S1B4 - Study Questions 01

Question 1

How do interferons inhibit a viral infection?

Answer 1

Interferons bind to host cell surface receptors and initiate an anti-viral state in uninfected cells, e.g., viral RNA and DNA synthesis is blocked; a ribonuclease is induced that degrades viral mRNA.

Question 2

What signal induces interferon?

Answer 2

Interferons are induced by the presence of double-stranded RNA in a virally-infected cell.

Question 3

What is the specificity of interferon?

Answer 3

Interferons are cell-specific in both their production and their effects, but are virus non-specific.

Question 4

What are the major types of interferons?

Answer 4

There are 3 classes on interferons, which basically stimulate the immune response and promote immune clearance of viral infection.

• IFN-alpha (20 types) and IFN-β (2 types) - inhibit viral protein synthesis, & activate leukocytes to kill viruses
• IFN-gamma (3 subtypes) - up-regulate MHC I and II expression and antigen presentation, and activate NK and Tc cells to kill virus-infected host cells

Question 5

How are symptoms of viral diseases associated with interferons?

Answer 5

Flu-like symptoms. Fever, myalgia, and headache are side-effects of interferon therapy. These common features of viral infections may reflect the effects of the patient’s own interferon synthesis in response to viral infection.

Question 6

Differentiate prions, viruses, degenerate bacteria and bacteria (prokaryotes) from each other and from eukaryotes on the basis of size, nuclear material and organization, reproduction and cellular organization.

Answer 6

Prions

• Viral-like deviant and disruptive proteins, e.g., Mad Cow disease

Viruses

• Smallest infectious particles
• RNA or DNA
• membrane or no membrane
• replication depends on host (true obligate parasites)

Bacteria

• small unicellular
• single, circular chromosome and plasmids
• lack nuclear membrane
• lack ER and mitochondria
• asexual reproduction

Degenerate bacteria

• have lost free-living ability
• obligate intracellular parasites

Fungi

• eukaryotic
• unicellular or multicellular
• asexual and sexual reproduction

Parasites

• complex unicellular (protozoans) or multicellular (helminths)
• eukaryotic
• can be small or large
• includes some arthropods (mites, fleas)

Question 7

How could one sterilize a heat-labile solution to be injected into a patient?

Answer 7

Non-heat sterilization techniques

• Filtration
  
  • HEPA filters, remove microorganisms; not so effective for viruses.
• Radiation
  
  • germicidal UV, ionizing (gamma), produces DNA damage, blocks replication
• Ethylene oxide gas
  
  • toxic alkylating agent used for sterilizing heat-sensitive materials, however, toxic or mutagenic by-products must be dissipated

Question 8

Why is 70% ethanol better than 95% for antiseptic usage?

Answer 8

ANTISEPSIS - use to kill most organisms on skin or in tissue, spores not killed.

• Alcohols
  
  • kills most organisms including mycobacteria, but not spores
  • non-toxic to skin but has a drying action
  • more effective in presence of water, thus 70% is more effective than 95%

Question 9

What is the phenol coefficient and how should one apply it?

Answer 9

ANTISEPSIS - use to kill most organisms on skin or in tissue, spores not killed.

• Phenolic compounds
  
  • attacks lipid membranes, effective against mycobacteria
  • action is improved by halogens, e.g., hexachlorophene
  • phenol coefficient is a rating scheme for antibacterial agents, 5 means 5 times as effective as phenol

Question 10

What are base substitutions, transitions, transversions, missense, nonsense, frameshifts, deletions, insertions, silent and null mutations?

Answer 10

Substitutions - one nucleotide base for another

Transitions - purine for a purine, pyrimidine for a myrimidine

Transversions - purine for a pyrimidine or vice versa

Missense - point mutation, codon codes for new amino acid

Nonsense - point mutation, results in premature stop codon

Frameshifts - reading frame shifted

Deletions - nucleotide base deleted

Insertions - extra nucleodide base inserted

Silent mutatons - results in no change to end protein

Null mutations - results in a non-functional protein

Question 11

Differentiate genetic shift from drift; reassortment and mutation?

