Those viruses that penetrate via membrane fusion and
endocytosis must then undergo uncoating, either
in the cytoplasm orwhile within the endocytitic vesicle. The host
cell's own enzymes cause this to occur, though the mechanism that triggers
this process is not well understood.
dsDNA Virus Replication
dsDNA forms such as herpes and papilloma viruses use this strategy. Pox viruses replicate in the cytoplasm only, while the genome of hepatitis B is first transcribed to RNA, then the RNA template is used to generate new dsDNA using reverse transcriptase, an enzyme carried by the virus.
ssDNA forms, such as the parvovirus replicate using this strategy.
The replication of the complementary strand occurs in the nucleus, but
the complementary strand is not incorporated into the nucleocapsid during
(+) ssRNA viruses such as polio and rhinoviruses have RNA that can act
immediately as mRNA to synthesize viral proteins. New (+) strands
are synthesized from a (-) strand, then assembled as nucleocapsids prior
viruses such as filoviruses, reoviruses and arenaviruses must first synthesize
a new (+) strand following uncoating. The (+) strand can then be
used to synthesize new complementary (-) strands and as mRNA to synthesize
Release of Animal Viruses
Nonenveloped viruses are released by cell lysis, but enveloped forms are released by budding. Budding occurs when the nucleocapsid migrates to and is surrounded by a portion of host cell membrane from either the nucleus, endoplasmic reticulum or outer membrane. During synthesis, viral protein spikes such as neuraminidase and hemagluttinin are added to the host membrane.
Latency and Persistance in Animal Viruses
Some animal viruses such as Herpes Zoster (chickenpox, shingles), Herpes Simplex I and II and HIV have the ability to remain in the body for extended periods of time without expressing themselves in a lytic cycle. The process, called latency, occurs two ways:
Herpes viruses produce lytic cycles when infecting mucosal or epithelial cells, but have the ability to reside in the ganglia of nervous tissues without killing the cells, migrating to the cells they are specific for after being stimulated by some environmental factor such as exposure to radiation or by lowering of resistance due to another disorder (gastroenteritis, influenza, AIDS).
The HIV virus becomes a provirus
(viral DNA incorporated with a host cell chromosome) and is held inactive
by a repressor protein for long periods of time. Stimulated by environmental
or host stimuli as above, the proviral DNA is used to transcribe viral
RNA, which begins the replication cycle.
Persistant viruses are those that are retained in the body in small numbers that continually replicate. The accumulation of tissue damage over long periods of time plays a major role in pathogenicity. Measles viruses, for example, can persist in nervous tissue for decades, eventually leading to a disorder called subacute schlerosing panencephalitis.