CSIM 1.14 Case 40 Launch A Spotty Problem The Biology of Viruses

Question 1

What is a virus?

Answer 1

A SUBmicroscopic entity consisting of a single nucleic acid surrounded by a protein coat capable of replication only within host cells

Question 2

What does submicroscopic mean?

Answer 2

Cannot be seen down a microscope

Question 3

Describe how the genetic material of viruses can vary

Answer 3

• RNA or DNA
  
  • Single or double stranded
  • One molecule or segmented
  • Few to hundreds of proteins coded

Question 4

Recall the types of proteins coded for by viruses

Answer 4

Structural proteins
• Capsid proteins
• Surface spikes

Non structural proteins
• Enzymes
• Block host immune system

Question 5

What is the most common structure/shape for viral capsids?

Answer 5

Icosohedron

Question 6

What is the function of:

1) Capsids?
2) Surface spikes?

Answer 6

1) Protects the DNA/RNA from degradation

2) Allows attachment to cells

Question 7

What is the difference between capsids and envelopes?

Answer 7

Envelopes are made of lipids and are acquired from cell membranes during exocytosis

Question 8

What is the function of lipid envelopes

Answer 8

Contains viral proteins which are required for the virus to attach to and infect a new cell

Question 9

Which viruses have surface spikes?

Answer 9

All those without a lipid envelops (as these act as replacements for the viral proteins in the lipid bilayer)

Question 10

Name the surface proteins found on viruses

Answer 10

• Haemagglutinin
  
  • Neuraminidase

(where H1N1, etc. comes from)

Question 11

What are viruses classified by?

Answer 11

• Nucleic acid
  
  • Replication strategy
  • Similarity of nucleic acid sequence

Question 12

Describe the Baltimore classification of viruses

Answer 12

• Single stranded DNA
  
  • Double stranded DNA
  • Positive sense single stranded RNA
  • Negative sense (antisense) single stranded RNA
  • Single stranded RNA viruses which use reverse transcription (RETROVIRUSES)
  • Double stranded RNA

Question 13

How are viruses grouped together in the baltimore system further classified?

Answer 13

• Classified into families based on nucleic acid sequence

• Within families, classified into genotype and serotype:
> Genotype based on genome organisation and sequence differences
> Serotype based on reactivity to antibodies

Question 14

How do double-stranded DNA viruses replicate?

Answer 14

• Replicates DNA and transcribes a positive sense mRNA strand (for the proteins) in exactly the same way human cells do.
  
  • Uses viral DNA polymerase for DNA replication, but hijacks cellular machinery for protein synthesis

IMG 37

Question 15

What are the three main families of double stranded DNA viruses?

Answer 15

• Papovaviridae
  
  • Adenoviridae
  • Herpesveridae

Question 16

How do negative sense (antisense) single stranded RNA viruses replicate?

Answer 16

• Makes a positive sense mRNA molecule, using the viral antisense RNA as a template
  
  • The mRNA is used to transcribe viral proteins
  • The mRNA is also used as a template to create more antisense viral RNA for the progeny virus

IMG 38

Question 17

What are two main families of negative sense single stranded RNA viruses? Give examples of these

Answer 17

Orthomyxoviridae
  •  Infuenza
Paramyxoviridae
  •  Measles
  •  Mumps

Question 18

How do positive sense single stranded RNA viruses replicate?

Answer 18

• The viral RNA is used as mRNA directly for translation of a polyprotein, which is cleaved by viral or host enzymes
  
  • To replicate, the viral RNA makes an antisense intermediate, which is then used as a template to make more positive sense RNA for the progeny virus

IMG 39

Question 19

What are two main families of positive sense single stranded RNA viruses? Give examples of these

Answer 19

Picornaviridae
• Rhinovirus
• Hepatitis A
• Poliovirus

Hepaciviridae
• Hepatitis C

Question 20

What is the main family of single stranded DNA viruses? Give an example

Answer 20

Parvoviridae

• Parvovirus

Question 21

What is the main family of double stranded RNA viruses? Give an example

Answer 21

Rotaviridae

• Rotavirus

Question 22

Which Baltimore virus class integrates its genetic information into the human genome? Describe how these replicate

Answer 22

Retroviruses

• Viral (sense) RNA is acted upon by reverse transcriptase to build a double-stranded DNA copy
• This DNA is integrated into the genome of the host cell
• From there, the DNA creates (sense) mRNA (NB: remember transcription uses antisense strand as a template)
• This sense mRNA is used to make the viral polyproteins, and some of it is preserved to be packaged into the virus (remember we started with sense RNA)

Question 23

Which viruses create polyproteins

Answer 23

All RNA viruses which begin with a ‘sense’ strand:
• Retroviruses
• Positive sense single stranded RNA viruses

Question 24

What are the stages of the viral life cycle which may be targeted by antiviral therapies?

Answer 24

1) attachment
2) entry
3) replication
4) integration
5) protein synthesis
6) assembly
7) release

Question 25

Which viral DNA polymerase inhibitor acts against herpes simplex virus?

Answer 25

Aciclovir

Question 26

Give examples of retrovirus

Answer 26

HIV

HTLV (human T-lymphocyte virus)