Competent cells: why calcium chloride, then heat shock at 42 degrees, then ice? And which gene transfer method goes with which cell?
Ishita GoyalDropper Sample
17 Sept 2026 · 1 min read
Planner update: the "gene transfer" page has been sitting in red in my tracker for three days. I'd like it to turn green 😅
From NCERT: DNA is a hydrophilic molecule, so it can't pass through cell membranes. To make bacterial cells competent to take up DNA we treat them with a specific concentration of a divalent cation such as calcium, which increases the efficiency of DNA entering through pores in the cell wall. Then the cells are incubated with the recombinant DNA on ice, followed by placing them briefly at C (heat shock), and then putting them back on ice.
What I don't get:
- Why that specific sequence of ice, heat, ice? What is the heat shock doing? And would any other cation work as well?
- NCERT lists other methods: microinjection, biolistics (gene gun), and "disarmed pathogens" as vectors. I can't keep the pairing of method and cell type straight: which is for plants, which for animals?
- What does "disarmed" actually mean? Is the vector pathogen modified or is it just a harmless strain?
I'd like to finish this with a clean table for my planner.
6 answers & replies
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[+][–]Rahul DubeyDropperSample· 1mo ago
Calcium (a divalent cation) is the NCERT answer and your sequence is correct: chill with the recombinant DNA on ice, a short heat shock at 42 degrees, back on ice. The commonly given explanation is that Ca2+ reduces the repulsion between the negatively charged DNA and the negatively charged phosphate heads of the membrane so that DNA can stick to the cell surface, and the sudden jump in temperature creates a thermal imbalance that pushes the DNA into the cell. You will not be asked the finer mechanism in NEET; you need the sequence and the word "divalent cation like calcium".
[+][–]Mehak AroraDropperSample· 1mo ago
The pairing table I made after having the same problem:
Method How Cells Microinjection recombinant DNA injected directly into the nucleus animal cell Biolistics / gene gun cells are bombarded with high-velocity micro-particles of gold or tungsten coated with DNA plant cells "Disarmed pathogen" vector the pathogen delivers the gene into the host Agrobacterium for plants, retroviruses for animals Calcium chloride + heat shock competent cells bacteria [+][–]Ishita GoyalDropperSample· 1mo agoAuthor
thank you all, table is going straight into the planner. my tracker is now green 💚
[+][–]Pooja Iyer12thSample· 1mo ago
my version is: microinjection is for plant cells because plants are the tough ones, you have to physically inject! And the gene gun is for animal cells like eggs, since animal cells are soft and you can just shoot them!! logic 🙈
[+][–]Nisha Verma12thSample· 1mo ago
Pooja, it is the other way round, and the logic also flips: plant cells have a rigid cell wall, which is exactly why the gene gun uses high velocity particles of gold or tungsten to get through it. Microinjection is for animal cells, where DNA goes into the nucleus through a very fine needle. Memory hook: gene gun = greens (plants), microinjection = mammal/animal cell.
[+][–]Siddharth RaoDropperSample· 1mo ago
About "disarmed": Agrobacterium tumefaciens is a pathogen of several dicot plants. In nature it delivers a piece of its DNA called T-DNA into plant cells and turns them into tumour cells that make chemicals the pathogen needs. Similarly, retroviruses can transform normal animal cells into cancerous cells. Researchers took advantage of this delivery trick, then removed the disease-causing (oncogenic) genes, and put the gene of interest in their place. So yes, the vector is modified, and the pathogen is "disarmed": it still delivers DNA but can no longer cause tumours.
