Choose the correct statements about making bacterial cells competent to take up recombinant DNA:
A. Since DNA is a hydrophilic molecule, it cannot pass through cell membranes.
B. Bacterial cells are made competent by treating them with a specific concentration of a monovalent cation such as sodium.
C. Recombinant DNA is forced into competent cells by incubating them with the DNA on ice, placing them briefly at a higher temperature and then putting them back on ice.
D. The brief heat shock is given before the cells are incubated with the recombinant DNA on ice.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → A and C only
DNA cannot cross cell membranes on its own, so bacterial cells are first made ‘competent’ to take it up. They are treated with a divalent cation such as calcium, then kept with the recombinant DNA on ice, given a brief heat shock and put back on ice.
A. Since DNA is a hydrophilic molecule, it cannot pass through cell membranes. — True. This is why the cells have to be made competent first.
B. Bacterial cells are made competent by treating them with a specific concentration of a monovalent cation such as sodium. — False. The treatment uses a divalent cation such as calcium.
C. Recombinant DNA is forced into competent cells by incubating them with the DNA on ice, placing them briefly at a higher temperature and then putting them back on ice. — True. This ice, heat shock, ice sequence lets the bacteria take up the recombinant DNA.
D. The brief heat shock is given before the cells are incubated with the recombinant DNA on ice. — False. The order is reversed. The cells are first incubated with the DNA on ice, and the heat shock comes after this.
So only statements A and C are correct.
NCERT Extract:
Competent Host (For Transformation with Recombinant DNA) – “Since DNA is a hydrophilic molecule, it cannot pass through cell membranes. Why? In order to force bacteria to take up the plasmid, the bacterial cells must first be made ‘competent’ to take up DNA. This is done by treating them with a specific concentration of a divalent cation, such as calcium, which increases the efficiency with which DNA enters the bacterium through pores in its cell wall. Recombinant DNA can then be 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.”