Lecture 6: Virus Structures and Functions

Question 1

Defining Features of Viruses

Answer 1

• obligatory intracellular parasites
• contain DNA or RNA
• genomes are very small
• contain a protein coat
• infect only specific type of tissue

Question 2

Capsid

Answer 2

protein shell that surround the genome of a virus particle

-composed of highly repetitive pattern of protein molecules around the nucleic acid genome

Question 3

Nucelocapsid

Answer 3

complete complex of nucleic acid and protein packaged in a virion

Question 4

Capsid Patterns:

Answer 4

1. Rotational Symmetry: form closed structures, polyhedral viruses, inner volume maximized
2. Helical symmetry: open-ended structures

Question 5

Enveloped Viruses

Answer 5

• Have membranes, surround nucleocapsids and are lipid bilayers with embedded proteins
• Make initial contact with the host cell

Question 6

Enzymes included in a virion

Answer 6

• Lysozome
• Nucleic Acid Polymerases
• Neuramidases (cleave glycosidic bonds in glycoproteins, allow liberation from host)

Question 7

Viruses only replicate in certain types of cells or whole organisms

Answer 7

-bacterial viruses easiest to be grown

Question 8

Bacterial viruses

Answer 8

cultured either in batch culture (liquid) or as isolated plaques on a bacterial lawn

Question 9

Animal viruses

Answer 9

cultivated in living animals, embryogenated eggs, or tissue cultures

• can be detected by assaying foci (groups of cells infected by a virus)
  1. Attachement
  2. Penetration (endocytosis or fusion)
  3. Uncoating but viral or host enzymes
  4. Biosynthesis
  5. Maturation
  6. Release by budding or rupture

Question 10

Titer

Answer 10

number of infectious units per volume of fluid

Question 11

Plaque Assay

Answer 11

analogous to the bacterial colony

• measure virus infectivity
• Plaques are clear zones that develop on lawns of host cells

Question 12

Identification

Answer 12

1. Distinct patterns of cytopathic effects
2. Serological Tests (Antibodies used to identify virus antigens)
3. Nucleic Acid Based Tests (RFLP, PCR)

Question 13

Bacterophages

Answer 13

1. Lytic or replication cycle

2. Lysogenic Life cycle

Question 14

Lytic Cycle

Answer 14

• causes lysis and death of the host cell
• Generalized transduction (DNA derived from ANY portion of the host genome)
  1. Attachement
  2. Penetration (injection)
  3. Biosynthesis (synthesis of nucleic acids and proteins)
  4. Maturation (assembly and packing of phage particles)
  5. Release (Lysis) - phage lysozyme breaks down cell wall
• protein coat remains outside, only viral DNA enters
• Latent period through maturation step

Question 15

Lysogenic Life Cyle

Answer 15

• viral nucleic acids invorporated into the host DNA
• phage conversion (when lysogenized by a phage, the host cell becomes immune to further infection by the same type of phage)
• specialized transduction: DNA from a SPECIFIC region of the host chormosome integrated directly into virus genome, replacing some viral genes

Question 16

Syncytia

Answer 16

Syncytia formation allows viruses to spread from cell to cell and to escape antibody detections
-cells in this state are fragile and suspetible to lysis

Question 17

Nonlytic Infections

Answer 17

follow acute infections, occur in infected cells that are not killed by a virus

Question 18

Latent Infections

Answer 18

virus remains in asymptomatic host cells for a long period of time until reactivation
-cold sores, Herpes, Shingles

Question 19

Persistent (Chronic) Infections

Answer 19

• disease processes occur gradually over a long period of time and are often fatal
• measles virus

Question 20

ICTV taxonomy of viruses

Answer 20

Genome composition, Symmetry of the Capsid, Envelope, Size, Host Range

Question 21

Nomenclature and Species Definition

Answer 21

Order, Family, Subfamily, Genus, Species

Question 22

Viral Species

Answer 22

group of viruses sharing the same genetic information and ecological niche (host)

Question 23

Subspecies

Answer 23

strains and isolates not disntinguished are designated by a number

Question 24

Baltimore Classification

Answer 24

divided viruses based on seven groups

1. Double Stranded DNA
2. Single Stranded DNA
3. Double Stranded RNA
4. Single Stranded RNA (+)
5. Single Stranded RNA (-)
6. RNA Retroviruses
7. DNA Pararetroviruses

Question 25

Tropsisms

Answer 25

affinities toward specific types of tissues

-way to distinguish animal viruses

Question 26

Modes of Transmission:

Answer 26

• Foodborne/Airborne
• Vector-Borne
• Airborne
• Direct Contact

Question 27

Prions

Answer 27

Infectious proteins whose extracellular form does not contain nucleic acids

• Etiological agent of spongiform encephalopathies
• characterized by vacuolated neurons (loss of functions)
• lack of an immune response or inflammation

Question 28

PrPc

Answer 28

Normal Prion protein localized on cell surface
PrPcS= Scarpie protein, due to misfolding accumulates in brain cells, formes plaques, results in fusions of neurons and glial cells
-Prion misfolding leads to neurological symptoms of disease

Question 29

Prion Disease Mechanisms

Answer 29

1. Infectious Prion Disease (pathogenic form of prion folding is transmitted between animals or humans)
2. Sporadic Prion Disease (Random misfolding of a normal, healthy prion protein in an uninfected human)
3. Inherited Prion Disease (mutation in prion gene yields a protein that changes more often into a disease causing a PRPs form

Question 30

Pathogenic Features and Symptoms of Prion Disease

Answer 30

• No cytopathologic effects in VITRO
• long double time of 5
• long incubation time up to 30 years
• include loss of muscle control, shivering, tremors, dementia