Competent Host
திறனுள்ள புரவலன்
DNA is a hydrophilic molecule. It cannot pass through the lipid bilayer of a bacterial cell membrane. If you don't understand the physics of cell competence, you are wasting your time on recombinant DNA technology.
The idea
Bacterial cells are made competent by treatment with specific divalent cations, typically calcium. This increases the efficiency of DNA uptake through cell wall pores. The recombinant DNA is then introduced into these cells by incubating them on ice, followed by a heat shock at 42°C, and placing them back on ice. This thermal imbalance facilitates the entry of the DNA molecule.
Why it matters
DNA is a hydrophilic molecule and cannot pass through the hydrophobic cell membrane; competence forces the cell to accept the foreign DNA.
NEET lens
NEET focuses on the specific ion used (Calcium) and the exact temperature for heat shock (42°C). Do not confuse divalent cations with monovalent ones. Expect questions on the sequence of steps: Ice-Heat-Ice. Memorize these values; they are high-frequency data points.
Clinical significance
In clinical microbiology, this mechanism is the foundation for producing recombinant insulin and vaccines. If competence fails, the host cell cannot express the transgene, rendering the entire therapeutic production process useless. Precision in this step determines the yield of life-saving proteins.
Introduction
Since DNA is a hydrophilic molecule, it cannot pass through cell membranes. Bacterial cells must first be made 'competent' to take up DNA.
Core concept
Methods (NCERT): treating cells with a specific concentration of a divalent cation, such as calcium, increases the efficiency with which DNA enters the bacterium through pores in its cell wall. Recombinant DNA is then forced into such cells by incubating the cells with recombinant DNA on ice, followed by placing them briefly at 42°C (heat shock), and then putting them back on ice. Other methods: micro-injection — recombinant DNA is directly injected into the nucleus of an animal cell; biolistics (gene gun) — plant cells are bombarded with high-velocity micro-particles of gold or tungsten coated with DNA; 'disarmed pathogen' vectors — e.g. Agrobacterium or retroviruses which, when allowed to infect the cell, transfer the recombinant DNA into the host.
Go deeper
Heat shock briefly opens up the membrane and helps the DNA enter cells treated with calcium chloride.
From Class 10
Bacteria normally keep foreign DNA out. A quick calcium and heat-shock treatment opens a door for the new DNA to get in.