Heaphy 5 virology

Question 1

Some different shaped viruses examples

Answer 1

Tobacco mosaic virus. => Rectangles
Calici Virus – 1-2 particles enough to infect => diarrhea => star shape 50nm
Hep A- via faecal –oral 8-9mth orange jaundice => round
HIV => round + lipid bilayer
VSV. Vesicular stomatitis virus => cattle helical coat of proteins => rod envelope.
Bacteriophage T4. => infects e.coli hexagonal head w/ tube body & tails/ base plates. 100nm

Rhinvirus => T=3 related to poli virus, cause of common cold.

Question 2

Methods to determine structures

Answer 2

X-ray crystallography.
• NMRI for small molecules 30,000 viruses are millions
Beam through molecule => defraction pattern => molecules
• crystallization
a biochemical analysis can determine what constituents are present in what ratio
- by ‘building’ the virus with subunit structures from other techniques such as electron microscopy. => purify individual crystalised proteins.
model build from each subunits -> computers used too.
• Variation in individual particles stops it from being crystallised. i.e. HIV

Question 3

• Crick &Watson (1956), 1st to suggest that virus capsids

Answer 3

composed of numerous identical protein sub-units arranged either in helical or icosahedral symmetry after seeing Ems.

Question 4

• helical symmetry

Answer 4

simplest way to arrange multiple subunits

Question 5

identical subunits:

Answer 5

use rotational symmetry to arrange irregularly shaped proteins around the circumference of a circle to form a disc.
• Multiple discs stacked to form a cylinder,

Question 6

protein shell

Answer 6

virus genome coated by the protein shell or contained in the hollow centre of the cylinder.

Question 7

capsid

Answer 7

consists of helix rather than a pile of stacked disks. 20-30nm wide, length variable, 300nm. Sometimes flexible

Question 8

• Icosahedral structures

Answer 8

building capsid is to arrange protein subunits in the form of a hollow quasi-spherical structure, enclosing the genome within like football shape (encloses genome)
• 20 equilateral triangles arranged into a sphere.

Question 9

An icosahedron

Answer 9

has 2-3-5 rotational symmetry.

Question 10

bacteriophage ØX174

Answer 10

Simplist form 60 identical subunits form a capsid. 3 protein subunits per triangular face. Most have more.
⇨ 20 not enough to protect & be stable.

Question 11

T3 quasi equivalence

Answer 11

Contacts are different between subunits but similar.

Question 12

Enveloped viruses:

Answer 12

contain lipids
• Membrane envelopes acquired from a cellular structure during release. Matrix layer found in some enveloped viruses. The capsid underlying the envelope may be have helical or icosahedral symmetry.

Question 13

Intergrase:

Answer 13

Interts => permanently infects cell

Question 14

Nuclear capsid:

Answer 14

protein that interacts with P15 => helical symmetry

Question 15

Protein layer (P24):

Answer 15

to protect virus icosahedral symmetry

Question 16

Why have subunits?

Answer 16

• Major function of the CAPSID. outer shell, of a virus is to protect the fragile nucleic acid genome from:
• Physical damage - Shearing. Even by water, sunlight,
• Chemical damage- UV irradiation leading to chemical modification.
• Enzymatic damage - Nucleases from dead or leaky cells or deliberately made as defence against infection.
• Protein subunits in a virus capsid are multiply redundant, Damage to one subunit does necessarily destroy the infectivity of the whole particle.

Question 17

Small genes advantage?

Answer 17

=> small proteins => more error resistant

- More contacts => more stable

Question 18

Spontaneous formation from purified subunits indicated

Answer 18

that particle was in the free energy minimum state i.e. the favoured structure of the components. This intrinsic stability is an important feature of the virus particle.
• Only goes together in one way

Question 19

• Other advantages?

Answer 19

Self assembly. Easier w/ one/ few subunits

• > Fidelity. smaller protein/gene it means there is less chance of an error occurring….
• Economy. And less waste when it does
• Stability. The larger the number of subunits the more stable the virus particle becomes.