Once viruses have attached, they penetrate across the host cell's
plasma membrane by either endocytosis or membrane fusion. In endocytosis,
viruses enter the cytoplasm within endosomes derived from the
invagination of the plasma membrane. Alowering of pH in endosomes is frequently the trigger that leads to capsid disintegration and nucleic acid
release, a process known as uncoating. The genomes of most DNA viruses
find their way to the nucleus by an unknown mechanism. In the nucleus,
the genome is replicated. The RNA viruses generally replicate in
the cytoplasm, but there are exceptions including the unique case of retroviruses,
|Figure 10-1 Enveloped viral infection of a eukaryotic cell.
Once the viral nucleic acid reaches the appropriate compartment, it
is transcribed and the transcripts are translated. This usually happens before
nucleic acid replication is initiated, since some of the proteins
synthesized early in infection play a role in viral nucleic acid replication.
Following genome replication, the nucleic acid becomes packaged, the
virions are assembled, and then release occurs. In nonenveloped viruses,
accumulation of virus particles in the cytoplasm causes the host cell to
rupture. Assembly of enveloped viruses is closely tied to their release.
Nucleocapsids binding to viral proteins in the plasma membrane trigger
budding, resulting in enveloped virions.
The replication of the nucleic acid is extremely diverse, however,
some general features can be outlined. Single-stranded +RNA viruses,
such as picornviruses and togaviruses, maybe directly translated. Aviralencoded
RNA-dependent RNApolymerase is produced early on and catalyzes
the transcription of intermediate complementary -RNA strands.
The minus strands then serve as templates for the synthesis of genomic
Retroviruses are an unusual group of +RNAviruses in that they synthesize
new +RNA using a DNA template. The plus genome is converted
in a step-wise manner to a dsDNA molecule by reverse transcriptase,
which is carried by the virus. Asecond enzyme, ribonuclease H, digests
the RNA in the intermediate RNA-DNA hybrid. Reverse transcriptase
subsequently synthesizes a DNA strand complementary to the first and
the resulting dsDNA integrates into the host genome.
Single-stranded -RNA viruses are those with nucleic acids (genomes)
that cannot be translated. The -RNA strand serves as a template for the synthesis of +RNA that functions as mRNA. Viral proteins translated
from the mRNApromote the synthesis of full-length +RNAstrands
that function as templates for the synthesis of full-length -RNA genome
strands. Usually the virus brings into the cell an RNA-dependent RNA
polymerase for making viral mRNA.
Examples of - RNA viral diseases:
Double-stranded RNA viruses, such as reoviruses, are segmented
and carry a dsRNA-dependent RNA polymerase which they use to transcribe
their genomes. The RNAsegments and the mRNAmolecules specified
by them only encode for one protein each. These viruses replicate
their dsRNAgenomes by producing many copies of plus strand RNAthat
are not translated, but serve as templates for the synthesis of complementary
Single-stranded DNA viruses are unusual in that that may have either
sense or antisense DNA . Sense DNA can serve as a template for
mRNAsynthesis, whereas antisense DNA is unable to function as such a
template. Sense DNA can be transcribed immediately, but antisense DNA
must serve as the template for the synthesis of the sense strand.
The dsDNA viruses can be divided into two categories: those that
replicate their DNA in the host's nucleus and those that replicate entirely
in the host's cytoplasm (the poxviruses). Although there are differences
in the processes of replication, generally, the viral DNA has to be transcribed
and viral proteins synthesized for DNA replication to occur.
Examples of dsDNA viruses:
HIV, which causes AIDS, is a retrovirus.