Viral Structure and Function

Question 1

Basic properties of viruses

Answer 1

• obligate intracellular bacteria
• not alive; do not undergo division
• virus replicates and reassembles w/in a host cell

Question 2

Viral strategies for survival

Answer 2

• house DNA or RNA genomes in small proteinaceous particles (capsids)
• genome contains info to carry out infectious cycle
• establish relationships w/population of hosts from benign to lethal

Question 3

Major classification systems of viruses

Answer 3

• classical system: viruses grouped according to shared physical properties
• baltimore system: based on the central dogma (based on how they produce mRNA)

Question 4

Criteria for classification via classical system

Answer 4

1. nature of genetic material (DNA vs. RNA)
2. symmetry fo the capsid (helical vs. icosahedral)
3. naked vs. enveloped
4. dimensions of the virion and capsid

Question 5

Methods for studying viruses (5)

Answer 5

• electron microscopy
• animal models
• sequence analysis
• cell culture
• serology

Question 6

Key principles of viral genomes (2)

Answer 6

• genomes are templates for synthesis of progeny genomes; thus there is finite number of nucleic acid copying strategies
• viral genome goal inside host is to make mRNA (that will be translated by host protein synthesis machinery); genomes must provide mechanisms for synthesis of mRNA

Question 7

Classes of viral genome configuration (7)

Answer 7

• dsDNA
• gapped circular dsDNA
• ssDNA
• dsRNA
• ss(+)RNA
• ss(-)RNA
• ss(+)RNA w/DNA intermediate

Question 8

Functions of virion proteins (3)

Answer 8

1. protect genome: recognize and package genome, assemble stable protein shell, interact w/ cell membrane if envelope formation needed
2. deliver genome: bind receptors, uncoat genome, induce fustion w/host cell membrane, direct transport of genome to appropriate site
3. mediate interactions w/host

Question 9

General characteristics and types of capsid proteins

Answer 9

• symmetrical arrangement of identical/highly similar proteins
• NON-COVALENT bonds (allows for stable assembly and ready reversal during genome delivery)
• sub-units are self-assembled by engaging in “identical” bonding contacts w/neighbors
• helical capsids
• icosahedral capsides

Question 10

Helical capsid properties

Answer 10

• multiple identical subunits arranged using rotational symmetry
• irregularly shaped proteins arranged around a circumference of a circle to form a disk

Question 11

Icosahedral capsid properties

Answer 11

• hollow, quasi-spherical structure
• 20 traingular faces w/12 vertices
• faces w/2-,3-, and 5-fold symmetry

Question 12

Properties of enveloped viruses

Answer 12

• envelope=lipid bilayers acquired during assembly of viral particles; usually have viral glycoproteins w/in membrane
• usually acquired through budding through a membrane of host
• don’t necessarily kill host cells in the course of replication

Question 13

General stages of “one-step growth curve” experiment

Answer 13

• every cultured cell is infected w/virus
• eclipse period (~0-12hrs): virus particles have broken down after penetrating cells; no detectable virus
• latent period: time between infection and release of new infections virus particles from the cell

Question 14

Processes taking place during latent period

Answer 14

1. attachment of virus to cell: virus-attachment protein binds to receptor
2. entry of virus into cell; uncoating of genome: energy-dependent, endocytosis, fusion (enveloped viruses)
3. viral gene expression
4. genome replication
5. assembly of new viruses
6. egress of new virus particles from cell

Question 15

DNA virus gene expression strategy

Answer 15

• must transcribe mRNA using (-) strand of the DNA genome

- uses host RNA pol II to fill gaps/replicate DNA and produce mRNA

Question 16

RNA virus gene expression strategies

Answer 16

-use viral enzyme RNA-dependent RNA pol (RdRp) for production of mRNA and replication of RNA genomes

Question 17

(+)RNA viruses gene expression strategy

Answer 17

genome may be directly translated by host ribosomes; RdRp is coded by and then replicates genome

Question 18

(-) RNA viruses gene expression strategy

Answer 18

• must transcribe (+)sense mRNA to express genes

- package RdRp w/in virus particle

Question 19

Retrovirus ((+)RNA w/DNA intermediate) gene expression strategy

Answer 19

• copy ssRNA into dsDNA via virus particle reverse transcriptase
• dsDNA incorporated into host genome
• host RNA pol II transcribes host genome into mRNA

Question 20

Viral genome replication strategies

Answer 20

• dsDNA: nuclear or cytoplasm
• ssDNA: nucleus. ds intermediate=template for ssDNA synthesis
• gapped circular dsDNA=viral reverse transcriptase to take viral mRNAs –> DNA
• RNA: RdRp. genome replicated using antigenome as a template

Question 21

Viral assembly strategies

Answer 21

• icosahedral:
  a. capsid assemble around genome
  b. genome fed into preformed capsids
• helical: genome coated w/capsid protein during synthesis

Question 22

Viral egress strategies

Answer 22

• naked: lysis
• enveloped: budding through a. cytoplasm –> extracellular environment or b. cytoplasm –> golgi/ER –> secreted from cell