Assertion (A): Before transformation, bacterial cells are treated with a specific concentration of a divalent cation such as calcium.
Reason (R): Calcium increases the efficiency with which DNA enters the bacterium through pores in its cell wall.
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Both A and R are true, and R is the correct explanation of A.
Assertion (A): Before transformation, bacterial cells are treated with a specific concentration of a divalent cation such as calcium. — True. A is about preparing bacterial cells before they take up recombinant DNA. DNA is a hydrophilic molecule and cannot pass through cell membranes, so the cells must first be made ‘competent’. This is done by treating them with a specific concentration of a divalent cation such as calcium.
Reason (R): Calcium increases the efficiency with which DNA enters the bacterium through pores in its cell wall. — True. R is about what calcium does. Calcium increases the efficiency with which DNA enters the bacterium through pores in its cell wall.
Why R explains A? A tells us that the cells are treated with calcium, and R tells us why. The calcium treatment helps DNA get in through pores in the cell wall, so more cells take up the DNA. So R is the correct explanation of A.
NCERT Extract:
Competent Host (For Transformation with Recombinant DNA) – “Since DNA is a hydrophilic molecule, it cannot pass through cell membranes. … 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.”