Lecture 23: Viruses

Question 1

What are viruses?

Answer 1

“Sub-microscopic particle (20-300nm) that can infect cells of biological organism”

Obligate intracellular parasites:
   1. cannot carry out any metabolic pathway
   2. Neither grow or respond to 
        environment
   3. Cannot reproduce independently

Question 2

What are the two forms that viruses can take on?

Answer 2

1. Extracellular:
   a. Virion
   b. Core nucleic acid (DNA or RNA)
   c. Protein coat: “capsid”
   d. Envelope (some virions):
   phospholipid membrane; allowing
   interaction with target cells
2. Intracellular:
   a. Virus exists as the nucleic acid
   (episomal or integrated)

Question 3

What dictates a viruses host cell?

Answer 3

1. Most viruses infect a particular type of
   cell. E.g., HIV & CD4 positive cells on
   surface of T cells
2. Specific affinity of viral surface
   proteins/glycoproteins to complementary
   proteins/glycoproteins on host surface
3. Few are “generalists” - E.g., Rabies

Question 4

What are the 5 stages of viral replication?

Answer 4

1. Attachment - chemical attraction
2. Entry - direct penetration (“poliovirus”),
   membrane fusion (“measles and
   mumps”), phagocytosis (“herpes”),
   uncoating of capsid may be involved
3. Synthesis - different viruses (ss/ds
   DNA/RNA) have different strategies
4. Assembly - DNA viruses assembled in
   nucleus, RNA viruses in cytoplasm
5. Release - enveloped viruses released
   by budding, naked viruses via
   exocytosis of cell lysis

Question 5

What viruses have we eradicated? Talk about them.

Answer 5

1. Small pox (“poxviridae”)
   a. middle ages (80% of European
   population go it)
   b. 18th century; was introduced to native
   Americans - up to 3.5 million died
   c. Edward Jenner showed immunization
   using mild cowpox
   d. Eradication possible due to:
   inexpensive & stable vaccine,
   specificity to infection, quick+obvious
   signs of infection (allowed
   quarantine), lack of carriers, close
   contact spreading
   e. CAUSED BY TWO VIRUSES: Variola major & Variola minor

2.

Question 6

What is polio?

Answer 6

1. ALMOST eradicated
2. ssRNA (acts as mRNA)
   3.

Question 7

What are the common viral diseases today?

Answer 7

1. Common cold: Rhinovirus
   a. as well as coronavirus, reovirus,
   adenoviruses
   b. affect epithelial cells of upper
   respiratory tract
   c. Gain entry after attach. to
   intracellular adhesion molecule
   ICAM-1
   d. Triggers inflammatory pathways
   e. Interleukin-1, 6, 8 responsible

2. Chicken pox: Varicella-Zoster virus   
    HHV-3 (Herpesviridae) 
   a. Enveloped polyhedral capsids
   b. linear dsDNA
   c. Enter via respiratory tract
   d. attach to receptor and fuse 
       envelope to cytoplasmic membrane
   e. replicate at site, traverse via blood
   f. Often "latent infection"
   g. can reactivate to produce shingles

3. Parvoviridae
   a. only human pathogens with ssDNA
   b. B19 - erythema infectiosum;
   reddening of skin
   c. No treatment

Question 8

Why do we have emerging/re-emerging diseases?

Answer 8

1. Infective agents evolution
2. Globalisation
3. Habitat modification

Question 9

What is acquired immunodeficiency syndrome (AIDS)

Answer 9

1. Enveloped + ssRNA viruses with 
   reverse transcriptase 
2. "Retroviridae"
3. Spike proteins:
   a. glycoprotein120 (gp120)
   b. gp41
4. Bind to CD4 receptor

Question 10

What is influenza?

Answer 10

1. Genesis of the swine-origin influenza
   virus A/H1N1
2. A merge of Classical swine (H1N1),
   North American avian (H1N1), human
   (H3N2), and Eurasian avian-like swine
   (H1N1)
3. N = neuraminidase: hydrolyse
   mucous in lungs
4. Hemagglutinin protein (H1-16)
   a. binds to human receptors
   b. triggers endocytosis
   c. Binds to sialic acid of target cell
   d. Acidification inside endosome
   causes HA conformational change -
   fusion peptide grabs endosome
   membrane
   e. Membranes pulled together
5. antigenic drift and shift responsible
   for new strains