Viruses

Question 1

What are the types of parasites?

Answer 1

Endoparasites- protozoa/ parasitic worms/ parasitic flukes/ others
Ektoparasites

Question 2

What are the relative sizes of bacteria and viruses?

Answer 2

E. Coli: 1-2m x 0.5-1m
Virus size: 18-300nm
Polio virus the smallest, smallpox the biggest

Question 3

What is the structure of a virus?

Answer 3

not able to replicate outside their host (target) cell
DNA or RNA enclosed within a virus-encoded protein coat (nucleocapsid) and (some families) an outer-most host cell membrane-derived envelope

Question 4

What is the action of a virus?

Answer 4

attach to host cell using receptor-binding proteins targeting host cell surface molecules that also serve as virus-specific receptors
gain access to cellular biochemical machinery following entry and replicate to generate progeny

Question 5

What is a virion?

Answer 5

Infectious viral particle

Question 6

What are the components of a viral particle?

Answer 6

Nucleic acid- DNA/RNA, circular/linear, ss/ds, piece/segmented
Structural proteins- determine symmetry, protect nucleic acid, role in target cell infection
Enzymes
Glycoprotein- in envelope, role in target cell infection

Question 7

What is the symmetry determined by?

Answer 7

virus structural proteins arranged as morphological units called capsomeres

Question 8

What are the types of symmetry?

Answer 8

Icosahedral/cubical- herpesvirus
Helical- Capsomeres surround the nucleic acid in a spiral (influenza A)
Complex- poxvirus

Question 9

How are viruses classified?

Answer 9

• type of nucleic acid (DNA or RNA) and its sequence
• number, configuration (ss or ds) and polarity (sense/positive sense or anti-sense/negative sense) of —nucleic acid strands
• particle size, structure and symmetry
• mode of replication

Question 10

How are viruses propagated?

Answer 10

• In vitro cell culture= Primary cell lines (derived from healthy tissue; finite number of cell divisions)/ Continuous cell lines (transformed/immortalised cells; infinite number of cell divisions- cancer)
• Embryonated eggs= Amniotic fluid (eg, influenza virus )
  Chorio-allantoic membranes (eg, poxviruses)
• Animal models= Suckling mice/ Neurotropic viruses (eg, poliovirus)

Question 11

What are the different modes of viral infection?

Answer 11

• Oral transmission= contaminated food and drink/ saliva
• Direct skin contact
• Trans-placental
• Droplet transmission= inhalation
• Direct inoculation= injections/ trauma/ insect bites
• Sexual transmission

Question 12

What is the process of infection?

Answer 12

• Attachment to receptor
• Penetration by receptor-mediated endocytosis/ fusion
• Uncoating where capsid is shed
• Replication- synthesis of viral messenger RNA
• Assembly- capsids form around nucleic acid
• Release by budding to form envelope/ cytolysis for no envelope

Question 13

What proteins are made first?

Answer 13

Early- Mostly non-structural proteins (enzymes and regulatory molecules for replication)
Late- Structural proteins (for formation of capsid)

Question 14

How is the genome converted into mRNA?

Answer 14

Transcription- negative strands into positive
SS Retro RNA- by viral reverse
transcriptase

Question 15

What is the process of replication?

Answer 15

• Adsorption, penetration and uncoating of virion

- mRNA to protein to maturation to capsid/ nucleic acid replication to capsid

Question 16

Describe dsDNA replication

Answer 16

Genome transcribed into early mRNA translated into early proteins (mainly enzymes)
New progeny virus DNA is synthesised off the DNA genome of the input parental virus
Progeny genomes transcribed into late mRNA translated into late structural (capsid) proteins, assembled and released

Question 17

What is the type of transcription with single stand positive sense RNA?

Answer 17

The virus genome is the virus mRNA

Question 18

Describe ss+ve replication

Answer 18

The virus genome is translated into one very large polypeptide which is almost immediately cleaved into smaller proteins like structural viral capsid proteins, RNA-dependent RNA polymerase, protease for cleaving precursor polypeptide and genome linked terminal protein
New progeny +ve RNA strands synthesised off -ve RNA strand in replicative form
RNA polymerase synthesises both replicative form and new +ve sense RNA strand genomes
Assembled on ribosomes

Question 19

What is the type of transcription with single stand negative sense RNA?

Answer 19

virus mRNA is transcribed from the parental genome

Question 20

Describe ss-ve replication

Answer 20

Viral mRNA synthesised off parental genome using transcriptase contained in virus particle, separate mRNAs for each viral protein
Progeny genomes by transcriptase using +ve complementary to parental genome as template
Budding through plasma membrane

Question 21

What is a retrovirus?

Answer 21

an RNA virus whose genome alternates between RNA and DNA, inverted dimers of two complete genomes

Question 22

What is the type of transcription with retroviruses?

Answer 22

Virion ssRNA transcribed into dsDNA, integrated into host chromosome as provirus, virus RNA later transcribed

Question 23

Describe retrovirus replication

Answer 23

Transcription by reverse transcriptase to produce DNA/RNA genome heteroduplex
Conversion into dsDNA
Integration into chromosome
Provirus DNA transcribed 
Proteins and genomes transcripts
Budding cell membrane

Question 24

Describe Egress by budding (influenzas A)

Answer 24

Envelope proteins and glycoproteins, translated from viral mRNA, are inserted into areas of the host cell membrane (usually the plasma membrane)
Modified region of host membrane pinched off enclosing viral matrix protein and RNA within capsid

Question 25

Egress by Cytolysis (CMV)

Answer 25

Release by cytolysis (cell destruction) results in a cytopathic effect (CPE)
CPE is used in diagnostic virology (virus detection in tissue culture systems)
cell bursts due to an osmotic imbalance that has caused excess water to diffuse into the cell.