Paper 2 · genetic engineering / recombinant DNA technology
A recombinant plasmid carrying an antibiotic resistance gene is used to transform bacterial cells. Which of the following is a standard method for identifying which bacterial colonies have successfully taken up the recombinant plasmid?
AStaining bacterial colonies with Gram stain and selecting only the colonies that stain gram-negative, since this is assumed to indicate plasmid uptake.
BSelecting the fastest-growing, largest colonies on ordinary nutrient agar, since plasmid uptake is assumed to speed up bacterial growth.
CExamining colonies under a light microscope for a visible change in cell shape or size, based on the assumption that taking up any recombinant plasmid physically alters the host bacterium's cell wall.
DCulturing the bacteria on a medium containing the antibiotic to which the plasmid's marker gene confers resistance; only bacteria that took up the plasmid will survive and grow.
Explanation: The recombinant plasmid carries a gene for resistance to a particular antibiotic. If transformed bacteria are grown on a medium containing that antibiotic, only cells that took up the plasmid express the resistance gene and survive, while untransformed cells are killed. Gram stain reaction, colony growth rate or size, and cell shape are not affected by plasmid uptake, so none of them can distinguish transformed from untransformed colonies.
Derived from ZIMSEC Biology Paper 2, November 2008, Q1 (Topic 5 Genetic Engineering)
