Virus Structure and Life Cycle

Question 1

What are viruses made of?

Answer 1

• Nucleic acid, protein and sometimes lipids
• Virus genome is often packed around proteins
• Nucleic acid is surrounded by a protective coat known as a Capsid
• Animal viruses have a membrane outer layer made up of lipids and proteins which surround the capsid called an Envelope

Question 2

What is the viral capsid?

Answer 2

Protein coat which provides protection for viral nucleic acid

• Means for attachment to target host’s cells
• Made of protein subunits called Capsomeres

Question 3

What is the Viral Envelope?

Answer 3

Acquired from host cell during viral release

• Envelope is a portion of the membrane system of the host
• Contains virally encoded proteins called Spikes (important in binding of a virion to host cell)
• Virus with no envelope = naked virus

Question 4

What are the types of viruses?

Answer 4

1) Helical (tobacco mosaic)
2) Polyhedral (adenoviruses)
3) Spherical (influenza)
4) Complex (bacteriophage T4)

Question 5

What is the general life cycle of viruses (4steps)?

Answer 5

1. Attachment - recognize and attach to host cell. Most viruses infect only a cetain type of host (cell tropism).

>Specificity due to the affinity of viral surface proteins for host cell molecules.

2. Entry - infect host cell (get inside).

>VIRUS ENTER THE CELL THROUGH DIRECT PENETRATION, ENDOCYTOSIS or FUSION

3. Replication - Force the cell to manufacture virus components and self-assemble to produce virions.
4. Release - new virions exit the host cell

Question 6

What are the factors that determine how severe a viral infection is?

Most viral infections are asymptomatic (show no symptoms)

Answer 6

• Number of infecting viral particles
• Speed of viral multiplication
• Effect of the virus on cellular functions
• Is virus lytic or lysogenic? (Lytic:** Rupture/kill cell, **Lysogenic: doesn’t kill cell)
• Secondary responses of host to cellular injury (inflammation and oedema)
• Host’s defences also contributes to viral pathogenesis

Question 7

What are the types of transmission of virues and give an example of each?

Answer 7

Natural means:

Horizontal transmission:

• Direct person to person contact or aeorosols (Influenza, Measles, Polio)
• Mechanical - animal bites e.g. yellow fever, dengue, rabies

Vertical transmission:

• Mother to offspring - HIV

Unnatural means:

• Transfusion products, sharing IV equipment
• Transplantation

Question 8

How do viruses damage cells?

Answer 8

Direct cell damage and death caused by:

• direct lysis
• re-direction of cells energy
• shutdown of important pathways
• competition by viral nucleic acids for proteins/enzymes involved in gene expression

Indirect cell damage can result from:

• integration of the viral genome into the host DNA-mutation
• inflammation and the host immune response

Question 9

The affinity of a virus for a specific body tissue (tropism) is determined by?

Answer 9

Extracellular structures: Spikes on the virion - bind to specific molecules on the target cell surfaces

i.e. receptor-ligand or lock and key analogy.

Intracellular systems:

• genes of a virus may be more efficiently expressed in the target cell type

Physical barriers: skin and mucuous membranes

Environmental:

• can virus survive (e.g. local temp, pH )
• In the GIT, enztmes and bile that may inactivate some viruses

Question 10

What are the viral patterns of infection?

Answer 10

Acute infection

Rapid, clinically apprarent disease with short incubation period.

> Associated with epidemics (measeles, smallpox

Outcomes: Recovery with residue/no residue effects, death, proceed to persistent infections

Sub-clinical infection

Many viruses cause acute infections with symptoms in only a portion of infected patients (influenza, yellow fever, polovirus)

Persistent infection(two types)

Occurs when host defences are either targeted by the virus or bypassed

Chronic infections: continuous viral replication and virion production with moderate or no symptoms (hepatitis B)

Latent infections: virus is generally hidden in a non-replicating form. >Virions are not detectable

>Periodic flareups may occur

Question 11

What are Oncogenes and how do they work?

Answer 11

Viruses that cause cancer are known as oncogenic

> Retroviruses can inactivate genes responsbile for suppressing tumour formation / activate genes involved in cell growth

>Some viruses can acquire oncogenes ( genes coding for proteins involve in cell growth which can cause cancer)

• Virus associated with human cancers include (PPV and epstein-barr virus)