Answer 11

Genetic drift

• frequent, but subtle changes.
• mutation

Genetic shift

• infrequent, but large and sometimes devastating changes
• reassortment

Reassortment

• Viruses with segmented genomes can exchange nucleic acid segments between strains. This is seen only in some RNA-containing viruses (Orthomyxoviridae, Arenaviridae, Reoviridae and Bunyaviridae). This tends to be the mechanism of “genetic shift,” whereby influenza viruses rapidly acquire new hemagglutinin and neuraminidase antigens, and it has been the initiating factor in some serious Influenza epidemics. Chromosome mixing: homopolyploidy, 2 strains. heteropolyploidy: 2 species (e.g., human, swine)

Mutation

• Mutations in viruses are frequent because of the poor fidelity shown by viral polymerases and the rapid rate of genome replication. One virally-infected cell may produce up to 100,000 viruses. RNA viral RNA polymerases are error-prone due to the absence of a proofreading function. This is the mechanism of “genetic drift,” which results in frequent, but subtle changes.

Question 12

Differentiate conjugation (and sex factors and pili), transposition, transformation (plasmid and chromosomal DNA types), and transduction (generalized, specialized) in terms of how donor DNA is brought into the recipient cell?

Answer 12

Conjugation

• fertility plasmid-facilitated transfer of a plasmid or of host chromosome to a recipient cell
• Conjugation occurs only between strains of the same or closely-related species.
• involves a sex pilus

Transposition

• “jumping genes”
• genetic transpositions provide a mechanism involving non-homologous or illegitimate recombination for plasmid-borne genes to move and integrate into the host chromosome, especially when facilitated by selective pressure.

Transformation

• uptake of extracellular DNA by bacteria in a particular physiological state (competency) induced experimentally or occurring naturally at a particular stage in their growth cycle.
• DNA binds to bacterial cell surface, is taken up through cell mambrane, is integrated into the chromosome

Transduction

• involves a bacteriorphage

Question 13

What are recombination and the Holliday structure, and how do they relate to the processes listed above?

Answer 13

Recombination

• breakage and reunion of homologous regions of donor and recipient double-stranded nucleic acid molecules that have been brought into close proximity following transformation, conjugation or transduction (or polyploid reassortment).

Holiday structure

• fancy name for the point of crossing over

Question 14

What is MIC used for and how is it determined?

Answer 14

Tests to determine antibiotic susceptibility: 

• Diffusion tests, e.g., Mueller-Hinton, can be used to identify antibiotic resistant bacteria 
• Dilution tests can determine the MIC (minimum inhibitory or bactericidal concentration) useful for treatment options

Question 15

What is required to culture a virus?

Answer 15

Because viruses are obligate parasites, a host is needed to culture a virus.

Question 16

What does icosahedral mean; what are its characteristics?

Answer 16

Icosahedral – Protomers, the basic building block, aggregate in groups of 5-6 to form the capsomer. In electron micrographs, capsomers are recognized as regularly spaced rings with a central hole. The shape and dimensions of the icosahedron depend on characteristics of its protomers. All icosahedral capsids have 12 corners each occupied by a penton capsomer and 20 triangular faces, each containing the same number of hexon capsomers. Size is increased between species by adding more hexons between the pentons to form an icosadeltahedron. Icosahedral symmetry is identical to cubic symmetry and looks spherical, e.g., poliovirus. Adenovirus possesses a long fiber at the center of each penton, which serves as a viral attachment protein. The capsid can be empty or full with the presence of chromosomal material.

Question 17

How does an envelope affect the ability of a virus to survive in the environment? Is a “naked” virus more or less resistant to desiccation; more or less likely to be involved in fecal-oral transmission?

Answer 17

Envelope – viruses that bud out of the host cell will be surrounded by host cytoplasmic membrane (lipid bilayer) studded on the surface with viral proteins, (the envelope). The presence of this structure confers lability to the virus to drying, acids, bile, detergents, and lipid solvents. Enveloped viruses are often transmitted wet (via blood, tissues, fluids or respiratory droplets).

