PRINCIPLES OF MODERN MICROBIOLOGY
Mark Wheelis
ANSWERS TO STUDYQUESTIONS
Chapter 6
Viruses and Other Acellular Entities
2. In order to calculate ratios, we have to have volumes in the same units; we could use either
 m
3
or nm
3
; here we'll calculate volumes in
 m
3
.
Volume of a virus:
V = (4/3) (3.1) (2 X 10
-2  m)
3
= 3.3 X 10
-5  m
3
Volume of a bacterium:
V = (4/3) (3.1) (0.5
 m)
3
= 0.51
 m
3
Volume of a yeast cell:
V = (4/3) (3.1) (5
 m)
3
= 510
 m
3
Thus the ratio of the volume of a virus to a bacterial cell is:
(3.3 X 10 -5 ) / (0.51) = 6.5 X 10 -5
And the ratio of the volume of a virus to a yeast cell is:
(3.3 X 10
-5
) / (5.1 X 10
2
) = 6.5 X 10
-8
4.
A typical plant virus enters a host cell through a wound, or is injected by an insect with viruscontaminated mouth parts during feeding. The nucleic acid is then replicated. and viral proteins synthesized. Viral nucleic acid spreads through plasmadesmata to adjacent cells, and virions may spread through the plant's vascular tissue to distant regions. Transmission to new plants requires release of virions and their spread by wind or insects.
6.
Replication of viral chromosomes occurs through any one of several mechanisms, depending on the specific virus and on the nature of its genome. Double-stranded DNA viruses may replicate their
DNA by bidirectional replication, like host cell DNA replication, or they may use rolling circle replication. Some ds-DNA viruses use both mechanisms--bidirectional replication to produce multiple circular chromosomes, then rolling circle replication to produce linear daughter chromosomes.
Single-stranded DNA viruses normally use rolling circle replication.
Single-stranded RNA viruses normally act as mRNA if they are the right sense (plus strand).
Translation of the viral chromosome produces an RNA replicase that makes complementary minus strands, which then act as templates for the production of progeny plus stands.
One group of plus stand ss-RNA viruses are the retroviruses, which include several enzymes in the virion along with their chromosome. One of these enzymes, reverse transcriptase, makes a ds-

DNA copy of the chromosome, which is then integrated into the host chromosome by the second enzyme, integrase. Transcription of the integrated provirus then produces daughter chromosomes.
Both minus-strand ss-RNA viruses and ds-RNA viruses normally include an RNA replicase within the virion, which can produce mRNA from the viral RNA, allowing the synthesis of additional RNA replicase to produce daughter chromosomes.
8 . Virus taxonomy is artificial because to date there is no reliable method of determining relationships among all but the most closely related viruses. This makes a natural, or phylogenetic, classification impossible. This is in contrast to cellular organisms, for which nucleic acid sequences are more informative (presumably because they have changed more slowly, or recombined less often).
10.
Prions, like viruses are infectious. However, this is essentially the only similarity. Prions consist of an aberrant conformation of a normal host protein that can induce the normal form of the protein to shift into the abnormal form, thus replicating.