Biology · Biotechnology: Principles and Processes · NEET
DNA is a hydrophilic (water-loving) molecule because of its negatively charged phosphate backbone. The cell membrane has a hydrophobic (water-fearing) core, so the charged DNA is repelled and cannot pass through on its own. This is exactly why we must first make the cell 'competent' before transformation.
The bacterial cells are treated with a specific concentration of a divalent cation, such as calcium (Ca2+). The positive calcium ions neutralise the negative charges on both the DNA and the cell wall/membrane, reducing repulsion. This increases the efficiency with which DNA enters the bacterium through pores in its cell wall.
After mixing competent cells with recombinant DNA, the cells are incubated on ice, then placed briefly at 42°C (this is the heat shock), and then put back on ice. The sudden temperature change momentarily disturbs the membrane and helps the DNA pass in. Remember the order: ice → 42°C → ice (cold–hot–cold).
No. Micro-injection is used for animal cells. Here the recombinant DNA is injected directly into the nucleus of an animal cell using a fine needle. The calcium + heat-shock method is the one used for bacteria. Mixing these up is a common exam mistake.
For plant cells, the biolistics or gene gun method is used. Cells are bombarded with high-velocity micro-particles of gold or tungsten coated with DNA. So: gene gun (biolistics) → plants; micro-injection → animals; calcium + heat shock → bacteria.
The last method uses disarmed pathogen vectors. These are pathogens (like Agrobacterium's Ti plasmid for plants or retroviruses for animals) whose disease-causing ability has been removed, but which can still deliver the recombinant DNA into the host cell when they infect it.
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:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
A competent cell is a bacterial cell that has been treated (usually with calcium chloride) so that its wall and membrane become leaky enough to take up foreign DNA from its surroundings. Without this treatment, the cell cannot absorb the recombinant DNA.
Transformation is the uptake of foreign (recombinant) DNA by a competent host cell. If the DNA carries, for example, an ampicillin-resistance gene, the E. coli host becomes an ampicillin-resistant, transformed cell.
(1) Calcium treatment + heat shock for bacteria, (2) micro-injection into the nucleus of animal cells, (3) biolistics or gene gun for plant cells, and (4) disarmed pathogen vectors that infect the cell and deliver the DNA.
NCERT gives a very precise sequence (ice → 42°C → ice) and specific method-to-organism pairings. NEET frequently frames one-line and match-the-column questions on transformation and these methods, as seen in the ReNEET 2026 paper.
