Biology · Biotechnology: Principles and Processes · NEET
A competent host cell is a bacterial cell (like E. coli) that has been specially treated so it can take up DNA from its surroundings. Normally DNA is hydrophilic (water-loving) and cannot cross the cell membrane, so the cell must first be made "competent". Memory hook: "Competent = the cell is now READY to receive DNA." This uptake of DNA by a competent cell is called transformation.
A normal bacterium cannot absorb hydrophilic DNA. Treatment with a divalent cation (calcium) opens pores in the cell wall, making the cell competent. With recombinant DNA plus a brief heat shock (ice to 42C to ice), the cell takes up the DNA and becomes transformed, then grows into selectable clones.
Your doubts, answered
What does 'competent' actually mean here?
'Competent' means the host cell has been treated so it is now ABLE to take up DNA from outside. A normal untreated bacterium cannot easily absorb DNA. After treatment (with a divalent cation like calcium), pores in the cell wall become more open, so DNA can enter. So competent = ready/able to take up DNA.
Why can't DNA just enter a normal bacterial cell?
Because DNA is a hydrophilic (water-loving) molecule. The cell membrane is made of lipids, so a hydrophilic molecule cannot pass through it easily. This is exactly why the cell must first be made competent, before you can push recombinant DNA into it.
Is a competent cell the same as a transformed cell?
No. A competent cell is one that is READY to take up DNA. A transformed cell is one that has ALREADY taken up the DNA. First you make the cell competent, then transformation (DNA uptake) happens. NEET often mixes these two up, so keep the order: competent first, transformed after.
Which cation is used and why calcium?
A divalent cation such as calcium (Ca2+) is used. It increases the efficiency with which DNA enters the bacterium through the pores in its cell wall. NCERT specifically names calcium as the example, so remember 'divalent cation, e.g. calcium'.
Where does the competent host cell step fit in rDNA technology?
It comes at the INSERTION step, after the recombinant DNA (gene ligated into a vector) is ready. The competent host cell takes up this recombinant DNA, and inside the cell the DNA multiplies. This is the step called 'insertion of recombinant DNA into the host cell'.
⚠️ The NEET trap ✗ A competent host cell already contains the foreign gene. ✓ A competent host cell is only PREPARED to take up DNA; it has not received the DNA yet. Only after it takes up the recombinant DNA is it called a transformed cell. 🧠 Competent = READY to receive. Transformed = HAS received. Do not swap the order.
Real NEET questions
ReNEET 2026
Match List-I with List-II:
List-I: A. Transformation B. Cloning site C. Selection D. Ori
List-II: I. Restriction enzyme II. Transfer DNA to host bacteria III. Replication IV. Antibiotic
Choose the correct answer:
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 = introducing/transferring DNA into host bacteria (II) - this is exactly what a competent cell allows. Cloning site is acted on by a restriction enzyme (I). Selection of recombinants uses an antibiotic marker (IV). Ori controls replication (III). So A-II, B-I, C-IV, D-III.
NEET 2017
A gene whose expression helps to identify transformed cell is known as
A · Selectable marker ✓
B · Vector
C · Plasmid
D · Structural gene
Solution: A selectable marker (e.g. an antibiotic-resistance gene) helps identify and select the cells that have been transformed - that is, the competent cells that actually took up the recombinant DNA. The vector/plasmid is only the carrier, and a structural gene does not report transformation.
Solved Biotechnology: Principles and Processes NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
