What is the main purpose of heat shock?

Correct answer: To help plasmid entry

The direct answer is option B: to help plasmid entry into the host cell. In a common bacterial transformation procedure, competent cells are mixed with plasmid DNA and exposed briefly to heat shock, followed by recovery under suitable conditions. The sudden temperature change can temporarily alter membrane permeability and help plasmid DNA cross the cell envelope. Heat shock is not simply a drying step, and it must be controlled because excessive heat or too long an exposure can damage or kill cells. Option B is correct because the intended purpose is to improve DNA uptake. Option A is wrong because drying the host would damage cells and is not the goal. Option C is wrong because heat shock does not always destroy DNA; the plasmid is intended to enter the cell, though poor conditions can harm DNA or cells. Option D is wrong because bacterial transformation does not make the cell produce chlorophyll. The exact temperature and duration depend on the organism and protocol, and cells are usually allowed to recover before selection. Memory cue: heat shock briefly opens a pathway for plasmid entry; it is not a sterilization treatment.