Composition and structure

Question 1

what is the 4 basic chemical composition components of viruses?

Answer 1

1. genetic material (RNA or DNA)
2. proteins (structural & non-structural, ie. for replication)
3. lipids (for enveloped viruses only, derived from most commonly plasma membrane)
4. carbohydrates (glycoproteins and glycolipids, involved in virus attachment to cells)

Question 2

what are ssRNA virus genomes like?

Answer 2

can be +ssRNA or -ssRNA, and can be segmented or non-segmented

Question 3

what are dsRNA virus genomes like?

Answer 3

almost always segmented

Question 4

what are ssDNA virus genomes like?

Answer 4

can be linear or circular

Question 5

what are dsDNA virus genomes like?

Answer 5

can be linear or circular

Question 6

what type of microscope can viruses be seen and not seen by?

Answer 6

too small to be seen by a light microscope, but can be seen by an electron microscope or x-ray microscope

Question 7

what are the 4 types of morphologies of viruses?

Answer 7

1. rigid rods or flexuous filaments (14x71 nm - 80x14,000 nm)
2. spherical/isometric (17-300nm in diameter)
3. complex morphology (ex. T-even phages have a spherical head and helical sheath)
4. irregular morphology (ex. baculoviruses can be in an occluded form (multiple virion particles together) to survive in the environment, or in a budded form (1 virion particle) to spread within insects)

Question 8

what is a nucleo-capsid?

Answer 8

the capsid proteins surrounding the genome enclosed in the membrane of enveloped viruses (in naked viruses the capsid is the only outer structure so it’s called a nucleo-capsid because it looks like a nucleus in an enveloped virus)

Question 9

what is a virion?

Answer 9

a complete virus particle

Question 10

what is a capsid?

Answer 10

a protein coat encasing the virus genetic material

Question 11

what is a structural unit?

Answer 11

a building block of a capsi/nucleo-capsid (may be one capsid protein or multiple)

Question 12

what is a subunit?

Answer 12

a single, folded polypeptide like a single capsid protein

Question 13

what is an envelope?

Answer 13

a lipid membrane encasing the nucleocapsid (only in enveloped viruses)

Question 14

what are structural proteins?

Answer 14

proteins that make up a virion’s structure

Question 15

what are non-structural proteins?

Answer 15

proteins encoded by the virus but aren’t a part of the virions structure, ie. proteins required for viral replication of movement

Question 16

what are the 2 basic symmetries to build a viral capsid?

Answer 16

icosahedral or helical

Question 17

what is the pitch of the helix and how is it calculated (formula)?

Answer 17

pitch is the height added to the virion by each axial turn of the helix

• Pitch = P
• mu = # subunits per helical turn
• p = axial rise per subunit = virion length/# CP subunits
• P = mu x p

Question 18

what is helical symmetry?

Answer 18

• the capsid subunits ARE equivalently bonded to eachother
• a helix is an OPEN structure (think of like a toilet paper tube) which therefore has unlimited packing capacity for genetic material
• usually virions are rigid rods or flexible filaments

Question 19

what is icosahedral symmetry?

Answer 19

• icosahedral = 20 triangular faces
• CLOSED structure, therefore limited size to package genetic material
• takes 3 CP subunits to make each triangle face, therefore a basic 20-faced icosahedron has 60 CP subunit copies
• 3 types of rotational symmetry: 2-fold, 3-fold, and 5-fold symmetry

Question 20

what are the 3 pillars of icosahedral symmetry?

Answer 20

1. triangulation number (T) = defines the icosahedral surface lattice (number of triangles that exist within each of the 20 triangular faces)
2. quasi-equivalence = describes how the subunits are ALMOST equivalently bonded
3. CP subunits spontaneously self-assemble (without help of genome) into capsid shells

Question 21

what is the triangulation number?

Answer 21

• for a basic icosahedron with 20 triangular faces, the triangulation number is T = 1
• only multiples of 60 are allowed as triangulation numbers (T = 1, 3, 4, 7, etc)
• ex. if T = 3, that means there are 3 x 60 CP = 180 CP

Question 22

what is the architecture of a complex virus?

Answer 22

ex. lambda phage
1. icosahedral head
2. helical tail (sheath)
3. base plate with tail fibres (look like legs)
- each part (1,2,3) is assembled separately, and then assembles into a complete virion

Question 23

what are the 2 BROAD categories of viruses? (hint: not having to do with genome)

Answer 23

enveloped vs naked viruses

Question 24

what is the architecture of TMV?

Answer 24

a rod-shaped virion that is 95% protein and 5% RNA