Less likely to be involved in fecal-oral transmission.

Question 18

Do all viruses have spikes? What is their purpose?

Answer 18

Spikes (Peplomers) – highly antigenic, glycoprotein projections through the envelope, which facilitate attachment to host cell surface receptors. They also serve as hemagglutinins, neuraminidases or have other functions. All negative stranded (antisense) and some positive (sense) RNA viruses are enveloped. The envelope may contain matrix proteins for genome packaging and may contain replication enzymes.

Not all viruses have spikes.

Question 19

What are the basic types of capsid morphology?

Answer 19

• Icosahedral
• Helical
• Complex

Question 20

Where do DNA and RNA viruses replicate and what are the exceptions?

Answer 20

All DNA viruses replicate in the host cell nucleus except for poxviruses.

All RNA viruses replicate in the host cell cytoplasm except for orthomyxoviruses and retroviruses.

Question 21

Differentiate entrance, adsorption, penetration, and uncoating? What is the eclipse phase?

Answer 21

Entrance

• Viruses enter the body via respiratory droplets (rhinovirus), food or water (HAV), direct transfer (e.g., tissue to tissue, HIV), or arthropod vectors (yellow fever).

Adsorption

• Viruses can enter cells via phagocytosis, viropexis (like pinocytosis) or absorption, which is the most common process, and requires the interaction of a specific viral attachment protein with a specific receptor site on the surface of the host cell. This process is usually species- and tissue-specific.

Penetration

• Getting past the host cell membrane. This occurs by one or more processes. Enveloped viruses fuse their envelope with the membrane of the host cell. This involves local digestion of the viral and cellular membranes, fusion of the membranes and concomitant release of the nucleocapsid into the cytoplasm. Naked viruses bind to receptor sites on the cellular membrane, digest the membrane and enter into the cytoplasm intact. Both naked and enveloped viruses can be ingested by phagocytic cells. However, in this process they enter the cytoplasm enclosed in a cytoplasmic membrane derived from the phagocytic cell.

Uncoating

• during this stage cellular proteolytic enzymes digest the capsid away from the nucleic acid. This always occurs in the cytoplasm of the host cell. The period of the replication cycle between the end of the uncoating stage and maturation of new viral particles is termed the eclipse stage. During the eclipse stage, no complete viral particles are seen within the cell, and the virus is non-infective.

Question 22

Compare and contrast lytic, lysogenic and latent infections? Is there a difference between DNA and RNA viruses? What is a plaque?

Answer 22

Lysis (lytic cycle). The rapid production and release of viruses may lyse the host cell membrane, killing it. A plaque (viral colony) is formed as the lytic process spreads radially from the original infected host cell.

Lysogenic - active replication

Latency. Some viruses can remain quiescent in the host cytoplasm (e.g., herpesviruses) or genome (e.g., retroviruses) until a signal stimulates the virus to actively replicate (Lysogenization), e.g., stress can induce the herpesvirus cold sore. Alternate explanations for latent or dormant infections are low pathogenicity of the virus, and suppression of viral propagation by the host immune system (humoral, cell mediated, interferon).

Question 23

How does a virus develop an envelope?

Answer 23

Enveloped viruses - In the maturation of enveloped viruses, a capsid must first be assembled around the nucleic acid to form the nucleocapsid, which is then surrounded by the envelope. During the assembly of the nucleocapsid, virus-coded envelope proteins are also synthesized. These migrate to the plasma membrane (if assembly occurs in the cytoplasm) or to the nuclear membrane (if assembly occurs in the nucleus) and become incorporated into that membrane. Envelopes are formed around the nucleocapsids by budding of cellular membranes. NOTE: Enveloped viruses have an antigenic mosaicism characteristic of the virus and the host cell. Viruses are slowly and continuously released by the budding process with the results that: (a) the cell is not lysed; and (b) little intracellular accumulation of virus occurs; and (c) inclusion bodies are not as evident as with naked viruses.