the replication of the plasmid and helps control its copy number.

Why two resistance genes? Because pBR322 uses one marker to confirm the plasmid got in, and the other to detect whether foreign DNA was successfully inserted. That two-marker system is the entire point of the vector — and it's what insertional inactivation exploits.

pBR322 Restriction Sites — The Complete Table

A restriction site is a specific sequence where a restriction enzyme cuts the DNA. In pBR322, the key sites fall inside the two resistance genes — which is exactly why this vector is so useful.

Restriction Enzyme	Position (bp)	Located In	Effect of Inserting DNA Here
EcoRI	0 / 4361	Outside both genes	Neither resistance affected
ClaI	23	tetR promoter region	Can affect tetracycline resistance
HindIII	29	tetR promoter region	Can affect tetracycline resistance
BamHI	375	Within tetR	Inactivates tetracycline resistance
SalI	651	Within tetR	Inactivates tetracycline resistance
PvuII	2066	Outside both genes	Neither resistance affected
PstI	3609	Within ampR	Inactivates ampicillin resistance
PvuI	3735	Within ampR	Inactivates ampicillin resistance
ScaI	3846	Within ampR	Inactivates ampicillin resistance

The pattern to remember: BamHI and SalI cut inside tetR. PstI, PvuI, and ScaI cut inside ampR. EcoRI and PvuII sit outside both genes. This placement is the foundation of the next concept.

Insertional Inactivation — The Concept NEET Tests Most

This is the single most important pBR322 concept for NEET. Here's the logic, step by step:

📌 Insertional Inactivation — How Recombinants Are Selected
Step 1: Cut at BamHI (inside tetR)	You cut pBR322 with BamHI, which cuts inside the tetracycline resistance gene (tetR), and insert your foreign DNA there.
Step 2: tetR is now broken	The foreign DNA interrupts the tetR gene — so the gene no longer works. The recombinant plasmid has LOST tetracycline resistance but KEPT ampicillin resistance (ampR is untouched).
Step 3: Grow on ampicillin first	Plate the bacteria on ampicillin medium. Only cells that took up the plasmid survive (they have ampR). Cells with no plasmid die.
Step 4: Replica-plate on tetracycline	Transfer colonies to tetracycline medium. Recombinants (with inserted DNA) DIE on tetracycline (tetR is inactivated). Non-recombinants (no insert) SURVIVE (tetR intact).
The result	Colonies that grow on ampicillin but DIE on tetracycline are your recombinants. This is how you identify cells carrying the foreign gene.

🎯 Insertional inactivation is hard to follow on paper — but it clicks instantly when you watch it happen.

The two-plate selection — survive on ampicillin, die on tetracycline — confuses students every year because it's a process, not a fact. Logic Bloom's Playground lets you actually run it: cut pBR322 at BamHI, drop in foreign DNA, watch tetR break, then plate the colonies and see which survive on each antibiotic. TarQ walks you through why recombinants die on tetracycline. Then practice every Biotechnology PYQ that tests this — line by line from NCERT + 10 years of PYQs, with your Mistake Book tracking where the selection logic trips you up.

Play the process → Free to start.

Why pBR322 Was Improved: The Blue-White Limitation

Insertional inactivation with antibiotics has one drawback NCERT highlights: it's cumbersome, because it requires replica plating onto two different antibiotic plates. This led to better selection methods using a different marker — the lacZ gene — which gives blue-white screening (recombinants are white, non-recombinants are blue) on a single plate. NEET sometimes contrasts the two methods, so know that:

Method	Marker	How Recombinants Show	Drawback / Advantage
Insertional inactivation (antibiotic)	tetR / ampR	Die on the second antibiotic	Cumbersome — needs replica plating on two plates
Blue-white screening	lacZ (α-galactosidase)	Appear white (vs blue colonies)	Faster — single plate, visual selection

pBR322 & Cloning Vector NEET PYQs — Questions You Must Attempt

pBR322 appears in the Biotechnology chapters, which deliver 8-10% of NEET Biology. These are the question types NTA repeats. For each, the trap is explained.

📌 Must-Attempt pBR322 PYQs — With the NTA Trap Explained
1. Restriction site location	Which restriction sites lie within the tetracycline resistance gene of pBR322? Answer: BamHI and SalI. Trap: Students confuse these with PstI (which is in ampR, not tetR). Know the gene each site sits in.
2. Insertional inactivation outcome	Foreign DNA is inserted at the BamHI site. On which antibiotic do recombinants fail to grow? Answer: Tetracycline. Trap: Recombinants still grow on ampicillin (ampR intact) — students pick ampicillin by reflex.
3. Selectable marker identification	The selectable markers in pBR322 are? Answer: ampR and tetR (the two antibiotic resistance genes). Trap: ori and rop are NOT selectable markers — they're replication features.
4. Function of ori	What does the ori sequence in pBR322 control? Answer: The initiation of replication and copy number. Trap: Students confuse ori (replication) with the resistance genes (selection).
5. Why antibiotic selection is "cumbersome"	Why is insertional inactivation using antibiotic resistance considered cumbersome? Answer: It requires replica plating onto two different antibiotic media. Trap: The answer is the procedure, not the cost — and blue-white screening using lacZ is the improvement NCERT names.
6. pBR322 size and nature	pBR322 is a ___ of ___ base pairs. Answer: Circular double-stranded DNA plasmid, 4361 bp. Trap: It's the first artificial cloning vector — not naturally occurring in its final form.

