Essential Cell Biology 5th edition

Mobile Genetic Elements and Viruses
319
double-stranded DNA
poxvirus herpesvirus adenovirus papillomavirus
DNA VIRUSES
Figure 9−28 Viruses come in different
shapes and sizes. Some of the viruses
are shown in cross section (such as
poxvirus and HIV). For others, the outer
structure is emphasized. Some viruses
(such as papilloma and polio) contain an
outer surface that is composed solely
of viral-encoded proteins. Others (such
as poxvirus and HIV) bear a lipid-bilayer
envelope (gray) in which viral-encoded
proteins are embedded.
100 nm
single-stranded RNA
poliovirus
HIV
(AIDS virus)
influenza
virus
coronavirus
(common cold)
rabies virus
mumps virus
RNA VIRUSES
the coat proteins and other proteins required to make a virus particle.
The RNA stage of its replicative cycle could then be packaged into a viral
particle that could leave the cell.
Like retrotransposons, retroviruses use the enzyme reverse transcriptase
to convert RNA into DNA. The enzyme is encoded by the retroviral
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genome, and a few molecules of the enzyme are packaged along with
the RNA genome in each virus particle. When the single-stranded RNA
genome of the retrovirus enters a cell, the reverse transcriptase brought
in with it makes a complementary DNA strand to form a DNA/RNA hybrid
double helix. The RNA strand is removed, and the reverse transcriptase
(which can use either DNA or RNA as a template) now synthesizes a
complementary DNA strand to produce a DNA double helix. This DNA
is then inserted, or integrated, into a randomly selected site in the host
genome by a virally encoded integrase enzyme. In this integrated state,
the virus is latent: each time the host cell divides, it passes on a copy of
the integrated viral genome, which is known as a provirus, to its progeny
cells.
The next step in the replication of a retrovirus—which can take place
long after its integration into the host genome—is the copying of the
integrated viral DNA into RNA by a host-cell RNA polymerase, which produces
large numbers of single-stranded RNAs identical to the original
infecting genome. These viral RNAs are then translated by the host-cell
ribosomes to produce the viral shell proteins, the envelope proteins, and
reverse transcriptase—all of which are assembled with the RNA genome
into new virus particles. The steps involved in the integration and replication
of a retrovirus are shown in Figure 9−30.
virus
DNA
coat protein
ENTRY OF DNA INTO CELL
TRANSCRIPTION
TRANSLATION
coat protein
DNA
REPLICATION
RNA
DNA
ASSEMBLY OF PROGENY
VIRUS PARTICLES
cell
Figure 9−29 Viruses commandeer the host cell’s molecular
machinery to reproduce. The hypothetical virus illustrated here
consists of a small, double-stranded DNA molecule that encodes just a
single type of viral coat 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 new virus particles, which escape from the cell
by lysing it.
EXIT
FROM CELL
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