Chapter 8

Question 1

Assembly

Answer 1

Package viral genome and structural proteins -> nucleocapsids
When viral molecules accumulate enough in infected cell
Coordinated interaction of viral molecules
Spontaneous (self-assembly)

Question 2

2 Assembly Mechanisms

Answer 2

Nucleation
Shell Stuffing(headful)

Question 3

Nucleation

Answer 3

Viral proteins coat viral genome sequentially
Viral proteins recognize specific genome sequence
Used by all helical viruses and some icosahedral

Question 4

Shell-Stuffing

Answer 4

Viral genome inserted into pre-formed procapsid
modification may occur: spherical -> Icosahedral
Scaffolding protein help to build procapsid
Portal protein
Used by most icosahedral viruses

Question 5

Procapsid

Answer 5

prohead in tailed phage

empty protein shell

Question 6

Portal protein

Answer 6

Protein inserted at “port”
Facilitate modification
Connect phage head and tail

Question 7

Packaging Signal

Answer 7

signal (sequence) in genome recognized by viral proteins
Viruses with ss genome, only the strand to be packaged has packaging signal
Not all viral genome copies are packaged
Some viruses also package host DNA or RNA

Question 8

How to overcome repulsion between (-) charges on Phosphate group during package?

Answer 8

Some viruses package (+) charged viral proteins

Some viruses package certain cations

Question 9

2 mechanisms to acquire envelope

Answer 9

Budding through modified cell membrane

De novo synthesis of viral membrane

Question 10

Budding through modified cell membrane

Answer 10

o Viral proteins (usually glycoproteins) insert into cell membrane
o Some of cell proteins are excluded in the region
o Nucleocapsids accumulate near the region
o Membrane pinch off
o Nucleocapsid -> put on “new cloth”
o Some viruses use M (membrane, matrix) protein
o -> “stitch” capsid protein and envelope protein together during budding
o A few viruses bud through

Question 11

De novo synthesis of viral membrane

Answer 11

Direct synthesis of viral membrane

Rare

Question 12

2 major mechanisms for exit from infected cell

Answer 12

Cell lysis

Membrane budding

Question 13

Cell Lysis

Answer 13

Cell rupture
Sudden release Most phages produce lysins -> break down peptidoglycan
Burst size varies
Often by naked virions

Question 14

Membrane Budding

Answer 14

pinch off from cell membrane
Not lyse host cell
Gradual release
By most enveloped viruses

Question 15

Plant virus exit

Answer 15

Spread between cells via plasmodesmata (channels span cell wall)

Question 16

Maturation

Answer 16

Changes in virion to confer infectivity
Structural change or protease cleavage of viral protein
Occur before, during or after exit
Not all viruses have this step