Question

A plasmid carries a cluster of genes that specify ability to conjugate and to replicate, and a cluster of genes that confer resistance to toxic metal ions. After conjugation into a "female" cell, the minimum number of crossing overs between the plasmid donor DNA and the host chromosome required to form recipient cells that stably express resistance to metal ions (is)are:

2

3

0

4

1

Answer 1

Option first that is 2 is the correct answer. A crossing over between plasmid donor DNA and the host chromosome is required to integrate the plasmid DNA into the recepient cell (host chromosome).  A single crossover between the plasmid DNA and host chromosome would produce a single long molecule that would be inviable. Therefore minimum 2 crossover are required between plasmid and recipient DNA to integrate a part of the donor genome into the recipient. Once a successful integration occurs, recipient cells will stably express resistance to metal ions.