L3: General Virology Flashcards
What are viruses?
- Viruses are not cells, i.e. they do not have a nucleus or organelles.
- The smallest infectious agents (20 to 300 nm in diameter).
- They are obligate intracellular parasites (they do not have ribosomes and canβt be planted on agar)
- contain one kind of nucleic acid (RNA or DNA)
What is the difference between prokaryotes and acellular organisms?
- Prokaryotes: they do not have a membrane-bound nucleus
- Acellular: do not have a cell membrane
What is a virus particle (virion) composed of?
- Protein coat (capsid)
- Nucleic acid core
- Envelope (in some viruses)
What are the characters of viral capsid?
o It surrounds viral nucleic acid.
o made up of subunits called capsomers.
o Each capsomer is consisting of one or several proteins.
o The capsid with the enclosed nucleic acid is called the nucleocapsid.
What are the functions of the capsid?
a) It protects the viral genome against inactivation by nuclease enzymes.
b) The arrangement of capsomers (symmetry) is either icosahedral, helical, or complex.
c) Participates in adsorption of virions to susceptible cells. It determines the antigenicity
D) has a role in attachment.
What is viral symmetry?
1- Icosahedral (enveloped and non enveloped)
2- Helical (enveloped and non enveloped)
3- Complex (rare)
What are the characters of viral nucleic acid?
ο DNA (single molecule) or RNA (single or segmented).
ο Single or double-stranded (but one molecule)
ο Linear or circular.
ο It is the infectious part of the virus and codes for viral structure and non-structural proteins.
What are the characters of the viral envelopes?
ο Lipoprotein, the lipid from the host cell membranes and protein is virus-specific.
ο Glycoproteins in the form of spike-like projections on the surface attach to the host cell receptors.
ο It determines viral antigenicity and specificity
What determines the antigenicity of the virus?
The envelope and the capsid
What are the spikes of HIV and the receptor it works on respectively?
Gp120 and cd4
What are the steps of the viral replication cycle?
1-Attachment (adsorption by glycoprotein spikes)
2-Penetration
3-Uncoating
4-Viral gene expression (transcription) and protein synthesis (translation for the synthesis of coat)
5-Viral nucleic acid synthesis (replication for the synthesis of more versions)
6. Assembly (reunion)
7. Release
Attachment step of virus
ο Attachment of the virus to the host cell
ο It is receptor-specific
Penetration step of the virus
ο In non enveloped viruses penetration occurs by crossing the plasma membrane directly or by receptor-mediated endocytosis.
ο In enveloped viruses penetration occurs by fusion of viral envelope with cell membrane or with the membrane of endosome at the cell surface
Uncoating step of the virus
ο It is the release of viral nucleic acid by cellular enzymes.
ο Uncoating renders viral nucleic acid accessible for transcription and replication.
Viral nucleic acid synthesis (replication) step of the virus
by using a strand of the parental nucleic acid as a template for the production of progeny DNA or RNA molecules.
Assembly step of the virus
Assembly of viral nucleic acid and protein coats to form mature virus particles.
Release step of the virus
Virus particles are released from the cell either by:
ο Budding through the outer cell membrane as in enveloped viruses (taking a part of the bilayer)
ο Rupture of the cell membrane and release of the mature particles in unenveloped viruses.
How are viruses laboratorally detected?
Direct or indirect (virus isolation) and serologic detection of antiviral antibodies
How are viruses directly detected in the laboratory?
1) Detection of Virus particles by (electron microscope), inclusion bodies by (LM) (in cytoplasm and nucleus)
2) Detection of viral antigens by (EIA, RIAβ¦)
3) Detection of the viral nucleic acid by (PCR) and other
molecular techniques.
How are viruses indirectly detected in the laboratory?
-Viruses are obligate intracellular parasites and can be cultivated on:
a) Tissue cultures:
ο Pieces of animal or human tissues are trypsinized to get separate cells.
ο Cells are grown in media containing amino acids, vitamins, calf serum, and antibiotics.
ο A monolayer or sheet of cells is formed on the flat side of the container within a few days. Viruses are inoculated on the monolayer (we notice the change on the cells done by the virus)
b) Embryonated egg.
c) Animal inoculation.
How are viruses serological detected in the lab?
By serological methods (ELISA, RIAβ¦).
How is the prevention of viruses?
Vaccination and public health measures.
How are viruses treated?
ο Antiviral drugs are medicines that cure or control virus infections.
ο antiviral agents tend to be narrow in the spectrum and have limited efficacy. (Unlike antibiotics)
What are the types of antiviral drugs?
1) Inhibitors of Herpesviruses
2) Inhibitors of Retroviruses
3) Inhibitors of other viruses
4) Interferon
What are examples of Inhibitors of Herpes viruses?
Nucleoside analogue : inhibits virus-specific DNA polymerase e.g. Acyclovir, Ganciclovir, and rhindisevir (for covid)
What are examples of Inhibitors of Retroviruses?
- Reverse transcriptase inhibitors
- protease inhibitors
- fuzeon
- integrate inhibitors
What is the function of reverse transcriptase inhibitors?
- inhibit reverse transcriptase of HIV
- e.g. Azidothymidine (AZT), dideoxyinosine and bcavar sulfate
What is the function of protease inhibitors
- saquinavir and indinavir for treatment of HIV
What is the action of fuzeon?
- blocks the viral and cellular membrane fusion step involved in the entry of HIV into the cell
What is the action of integrase inhibitors?
- inhibit integrase enzyme of the virus
Give examples for inhibitors of other viruses
- amantadine
- zanamivir and oseltamivir
- ribavirin
What is the function of amantadine?
inhibits Influenza A virus uncoating
What is the function of zanamivir and oseltamivir?
inhibit viral neuraminidase of influenza A and B viruses (inhibit viral release from an infected cell to other cells
What is the function of ribavirin?
inhibits both DNA and RNA polymerase enzymes. Used for treatment of HBV, HCV infections, and RSV pneumonitis
How are interferons produced?
Infection of cells with viruses induces the production of proteins that are known as interferons because they were found to interfere with viral replication in previously uninfected tissue culture cells.
What are the types of interferons?
There are 3 types of interferons:
- interferon-Ξ± (IFN-Ξ±)
- interferon -Ξ² (IFN-Ξ²),
- interferon-Ξ³ (IFN-Ξ³), which is induced by activated T cells.
What is the mechanism of action of interferons?
- IFN-Ξ± and IFN-Ξ² are secreted by the infected cell and then bind to a common cell-surface receptor, known as the interferon receptor, on both the infected cell and nearby cells.
- Interferon induces the synthesis of several host cell proteins e.g. RNA dependent protein kinase (PKR) that contributes to the inhibition of viral replication
Give example for DNA enveloped viruses
Herpesviruses
Hepatitis
Pox
Give examples for DNA non-enveloped viruses
Adenovirus
Parvovirus
Give example for RNA enveloped viruses.
Influenza
Corona
Give example for RNA non-enveloped viruses
Polioviruses
Reoviruses
All DNA viruses are DS except parvovirus.
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All RNA viruses are single-stranded except Reoviruses
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What is the function of drugs that are similar to adenine, guanine, or others in the nucleic acid chain?
Polymerase incorporate them instead of the nucleic acid bases so inhibition of polymerase happens
What is the function of interferons?
They stimulate infected cells and nearby cells to produce enzymes as protein kinase which compete with IF2 at the ribosome so it decreases protein synthesis of the virus and replication