🎯 These are 6 of the 200+ Biotechnology PYQs in the app. Drill all of them.

pBR322, restriction enzymes, insertional inactivation, and the full Biotechnology chapter are all inside Logic Bloom — line by line from NCERT + 10 years of PYQs, mapped into chapter topics you can learn and master. Play through the cloning simulation: cut the plasmid, insert DNA, run the selection. When you get a question wrong, TarQ teaches you the concept — not the answer. Your Mistake Book tracks exactly which steps of the cloning process trip you up. Then take it all into Battleground — 1v1 duels under real exam pressure. Get Logic Bloom — Free to start →

Quick Revision — Everything About pBR322 in One Box

📌 pBR322 — The Complete Cheat Sheet
Full form	plasmid Bolivar Rodriguez 322 (first artificial cloning vector, 1977)
Size	4361 base pairs, circular double-stranded DNA
ori	Origin of replication — controls copy number
Selectable markers	ampR (ampicillin) + tetR (tetracycline)
rop	Codes for proteins controlling plasmid replication
Sites in tetR	BamHI, SalI (used for insertional inactivation)
Sites in ampR	PstI, PvuI, ScaI
Sites outside genes	EcoRI, PvuII
Insertional inactivation	Insert at BamHI → tetR broken → recombinants survive ampicillin, die on tetracycline
Improvement	Blue-white screening (lacZ) — single plate, replaces cumbersome two-antibiotic method

Want to actually run the cloning, not just memorise it?

🎯 The diagram tells you the structure. The app lets you use it.
🎮 Playground Understand through games — with TarQ	Run the full gene-cloning process as an interactive game: cut pBR322 at any restriction site, insert foreign DNA, watch the resistance gene break, then plate colonies and run the antibiotic selection. The Biotechnology chapter map breaks every topic into concept games → readings → MCQs. Line by line from NCERT + 10 years of PYQs, all inside. When you're stuck, TarQ teaches the concept. Mistake Book catches the selection-logic slips. Get the app →
⚔️ Battleground Score through practice — 1v1 duels	Test the Biotechnology concepts you understood in Playground under real time pressure. Challenge a friend or get matched live. 10 timed questions per match across Physics, Chemistry, Biology. ELO climbs through 6 tiers: Bronze → Silver → Gold → Platinum → Diamond → Archeon. Get the app →
Understand through games. Score through practice. Get Logic Bloom — Free to start →

FAQs — pBR322

Q1: What is pBR322 and why is it important?
pBR322 is the first artificial cloning vector, constructed in 1977 and named after Bolivar and Rodriguez. It's a circular double-stranded DNA plasmid of 4361 base pairs. It's important because it has all the features of an ideal cloning vector — an origin of replication (ori), two selectable marker genes (ampR and tetR), and several unique restriction sites for inserting foreign DNA. NCERT uses it as the standard example to teach gene cloning.

Q2: What are the restriction sites in pBR322?
The key restriction sites are: EcoRI (outside both genes), HindIII and ClaI (in the tetR promoter region), BamHI and SalI (within the tetracycline resistance gene tetR), PvuII (outside both genes), and PstI, PvuI, ScaI (within the ampicillin resistance gene ampR). BamHI and SalI are the most important because they're used for insertional inactivation.

Q3: What is insertional inactivation in pBR322?
When foreign DNA is inserted at the BamHI or SalI site, it interrupts the tetracycline resistance gene (tetR), inactivating it. The recombinant plasmid then loses tetracycline resistance but keeps ampicillin resistance. Recombinant cells are identified because they grow on ampicillin medium but die on tetracycline medium — while non-recombinants survive both.

Q4: What is the function of ori and rop in pBR322?
The ori (origin of replication) is the sequence where DNA replication begins; it controls how many copies of the plasmid exist per cell (copy number). The rop gene codes for proteins involved in the replication of the plasmid. Neither is a selectable marker — those are the two antibiotic resistance genes.

Q5: Why was antibiotic-based selection in pBR322 replaced by blue-white screening?
Insertional inactivation using antibiotic resistance is cumbersome because it requires replica plating onto two different antibiotic media to identify recombinants. Blue-white screening using the lacZ gene is faster — recombinants appear as white colonies and non-recombinants as blue colonies on a single plate, allowing visual selection without replica plating.