Lecture 3: Virus Structural Diversity

Question 1

A virus is normally much __ than the host cells in which it replicates.

Answer 1

smaller

Question 2

How big is the Acathhameba?

Answer 2

15 to 25 uM

Question 3

How big is the world’s largest virus?

Answer 3

1.5 x 0.5 um. It is large enough to see with a light microscope.

Question 4

How big is E coli?

Answer 4

1 x 2 uM

Question 5

How big is the smallest animal virus polio?

Answer 5

24 x 24 nm

Question 6

How big is the largest animal virus (pox)?

Answer 6

0.3 x 0.3 uM

Question 7

Bacterial cells vary in size by ___ orders of magnitude the smallest being just ___ in diameter

Answer 7

a) 4 b) 0.15 uM

Question 8

The largest bacterial ___ are smaller than the smallest bacterial ___

Answer 8

a) viruses b) cell

Question 9

How were viruses first discovered?

Answer 9

First discovered as infectious agents that passed through filters which retained the smallest cells (bacteria) the pores of these filters were 0.3 uM in diameter.

Question 10

Are all viruses 0.3 uM (300 nm) in diameter?

Answer 10

No, there are some viruses in amoeba that cannot pass through a filter and are named giant viruses.

Question 11

Describe giant viruses

Answer 11

They behave in the same way as all other viruses they just are larger. Two examples are megavirus and pithovirus. (Giant viruses that replicate in animal cells or cause disease have not been discovered yet).

Question 12

Describe sub viral agents

Answer 12

At the lowest end of the size scale there are infectious entities that are smaller than a virus.
Ex. satellite/ crippled virus that can only replicate in the presence of a real virus (helper virus).

Question 13

Describe a satellite virus

Answer 13

Helper virus is required
Its genome is protein-encapsidated
Its genome encodes its capsid protein

Question 14

Describe a satellite nucleic acid

Answer 14

Helper virus is required
Its genome is protein encapsidated
Its genome does NOT encode its capsid protein

Question 15

Describe a viroid

Answer 15

No helper virus is required.
Genome is not protein encapsidated
Genome does not encode capsid protein
Viroid is just a piece of RNA
Simplest known infectious agent.
May have descended from a virus or vice versa

Question 16

What makes up the exterior of a viral structure?

Answer 16

Exterior: proteins for protecting the genome and attaching to the cell surface at the start of infection.

Question 17

What makes up the interior of a viral structure?

Answer 17

Interior: nucleic acid genome usually engaged with nucleic acid-binding proteins.

Question 18

What is the naked capsid virus?

Answer 18

exterior= protein shell: rugged release of new virons from cell requires cell lysis.

Question 19

What is the capsid?

Answer 19

name of exterior protein shell

Question 20

What is the core?

Answer 20

Interior nucleic acid (genome) maybe complexed with proteins. Core proteins if they exist are sometimes known as non-structural proteins (may be histone-like or include enzymes for genome expression/replication).

Question 21

What is an enveloped virus?

Answer 21

outer layer = lipid envelope
less rugged cell lysis generally not required for viron exit

Question 22

What is an envelope?

Answer 22

flexible lipid bilayer containing virus encoded transmembrane proteins (envelope is picked up from cell plasma membrane during virus exit).

Question 23

What is an matrix protein?

Answer 23

lines inside of the envelope = protein sac either rigid or flexible that supports envelope holds genome /NP

Question 24

What is an nucleocapsid?

Answer 24

Interior protein/nuclic acid (genome) complex like human chromatin. The major genome binding protein may be referred to as nucleoprotein.

Question 25

What are spikes (present on both naked capsid viruses and enveloped viruses)

Answer 25

protein or glycoprotein structures emanating from iron surface (for attachment to cell).

Question 26

Exceptions to the generic structures among DNA viruses

Answer 26

Ex. Herpesviruses have a family rigid capsid that contains the genome and outer envelope. But between these two layers is a soft matrix protein-like layer called a tegument.

Question 27

What are the functions of a capsid?

Answer 27

-Carry and protect the chemically labile genome and other critical components (enzymes).
-Facilitate virus entry into the cell (via spikes) delivery of critical components into the host cell.

Question 28

Depending on the virus family, a virus genome may be:

Answer 28

DNA/RNA:
-single stranded/double stranded/ partially ds
-linear/circular
-single molecule/segemented or diploid
-If single stranded:
(+) sense or
(-) sense (antisense)

Question 29

What is an example of the cylindrical virus shape?

Answer 29

Ex. Tobacco Mosaic Virus (TMV) in which naked capsid proteins are arranged as a helical nucleic acid protein complex

Question 30

What is a spherical virus shape?

Answer 30

typical shape actually = icosehedron or soccer ball.
but initially thought to be spherical in low resolution images.

Question 31

What are the two basic virus shapes?

Answer 31

Cylindrical
Spherical
Or a combination of the two
In a naked capsid virus the capes may contain many (> 60 several thousand) copies of a single type of capsid protein.

Question 32

Describe the icosahedron which is an important shape in virology

Answer 32

Has structural rigidity
Easy assembly
30 edges
20 triangular faces
12 vertices
Highly symmetrical structure
Each edge defines a two fold axis of rotational symmetry
Center of each of the 20 faces defines a 3 fold axis
Each vertex defines a 5 fold axis.

Question 33

What are the key points of lecture 3?

Answer 33

Viruses are generally smaller than cells.
Viruses may convert the host cell into a factory for making more viruses (increasing virus titer)
Virus structures come in the form of either naked capsid or enveloped with a fairly predictable generic morphology in each case.
Virus genomes come in more varieties than just the overall morphology.
Naked capsid viruses are either spherical or cylindrical in shape.
The icosedron and variations of it are central themes in the geometries of spherical naked capsids and possibly the internal structures of a few developed viruses.
It has multiple symmetries.