Viruses

Question 1

What is a virus?

Answer 1

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

Question 2

How can viruses be classified?

Answer 2

morphology
- size and shape

chemical composition and structure of genome

mode of replication

Question 3

What are the properties of viruses?

Answer 3

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.

Question 4

What is a virion?

Answer 4

vision - a fully assembled infectious virus

- simplest virion consists of nucleic acid and a capsid (protein coat)

Question 5

What is the structure of a virus?

Answer 5

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

Question 6

What are the types of viral morphology (size and shape)?

Answer 6

helical
- identical protein subunits self-assemble into helical array surrounding nucleic acid (spiral path)

icosahedral
- polyhedron have 20 equilateral triangular faces and 12 vertices

Question 7

What is the mechanism for viral pathogenesis?

Answer 7

pathogenic mechanisms include

• implantation of virus at entry site
• local replication
• spread to target organs
• shedding of virus into environment

Question 8

What are the factors affecting viral pathogenesis?

Answer 8

factors that affect pathogenic mechanisms are

• accessibility of virus to tissue
• cell susceptibility to virus multiplication
• virus susceptibility to host defences

Question 9

How does a virus replicate?

Answer 9

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

Question 10

What are the different types of vaccines? What cell has been used for its production?

Answer 10

chicken embryo eggs

• measles virus
• yellow fever virus
• mumps virus

human embryo eggs
- varicella-costa virus

monkey kidney cells

• polio virus
• german measles virus

Question 11

What are different types viral diseases? What type of infection do they cause?

Answer 11

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

Question 12

What is the type of treatment available for viral diseases?

Answer 12

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

Question 13

What are the modes of action for antiviral drugs?

Answer 13

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)

Question 14

What are the types of drug and their mode of action?

Answer 14

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

Question 15

How do nucleotide analogues inhibit viral replication?

Answer 15

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

Question 16

What is swine flu? What is its properties?

Answer 16

viral disease

• transmission by indirect, direct contact, inhalation of infectious droplets
• incubation is 1-4 days
• children, patients with lower RITs, elderly and immunocompromised patients - infectious for up to 10 days but normal is 1-7 days

symptoms
- fever, cough, sore throat, diarrhoea & vomiting, myalgia and joint pain

Question 17

What is the treatment for swine flue? What are complications that can occur with swine flu?

Answer 17

oseltamivir

• is a prodrug hydrolysed in liver
• neuraminidase inhibitor = prevents new virus release from infected cells

zanamivir

• administered by inhalation
• action similar to oseltamivir

pneumonia (lower respiratory tract infections) and dehydration could occur
death is usually associated with respiratory failure
- resulting from severe pneumonia and acute respiratory syndrome
renal or multi-organ failure is high