Essential Cell Biology 5th edition

318 CHAPTER 9 How Genes and Genomes Evolve
TABLE 9–1 VIRUSES THAT CAUSE HUMAN DISEASE
Virus Genome Type Disease
Herpes simplex virus double-stranded DNA recurrent cold sores
Epstein–Barr virus (EBV) double-stranded DNA infectious mononucleosis
Varicella-zoster virus double-stranded DNA chickenpox and shingles
Smallpox virus double-stranded DNA smallpox
Hepatitis B virus
Human immunodeficiency
virus (HIV)
part single-, part
double-stranded DNA
single-stranded RNA
serum hepatitis
acquired immune
deficiency syndrome
(AIDS)
Influenza virus type A single-stranded RNA respiratory disease (flu)
Poliovirus single-stranded RNA poliomyelitis
Rhinovirus single-stranded RNA common cold
Hepatitis A virus single-stranded RNA infectious hepatitis
Hepatitis C virus single-stranded RNA non-A, non-B type
hepatitis
Yellow fever virus single-stranded RNA yellow fever
Rabies virus single-stranded RNA rabies encephalitis
Mumps virus single-stranded RNA mumps
Measles virus single-stranded RNA measles
QUESTION 9–5
Discuss the following statement:
“Viruses exist in the twilight zone
of life: outside cells they are simply
dead assemblies of molecules; inside
cells, however, they are alive.”
Most viruses that cause human disease have genomes made of either
double-stranded DNA or single-stranded RNA (Table 9−1). However,
viral genomes composed of single-stranded DNA and of double-stranded
RNA are also known. The simplest viruses found in nature have a small
genome, composed of as few as three genes, enclosed by a protein coat
built from many copies of a single polypeptide chain. More complex
viruses have larger genomes of up to several hundred genes, surrounded
by an elaborate shell composed of many different proteins (Figure 9−28).
The amount of genetic material that can be packaged inside a viral protein
shell is limited. Because these shells are too small to encase the
genes needed to encode the many enzymes and other proteins that are
required to replicate even the simplest virus, viruses must hijack their
host’s biochemical machinery to reproduce themselves (Figure 9−29). A
viral genome will typically encode both viral coat proteins and proteins
that help the virus to commandeer the host enzymes needed to replicate
its genetic material.
Retroviruses Reverse the Normal Flow of Genetic
Information
Although there are many similarities between bacterial and eukaryotic
viruses, one important class of viruses—the retroviruses—is found only
in eukaryotic cells. In many respects, retroviruses resemble the retrotransposons
we just discussed. A key feature of the replication cycle of
both is a step in which DNA is synthesized using RNA as a template—
hence the prefix retro, which refers to the reversal of the usual flow of
information from DNA to RNA. Retroviruses are thought to have derived
from a retrotransposon that long ago acquired additional genes encoding