Viruses Flashcards
Major phases of the viral growth and replication cycle
- Attachment
- Penetration
- Uncoating
- Synthetic period
- Maturation and release
Attachment
First step in adsorption (adhesion to a surface).
The initial attachment of a virus particle to a host cell involving an interaction between specific molecular structures on the virus surface and receptor molecules on the host cell.
a. Specialized attachment structures like glycoprotein spikes found on viral envelopes.
b. Unique capsid protein folding forming attachment sites
Penetration
Passage of the virion from the surface of the cell across the cell membrane and into the cytoplasm.
a. Receptor-mediated endocytosis
b. Direct membrane fusion
Uncoating
Stepwise process of disassembly of the virion that enables the expression of the viral genes that carry out replication. For enveloped viruses, the cell penetration is the first step in uncoating. Full uncoating depends on cellular enzymes or in some cases of more complex viruses, requires additional synthesized viral proteins to complete the process.
Synthetic period
Mechanisms of Viral replication dependent on the type of genetic material. Types include: ssDNA (positive vs negative strand), dsDNA, ssRNA, dsRNA.
(ds = double stranded)
(ss = single stranded)
Maturation and release
Nucleocapsids are assembled in the host cell where the viral nucleic acid replication occurs (cytoplasm for most RNA, nucleus for most DNA viruses).
Hepadnaviruses
Cause liver infections, think Hepatitis B.
Genome is DNA (short strand/long strand).
Transcribed by host cell to an RNA intermediate,
then is converted back to DNA by viral reverse transcriptase.
mRNA is transcribed from DNA by host cell enzyme.
Single-stranded RNA viruses
(+) strand:
Same orientation as mRNA.
Translated directly by host ribosomes to form viral proteins: includes code to make RNA-dependent RNA polymerase for RNA replication
(-) strand:
Complementary to mRNA, cannot be directly translated;
Viral RNA-dependent RNA polymerase within virus synthesizes (+) strand/mRNA
Retroviruses
Genome is ssRNA.
Use viral RNA-dependent DNA polymerase or (“reverse transcriptase”) to convert viral (+) strand ssRNA to dsDNA.
dsDNA is incorporated into host cell DNA by viral enzyme “integrase.”
Once in dsDNA form, the rest of transcription and translation is carried out by host cell.
Double-stranded RNA viruses
Genome is dsRNA.
REQUIRES viral RNA-dependent RNA polymerase (host cell does NOT have this enzyme).
Transcribes RNA to mRNA; host cell takes it from there.
DNA viruses
(Note: there are single and double stranded DNA viruses, but both use cell’s machinery to carry out Central Dogma).
Genome is DNA.
Use host cell’s enzymes to create mRNA; host cells take it from there.
Central Dogma of Molecular Biology
DNA makes RNA; RNA makes protein.
Naken (un-enveloped) viruses
Virion is complete once capsids are formed. Release of progeny is usually a passive event resulting from the disintegration of the dying cell.
Enveloped viruses
Virus-specific glycoproteins are synthesized and transported to the host cell membrane which binds the nucleocapsid to the plasma membrane. The host membrane envelopes the nucleocapsid in a process called “budding” and releases it from the cell.
Adenovirus
Non-enveloped; dsDNA genome.
Wide range of illnesses, from mild respiratory infections in young children to life-threatening multi-organ disease in people with a weakened immune system.
Unusually stable to chemical or physical agents and adverse pH conditions, allowing for prolonged survival outside of the body and water. Adenoviruses are spread primarily via respiratory droplets, however they can also be spread by fecal routes.
Diagnosis: Antigen detection, polymerase chain reaction assay, virus isolation, and serology can be used to identify adenovirus infections. Since adenovirus can be excreted for prolonged periods, the presence of virus does not necessarily mean it is associated with disease.
Prevention: No longer vaccinating for it (stopped in the 1990s). Prevent it with sterile habits and chlorinating pools.
Treatment: Usually mild, require no therapy or only symptomatic treatment.
