2. Viruses

Question 1

What makes a virus different?

Answer 1

• Infectious agent
• not cellular
• cannot reproduce by itself
• needs a host cell to make copies of itself
• comprised of a genome (DNA or RNA)
• no organelles (or energy metabolism)

Question 2

General properties of viruses

Answer 2

• small ( ~20 to 300nm)
• obligate parasites (unculturable)
• infectious particles of nucleic acid and protein
• complete infectious particle is known as a virion

Question 3

Properties of a virion

Answer 3

Complete infectious particle 
• nucleic acid genome
• 6400 nucleotides in TMV
• protein coat: capsid
• sometimes a lipid envelope and enzymes

Question 4

What is protein capsid constructed from?

Answer 4

protein subunits

• self-assembling

Question 5

What are the different shapes of protein capsids?

Answer 5

• Helical array - tobacco mosaic virus
• icosahedral - papilloma adenovirus
• complex - bacteriophage
• other - brick-like, worm like (ebola), bullet-shaped

Question 6

Describe the steps in the replication of viruses

Answer 6

1. Taken up by injection or endocytosis
2. genome released
3. parasites of translation
4. RNA acts as a template for protein synthesis to make DNA (retrovirus)
5. DNA viruses replicate genome in host nucleus and direct capsid synthesis via mRNA

Question 7

What factors are virus classification based on?

Answer 7

• morphology
• type of nucleic acid
• single or multiple-particle virion
• host virus-vector relationships

Question 8

Define zoonosis

Answer 8

A pathogen that can jump from animals to humans

Question 9

What are the H and N particles of influenza virus?

Answer 9

Hemagglutinin and Neuraminidase

Question 10

Function of Hemagglutinin

Answer 10

• enables the virus to enter the cell

Question 11

Function of neuraminidase

Answer 11

• Enables the virus to leave the cell

Question 12

Describe the emergence of new flu strains

Answer 12

• multiple different version of Hemagglutinin and neuraminidase in different virus strains
• each flu virus carries one gene for H and another for N
• genes are in separate pieces of RNA
• two different strains of flu infecting one host cell can exchange versions of H&N to make new combinations
• immune system cannot recognise new combinations

Question 13

What are chemical treatments for flu and their modes of action

Answer 13

• relenza
• tamiflu
• both are neuraminidase inhibitors that prevent flu virus from exiting used host cells

Question 14

genetic material found in viroids

Answer 14

• single circular strand of naked RNA

* Rna is catalytic

Question 15

Prions

Answer 15

infectious proteins

Question 16

What is the mechanism of action of prions?

Answer 16

• PrP is the normal brain prion protein
• in infected animals, PrP is converted to PrPsc
• if we eat PrPsc, it spontaneously converts our PrP to PrPsc

Question 17

What is the effect of PrPsc

Answer 17

Forms insoluble plaques in brain disease

Question 18

What do viruses, viroids and prions have in common?

Answer 18

• They are infectious particles that rely on a host cell for their replication
• they lack energy metabolism but are capable of heritable change (Except prions)