Viruses Flashcards
What is a virus?
a virus is a
- sub microscopic entity consisting of a single nucleic (RNA or DNA) acid surrounded by a protein coat (capsid) - - it is capable of replication only within the living cells of bacteria, animals or plants = requires living host cells, are inert outside of the host cell
- carries only one or two enzymes which decode genetic instructions
How can viruses be classified?
morphology
- size and shape
chemical composition and structure of genome
mode of replication
What are the properties of viruses?
viruses are obligate intracellular parasites
- cannot reproduce outside of their host cell
during replication, they fully depend on biochemical machinery of host cell = hijack the host cell
their main purpose is delivery of genome into host cell to allow its expression (transcription and translation) by the host cell.
What is a virion?
vision - a fully assembled infectious virus
- simplest virion consists of nucleic acid and a capsid (protein coat)
What is the structure of a virus?
nucleic acid
- RNA or DNA
capsid = protein coat
envelope = outer membrane layer
- derived from the host cell
- has both host and viral specific antigens
- consists of a lipid bilayer = exterior consists of glycoslylated membrane proteins
spikes = glycoslyated proteins
- remain exposed in the exterior of the lipid bilayer
- essential for attachment to the host cell = has antigens which are host and viral specific
What are the types of viral morphology (size and shape)?
helical
- identical protein subunits self-assemble into helical array surrounding nucleic acid (spiral path)
icosahedral
- polyhedron have 20 equilateral triangular faces and 12 vertices
What is the mechanism for viral pathogenesis?
pathogenic mechanisms include
- implantation of virus at entry site
- local replication
- spread to target organs
- shedding of virus into environment
What are the factors affecting viral pathogenesis?
factors that affect pathogenic mechanisms are
- accessibility of virus to tissue
- cell susceptibility to virus multiplication
- virus susceptibility to host defences
How does a virus replicate?
1 - adsorption
- virus attaches to the host cell
- specific binding of glycosylated proteins (spikes) to cell receptors
2 - penetration
- virus is engulfed into a vesicle and enters the cell
3 - uncoating
- uncoating of the envelope and capsid
- releases the nucleic acid into the cell cytoplasm
4 - synthesis
- virus takes over the biochemical machinery of the host cell = produces RNA molecules, capsomers, spikes
- replication and protein production
5 - assembly
- new particles assemble into new viruses
- viral glycosylated proteins are inserted into the cell membrane for the viral envelope
- nucleocapsid forms from RNA and capsomers
6 - release
- enveloped viruses bud off the membrane carrying away an envelope with spikes
- the virion/virus is ready to infect another cell
What are the different types of vaccines? What cell has been used for its production?
chicken embryo eggs
- measles virus
- yellow fever virus
- mumps virus
human embryo eggs
- varicella-costa virus
monkey kidney cells
- polio virus
- german measles virus
What are different types viral diseases? What type of infection do they cause?
herpes viruses = Varicella-Zosta
- Chicken pox
herpes virus = herpes simplex
- cold sores
human papilloma virus
- genital warts
paramyxoviruses
- measles
retrovirus
- HIV
hepatitis viruses
- hepatitis B
What is the type of treatment available for viral diseases?
antiviral chemotherapy is divided into
- virucidal agents = directly inactivate viruses
- antiviral agents = inhibit viral replication at cellular level
- immunomodulators = alter host immune response
antiviral chemotherapeutic agents can affect viruses at different points in their replicative cycle
antibiotics cannot be used for viral infections
What are the modes of action for antiviral drugs?
fusion inhibitors
- prevent the virus from attaching to the host cell
- prevent the virus from uncoating (envelope and capsid)
= amantidine
ion channel inhibitors
- blocking viral cellular membrane H+ channels
- prevents the virus from uncoating (envelope and capsid)
= amantidine
polymerase inhibitors
- prevent replication of the genome
- antiviral nucleotide analogues interfere with nucleic acid replication, competes with NTP substrates
= ribavarin
blocking/inhibiting reverse transcriptase
- preventing change from ssRNA to dsDNA
protease inhibitors
- prevent the assembly of virus = virus is easily destroyed/inactive
neuroaminimidase inhibitors
- prevent the release of the virus from the host cell
- drugs are sialic acid analogues - Oseltamivir phosphate (Tamiflu)
What are the types of drug and their mode of action?
amantidine
- target is penetration, uncoating and assembly
- effective against influenza A
acyclovir
- DNA chain termination
- effective against herpes simplex
ribavirin
- inhibition of the 5’apping of viral mRNA
- effective against influenza A, B, C and measles
zidovudine
- inhibition of reverse transcriptase
- effective against HIV
How do nucleotide analogues inhibit viral replication?
selective inhibition of virus DNA replication
- competitive inhibition of viral DNA polymerase utilization of GTP
- viral chain termination (guanosine analogue substitute)
- inactivation of DNA polymerase
