Lecture 17

Question 1

Definition and properties of viruses

Answer 1

•  Viruses cannot make energy or proteins independently of
a host cell, which acts as a “factory”
•  Viruses are obligate intracellular parasites
•  Viral components are assembled and do not replicate by
“division”

•   Viruses are filterable agents
•   Genomes may be either DNA or RNA but not both
•   Viruses have a naked capsid or an envelope morphology

Question 2

Consequences of Viral Properties

Answer 2

•  Viruses are not living organisms
•  Viruses must be infectious to endure in nature
•  Viruses must be able to use cell processes
•  Viruses must encode any required proteins not
provided by the host cell
•  Viral components must self-assemble

Question 3

How are viruses classified?

Answer 3

• Structure: size, morphology, nucleic acid
  •  Biochemical characteristic: structure, mode of replication
  •  Disease:
  •  Means of transmission:
  •  Host cell (host range): animal, plant, bacteria
  •  Tissue or organ (tropism)
  •  location of first isolation
• members of a particular family

Question 4

•  Structure: size, morphology, nucleic acid

Answer 4

picornavirus (= small RNA)

Question 5

•  Biochemical characteristic: structure, mode of replication

Answer 5

togavirus (= mantle, envelope), retrovirus

Question 6

Disease:

Answer 6

e.g. encephalitis virus, hepatitis virus, poxvirus

Question 7

Means of transmission:

Answer 7

e.g. arbovirus (spread by insects)

Question 8

Host cell (host range): animal, plant, bacteria

Answer 8

(bacterio) phage

Question 9

Tissue or organ (tropism):

Answer 9

adenovirus, enterovirus

Question 10

members of a particular family

Answer 10

papovavirus

Question 11

•  location of first isolation

Answer 11

Marburg virus

Question 12

What is the basic virus structure?

Answer 12

•  Nucleic acid genome (DNA or RNA) 
•  Capsid (protective protein coat 
•  Capsids are made of capsomers 
•  Envelope is an outer membraneous 
 layer made of lipids and proteins 
•  Not all viruses have envelopes

Question 13

What is the morphology of a helical caspid?

Answer 13

1. nucleic acid genome protector and attaches there
2. capsomer (proteinaceous subunit)
3. Capsid (protection, host cell attachment)

Question 14

What are the spikes on the viral structures?

Answer 14

•   protein structures used to host cell binding

-   very specific (narrow host cell spectrum)

Question 15

What is the morphology of a polyhedral caspid?

Answer 15

•  self-assembly
•  no division
icosahedron - 12 pentamers

Question 16

What are the environmental properties of the naked capsid?

Answer 16

Environmentally stable to the following:

• temperature
• acid
• proteases
• detergents
• drying

Question 17

What are the consequence of the naked capsid due to environmental properties?

Answer 17

Consequences:

• can be spread easily (dust, hand-to-hand)
• can dry out and still contain infectivity
• can survive the adverse conditions of the gut
• can be resistant to poor sewage treatment

Question 18

What are the environmental properties of the envelope?

Answer 18

Environmentally labile and disrupted by:

• acid
• detergent
• drying
• heat

Question 19

What are the consequences of the environmental properties of the envelope?
HIV

Answer 19

Consequences:
- must stay wet
- cannot survive in gastrointestinal tract
- spreads in large droplets, body fluids (blood, saliva,
breast milk, transplants, blood transfusion

Question 20

How does viral replication occur?

Answer 20

1) interaction of spikes with cell receptors
2) endocytosis
3) get out of the vacuole and uncoats the envelope+capsid to release genome into the cytosol
4) uses cell machinary for replication + proteins
5) Some viruses assemble in the cell and then llyse and release
6) Others assemble on the cell membrane (spike then genome, capsid then the structures bud off creating the structure)

Question 21

Life cycle of lytic virus

Answer 21

Initially theres a drop in viral numbers as it disassemble in the cell. The numbers stays continues during synthesis and assembly. Then numbers increase during release, which is called the burst size.

