Exam 3: Study Guide Ch. 13 DONE

Question 1

Describe the general characteristics of viruses and explain how they differ from cellular organisms.

Answer 1

o Submicroscopic, acellular (no cell wall) infectious particle
o Obligatory intracellular parasites: Require living host cells to multiply because they lack the machinery for generating energy and large molecules.
o Contain DNA or RNA (not both)
o Contain a protein coat called capsid, that surrounds their nucleic acid
o Nucleic acid and capsid combined is called the nucleocapsid

Question 2

Describe the structure and morphology of viruses based on the shape of their capsid and the presence or absence of envelope.

Answer 2

o Helical viruses: hollow, cylindrical capsid
o Polyhedral viruses: many-sided
o Naked or non-enveloped viruses: composed only of the nucleocapsid
o Enveloped viruses: Pleomorphic
o Complex viruses: complicated structures (e.g., tail, sheath, plates and pins or spikes)

Question 3

Explain how viruses have host range and tissue specificity and what tissue tropism is.

Answer 3

o Host Range: Spectrum of host cells a virus can infect
 Bacteriophages: viruses that infect bacteria
 Plant viruses: infect plant cells
 Animal viruses: infect animal cell
o Tissue Tropism: Most viruses infect only specific types of cells in one host. It is determined by the composition of viral capsid and specific host attachment

Question 4

Describe how viruses are classified according to their genetic material.

Answer 4

o Bacteriophages: Contains only DNA as genetic material
o Animal Viruses: May contain ether DNA or RNA

Question 5

Differentiate between bacteriophages and animal viruses.

Answer 5

o Bacteriophages: Contains only DNA as genetic material
o Animal Viruses: May contain ether DNA or RNA
 RNA viruses contain single or double stranded RNA genomes

Question 6

Differentiate between lytic and lysogenic bacteriophages.

Answer 6

o Lytic (virulent) cycle:
 Phage causes lysis and death of the host cell
o Lysogenic (temperate) cycle:
 Phage DNA is incorporated in the host DNA
 Bacterial cell becomes immune to superinfection with the same phage
 Lysogenic conversion: host cell exhibits new properties such as change in surface antigens, antibiotic resistance and virulence factors (e.g., toxins)
 Specialized transduction

Question 7

Describe the multiplication cycle of lytic bacteriophages.

Answer 7

o Attachment:
 Phage attaches by the tail fibers to the host cell
o Penetration:
 Phage tail releases lysozyme to open the cell wall; tail sheath contracts to force the tail core and DNA into the cell
o Biosynthesis:
 Production of phage DNA and proteins
o Maturation:
 Assembly of phage particles
o Release:
 The exit of viral offspring (progeny) from the bacterial cell. Also known as the lysis stage due to rupture of host cell

Question 8

Define virion, progeny, eclipse period, burst time, and burst size.

Answer 8

o Virion: fully assembled virus
o Progeny: : offspring of a virus
o Eclipse Period: time during viral multiplication in which infectious phages are not yet present
o Burst time: time elapsed from phage adsorption to release (approx. 20-40 mins)
o Burst Size: Number of newly synthesized phage particles released form a single cell. Ranges from 50-200

Question 9

Describe the multiplication cycle of lysogenic bacteriophages.

Answer 9

o Adsorption
o Penetration
o Site-specific recombination:
 Occurs between a specific region of the virus and the host chromosome to allow integration
o DNA replication:
 Every time the bacteria replicates, the prophage replicates. May go on for many generations
o Excision:
 Carried out by reverse recombination. Can occur spontaneously or may be induced
o Lytic cycle:
 Not all infected bacteria undergo lysis at the same time

Question 10

Explain how the plaque forming assay can be used to determine the number of infectious viral particles present in a solution as well as if the virus as a lytic of lysogenic virus.

Answer 10

o Plaques: Clearings on a lawn of bacteria on the surface of agar
 Each plaque corresponds to a single virus; can be expressed as a plaque-forming units (PFU)

Question 11

Describe the different multiplication cycle of animal viruses with special emphasis on the different mechanisms of biosynthesis of nucleic acids.

Answer 11

o 1. Adsorption:
 Viruses attach to the cell membrane
o 2. Penetration:
 Entry by receptor-mediated endocytosis or fusion. Virus taken into the cytoplasm as intact capsids
o 3. Uncoating:
 Separation of the capsid from the viral genome. Envelope and capsid are destroyed by viral or host enzymes
o 4. Biosynthesis: Production of nucleic acid and proteins
 a. dsDNA - containing viruses D
* NA travels to nucleus where it is replicated and transcribed mRNA exit nucleus for translation of capsid proteins. Migration of capsid proteins back to the nucleus
 b. RNA - containing viruses
* 1. Single stranded (+) or sense RNA strand acts as mRNA of viral enzymes and capsid proteins and a template for the synthesis of an antisense (-) RNA strand, which in turns acts as template for more (+) strands; carried out by the RNA-dependent RNA polymerase coded by the virus. Transcription and translation follow
* 2. Retroviruses - carry gene for the RNA-dependent DNA polymerase (a.k.a. reverse transcriptase). It catalyzes the synthesis of viral DNA, which then integrates into the host cell DNA (provirus- host cell can make retroviral DNA)
o 5. Maturation:
 Nucleic acid and capsid proteins assemble. It is a spontaneous process
o 6. Release:
 Depends on if virus is naked or enveloped
* Rupture or lysis (naked viruses)
* Budding (enveloped viruses)

Question 12

List and explain River’s postulates and how they expand upon Koch’s postulates to identify pathogenic viruses.

Answer 12

o Rivers’ postulates expand upon Koch’s postulates to help identify viruses.
 Filtrates of infectious material shown not to contain bacterial or other cultivatable organisms must produce the disease.
 Filtrates must produce specific antibodies.
o 1. All the sick organisms should have the virus
o 2. Filtrates: Filtered and then there is liquid (serum)
o 3. Liquid is then injected into healthy animals (causes disease)
o 4. Diseased animal now has antibodies against viruses

Question 13

Describe the techniques used to detect and cultivate viruses.

Answer 13

o Serological tests - reaction of the virus with circulating antibodies in blood
 Fluorescence microscopy
 ELISA (Enzyme Linked Immunosorbent Assay)
o Nucleic acids
 PCR
o In living animals
o Injected into an egg

Question 14

Define oncogenic virus, explain the relationship between tumors and viruses and give examples of cancers associated with viral infections.

Answer 14

o Oncogenic virus: Cancer
 They are responsible for up to 20% of human tumors

Question 15

Differentiate between virus, viroids and prions.

Answer 15

o Virus: Infective agent that consist of a nucleic acid in a protein coat
o Viroids: short pieces of naked RNA
o Prions: Proteinaceous infectious particles

Question 16

List diseases associated with prion infection and describe the current hypothesis on how these diseases occur.

Answer 16

o Diseases: Spongiform encephalopathies (neurodegenerative that makes the brain look like a sponge)
 Animals:
* “Mad cow disease”
* Creutzfeldt-Jakob disease (CJD)
 Humans:
* Gerstmann-Sträussler-Scheinker syndrome
* Fatal familial insomnia
* Sheep scrapie
o Hypothesis:
 Eventually have holes

Question 17

Define cytopathic effects.

Answer 17

structural changes to host cells caused by a viral infection

Question 18

Explain the outcomes of lysogeny.

Answer 18

o Cannot be super-infected (immune to super infection: Can’t get the same viruses twice)
o Express viral proteins on host cells surface