Which of the following does NOT occur during bacterial conjugation? A. Direct contact between donor and recipient cells B. Shortening of the pilus C. Enzymatic cleavage of one strand at the origin of transfer D. Unidirectional transfer of both DNA strands E. Recipient cell becomes a donor cell after conjugation

The correct answer is D. Unidirectional transfer of both DNA strands.

During bacterial conjugation, only one DNA strand (the “transfer strand”) is typically transferred from the donor to the recipient. The other strand remains in the donor cell and is replicated to form a complete double-stranded DNA molecule. Therefore, both strands are not transferred.

Here’s a brief breakdown of the other options:

• A. Direct contact between donor and recipient cells: This is a fundamental part of conjugation, as the donor and recipient cells must physically connect through a pilus.
• B. Shortening of the pilus: After the initial contact, the pilus contracts to bring the cells closer together for DNA transfer.
• C. Enzymatic cleavage of one strand at the origin of transfer: This step is necessary for the initiation of DNA transfer. The enzyme nicking the DNA strand is crucial for the transfer process.
• E. Recipient cell becomes a donor cell after conjugation: After receiving the transferred DNA, the recipient cell may acquire the plasmid or DNA that allows it to become a donor in future conjugation events.