Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

1274 Chapter 23: Pathogens and Infection
Figure 23–10 A simple viral life cycle. The hypothetical simple virus shown
here consists of a small double-stranded DNA molecule that codes for
only a single viral capsid protein. To reproduce, the viral genome must first
enter a host cell, where it is replicated to produce multiple copies, which are
transcribed and translated to produce the viral coat protein. The viral genomes
can then assemble spontaneously with the coat protein to form a new virus
particle, which escapes from the host cell. No known virus is this simple.
virus
DNA
coat protein
ENTRY INTO HOST CELL
AND UNCOATING OF DNA
host cell
and (6) release of progeny virions (Figure 23–10). A single virus particle (a virion)
that infects a single host cell can produce thousands of progeny.
Virions come in a wide variety of shapes and sizes (Figure 23–11), and
although most have relatively small genomes, genome size can vary considerably.
The recently discovered giant viruses of amoebae, called pandoraviruses, are the
largest known viruses, with 700 nm particles and double-stranded DNA genomes
of over 2,000,000 nucleotide pairs. The virions of poxvirus are also large: they are
250–350 nm long and enclose a genome of double-stranded DNA of about 270,000
nucleotide pairs. At the other end of the size scale are the virions of parvovirus,
which are less than 30 nm in diameter and have a single-stranded DNA genome
of fewer than 5000 nucleotides.
Viral genomes are packaged in a protein coat, called a capsid, which in some
viruses is further enclosed by a lipid bilayer membrane, or envelope. The capsid is
made of one or several proteins, arranged in regular arrays that generally produce
structures with either helical symmetry, which results in a cylindrical structure
(for example, influenza, measles, and bunyavirus), or icosahedral symmetry (for
example, poliovirus and herpesvirus; see Figure 23–11). Some viruses instead produce
capsids with more complicated structures (for example, poxviruses). When
the capsid is packaged with the viral genome, the structure is called a nucleocapsid.
The nucleocapsids of nonenveloped viruses usually leave an infected cell by
lysing it. For enveloped viruses, by contrast, the nucleocapsid is enclosed within a
lipid bilayer membrane that the virus acquires in the process of budding from the
host-cell plasma membrane, which it does without disrupting the membrane or
killing the cell (Figure 23–12). Enveloped viruses can cause persistent infections
that may last for years, often without noticeable deleterious effects on the host.
Because the host cell performs most of the critical steps in viral replication, the
identification of effective antiviral drugs that do not harm the host can be difficult.
Probably the most effective strategy for containing viral diseases is through vaccinating
of potential hosts. Highly successful vaccination programs have effectively
TRANSCRIPTION
coat
protein
RNA
TRANSLATION
DNA
REPLICATION
ASSEMBLY OF PROGENY
VIRUS PARTICLES AND
EXIT FROM HOST CELL
MBoC6 m24.12/23.10
DNA
poxvirus herpesvirus adenovirus papillomavirus
100 nm
DNA VIRUSES
100 nm
poliovirus
HIV
(AIDS virus)
influenza
virus
coronavirus
(common cold)
RNA VIRUSES
rabies virus
mumps virus
Figure 23–11 Examples of viral
morphology. As shown, both DNA and
RNA viruses vary greatly in both size and
shape.