Intro to viruses

Question 1

Give characteristics of viruses

Answer 1

Small: 20 – 400 nm diameter

Non-cellular: Genetic element that cannot replicate independently of a living (host) cell

Obligate intracellular pathogens (can only replicate inside host cell)
Most viruses have a specific host range and only infect specific host cell types (tissue tropism)

Question 2

What different structures can viruses come in?

Answer 2

Icosahedral: 20 faces, each an equilateral triangle
Helical: protein binds around DNA/RNA in a helical fashion
Complex: neither icosahedral or helical

Question 3

What is a virion?

Answer 3

Virion: extracellular form of a virus
Exists outside host and facilitates transmission from one host cell to another
Contains nucleic acid genome surrounded by a protein coat and, in some cases, other layers of material

Question 4

How can virus families be classified?

Answer 4

Virion shape / symmetry
Presence / absence of envelope
Genome structure
Mode of replication

Question 5

Describe virion structure?

Answer 5

in middle have nucleic acids , polymerase (enzyme required for replication of virus) around it , protein capsid (hexagonal) around that and then outer lipid envelope

Question 6

Virus replicate steps?

Answer 6

virus attaches to cell wall
gets uncoated
goes inside host cell in order to utilise the structure or material inside host cell to allow itself to replicate
either direction of protein synthesis or genomic nucleic acid synthesis
assemble new virion which acquires coat from cell membrane

Question 7

What happens when coinfection occurs?

Answer 7

Coinfection of human and animal or bird strains in one organism may lead to recombination and generation of a new strain

Question 8

Consequences of viral infection?

Answer 8

Clearance of virus (with no, short or long term immunity)
Measles (long term immunity)

Chronic infection
HIV, hepatitis B, hepatitis C

Latent infection: Herpes Virus

Transformation (long term infection with altered cellular gene expression)
Epstein-Barr Virus, Human Papillomavirus

Question 9

Describe viral latency?

Answer 9

Following primary infection, some viruses lie dormant in the cell.
The full viral genome is retained in the host cell, but its expression is restricted, such that few viral antigen and no viral particles are produced.
Reactivation of viral replication can occur
Reactivations may or may not cause apparent disease
Reactivation more likely to occur and more severe in immunocompromised
Examples:
Herpes Simplex Virus
Varicella Zoster Virus

Question 10

How do viruses lead to cancer?

Answer 10

Modulation of cell cycle control (driving cell proliferation)

Modulation of apoptosis (prevention of programmed cell death)

Reactive oxygen species mediated damage (some persistent viral infections can cause persistent inflammatory processes which lead to cancer via reactive oxygen species)

Question 11

What are some viruses and the consequences?

Answer 11

EBV: Burkitt’s lymphoma, Hodgkin’s lymphoma, nasopharyngeal carcinoma, post-transplant lymphoproliferative disease (PTLD)
Human herpes virus 8: Kaposi’s sarcoma, primary effusion lymphoma, Castleman’s disease
Human T-cell Lymphotropic Virus (HTLV): Adult T-cell leukaemia/lymphoma
Human Papillomavirus (HPV): cervical, anal, oropharyngeal cancers
Hepatitis B and Hepatitis C: hepatocellular carcinoma