Biology, Eighth Edition (Raven)

Chapter 27: Viruses

Replication Cycle of a Retrovirus

Why do some viruses contain DNA and others contain RNA? The origin of viruses is unclear. As all known viruses require a cellular organism for reproduction they cannot persist on their own. Viruses may be organisms that have evolved the loss of their cellular structure over time after becoming intracellular parasites. Alternatively viruses may have begun as parasitic DNA within the genome of organisms and then evolved an ability to independently move between cells. In either case, the diversity of forms of viruses and virus-like structures indicates that viruses have probably evolved multiple times in the history of the earth.

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


1.	Retroviruses have

		A)	one copy of single-stranded RNA
		B)	two copies of single-stranded RNA
		C)	one copy of double-stranded RNA
		D)	two copies of single-stranded DNA

2.	The provirus state exists when

		A)	double-stranded viral DNA is integrated into host cell chromosome
		B)	single-stranded viral RNA is integrated into host cell chromosome
		C)	double-stranded viral RNA is integrated into host cell chromosome
		D)	single-stranded RNA is present as a plasmid in the host cell
		E)	double-stranded DNA is present as a plasmid in the host cell

3.	If the DNA of the provirus is transcribed

		A)	several single mRNA strands are produced
		B)	a plasmid is made
		C)	the provirus detaches and becomes lytic
		D)	one long polygenic mRNA is produced

4.	The polygenic mRNA codes for making one long polyprotein which is then cleaved into the separate individual virus proteins.

		A)	True
		B)	False

5.	If the virus is present as a provirus in the host cell, it remains latent and never produces new virus.

		A)	True
		B)	False