Question 22

What are bacteriophages?

Answer 22

= viruses that infect bacterial cells
have complex structures
Have tail fibres that are specific to specific proteins on species. The virus doesn’t get into the cell, rather it injects the DNA into the cell. The end plate attaches to the cell, the tail contracts and DNA goes into the cell.

Question 23

How do phage replicate?

Answer 23

Binding to a receptor on bacterial cell. Injection of DNA into cell. Phage genes are expressed genome replicated. Phage parts are assembled. Phage escapes by cell lysis.

Question 24

How does a viral disease progress?

Answer 24

1.   Acquisition (entry into the body)
2.   Initiation of infection (at primary site) - has to recognize specific cell for infection
3.   Incubation period (virus amplifies and spreads)
4.   Replication in target tissue (disease symptoms)
5.   Immune responses (limit and contribute to disease) - disease symptoms are mainly due to the immune system.
6.   Virus production in tissue, release, contagion. When virus is release from the tissue is the person is contagious.
7.   Resolution or persistent infection/chronic disease

Question 25

How are virus transmitted?

Answer 25

*   aerosols - coughing e.g. influenza virus, picornaviruses *   food, water, e.g. enteric viruses such as reovirus, hepatitis A *   fomites - non enveloped viruses (tissues, clothes), e.g. rhinovirus *   direct contact with secretion (saliva, semen), e.g. cyto-megalovirus, Epstein Barr virus/Kissing virus *   sexual contact, e.g. herpes simplex virus, papilloma virus, HIV *   maternal-neonatal, e.g. rubella virus, herpes simplex virus, HIV *   blood transfusion, organ transplant, e.g. hepatitis B virus, HIV *   zoonoses (animals, insects), e.g. influenza virus, rabies virus

Question 26

What are latent viruses?

Answer 26

*   persistent infection *   virus does not reproduce (no clinical symptoms) *   virus can occasionally activate and produce symptoms - happens by some stress *   latent infections are limited by immune response example chicken pox that is latent even after infection.

Question 27

How does the latent herpes virus work?

Answer 27

- latent in neurons (nucleic acid integrated in genome) - activated by fever, stress, sun light - causing cold sores

Question 28

How does the latent chicken pox virus work?

Answer 28

- resurfaces when immune system weakens by disease or old age - causes shingles

Question 29

How does the latent HIV virus work?

Answer 29

- latent in T cells, macrophages (integrated in genome) | - causes AIDS

Question 30

What are oncogenic viruses?

Answer 30

*   viruses that can cause cancer *   integrate into host genome close to proto-oncogen *   activation of virus also activated proto-oncogene.

Question 31

What are examples of oncogenic viruses?

Answer 31

Epstein-Barr virus: Burkitt’s lymphoma Human Papillomaviruses: cervical cancer Human Herpesvirus 8: Karposi sarcoma (common in AIDS)

Question 32

What are viroids and Prions?

Answer 32

even smaller and less complex than viruses Viroid: short naked fragments of ssRNA, infects plants Prion: - small infectious proteins - cause neurological disease - don't have DNA or RNA example - scrapi in sheep or Mad cow disease

Question 33

What are antiviral treatments?

Answer 33

•  antibiotics are NOT effective against viruses •  activity of most antiviral drugs is limited to certain virus family •  resistance to antiviral drugs is becoming more of a problem

Question 34

What is a therapy for HIV?

Answer 34

Nucleoside analogues phosphonoformates - i.e. this thing gets inserted into the RNA so that reverse transciption is stopped. protease inhibitors - inhibit proteases that chop the long polypeptide chain into functional units. dual therapy

Question 35

Neurominidase how it works and what stops it?

Answer 35

It is found in influenza virus that cleaves sugar of proteins- helps assemble proteins TAMIFLU