Biology, Eighth Edition (Raven)

Chapter 28: Prokaryotes

Integration and Excision of a Plasmid

Is integration or independent existence of a plasmid better for the cell? For the plasmid? Plasmids can exist independently of the chromosome or they can become integrated into the chromosome. In either case any genes on the plasmid will be expressed. Plasmids often contain genes that benefit the host cell. These genes are equally useful to the cell no matter where the plasmid is. In contrast, plasmids that are integrated into the chromosome can only be passed on to daughter cells during cell division. Integration into the chromosome disrupts conjugation so that only part of the plasmid is transferred. It is evolutionarily advantageous to the plasmid to remain independent.

View the animation below, then complete the quiz to test your knowledge of the concept.


1.	Which of the following statements best describes plasmid excision?

		A)	breakdown of the plasmid
		B)	joining of the plasmid to the bacterial chromosome
		C)	removal of the plasmid from the bacterial chromosome
		D)	entry of the plasmid into the bacterial cell
		E)	exit of the plasmid from the bacterial cell

2.	Which of the following chromosomal regions can form the largest single transient loop? (Each letters represents a nucleotide sequence)

		A)	A B C D
		B)	B C A D D’ C’ B’
		C)	B C D B’ C’ D’
		D)	B C D A B’ C’ D’
		E)	B A B’ C D D’ C’

3.	In this animation plasmid excision occurs by ________ recombination.

		A)	homologous
		B)	non-homologous
		C)	heterologous
		D)	constitutive
		E)	integrative

4.	Plasmid integration into the bacterial genome typically occurs via a single cross-over between the two molecules.

		A)	True
		B)	False

5.	Excision of a plasmid from a bacterial genome occurs by only one mechanism.

		A)	True
		B)	False