Virus structure, viral replication cycle & prions

Question 1

Viruses

Answer 1

• Viruses infect all living things
• Once infected with those viruses, it is for life
• We carry viral genomes as part of our own genetic material

Question 2

Virus general characteristics

Answer 2

• Acellular Infectious particles
• viruses are the smallest infectious agents - don’t need nutrient
• Do not grow or undergo division outside a cell
• Obligatory intracellular parasites requires host for replication
• Assembly of pre-formed viral components such as DNA and RNA

Question 3

Treatment for viruses

Answer 3

• Cannot be treated with antibiotics with antiviral agents

Question 4

Virus taxonomy

Answer 4

• Viridae - genus
• Virus - species

Question 5

Virus composition

Nucleic acid

Answer 5

• DNA or RNA can be single or double-stranded matrix proteins or enzymes

Question 6

Viral compostition

Capsid

Answer 6

Protein coat/shell to protect the
nucleic acid

Question 7

Virus composition

Envelope

Answer 7

• Lipid bilayer membrane on some viruses, in which
  some peripherical glycoproteins (e.g.
  Spikes) projects from outer surface

Question 8

Absence envelope

Answer 8

• External layer of a virion
• Not always present
• Not present in rhinoviruses, adenoviruses
• Whereas influenza has envelope

Question 9

Envelope

Answer 9

• External to viron
• A lipid bilayer derived from host cell
• Acquired by budding of capsid when
  viruses leaves the cell

Question 10

Envelope glycoproteins

Answer 10

• Transmembrane glycoproteins synthesised through the secretory pathway destined to the plasma membrane

Question 11

Mechanism of Envelope glycoproteins

Answer 11

• Involved in binding specific protein receptors on the external surface of the host’s cell (viral absorption, the first step)

Question 12

Capsid

Answer 12

• Large and protective shell that surrounds/protects viral nucleic acid
• Composed of many capsomers
• Has identical and reversible bonding contacts with its neighbours

Question 13

Capsid shape

Helical

Answer 13

• Capsomere subunit forms a capsid in hollow circular shape

Question 14

Capsid shape

Polyhedral

Answer 14

• Many-sided shape 3-D structure

Question 15

Viral genomes

Answer 15

• Either DNA or RNA which is double or single stranded
• Encodes for the structure of the virion and invading the host cells and hijacking/regulating its metabolic activity to produce more viral proteins
• Enzymes to reproduce/multiply their genomes

Question 16

Spectrum of a viral infection

Answer 16

• Mainly determined by the presence of specific receptors on the surface
• Cells should have cellular factors capable
  of replicating virus
• Different viruses can bind the same receptor
• Viruses of the same family may bind different receptors

Question 17

Viral replication cycle

Answer 17

• Attach to the target of the epithelial cell
• Cell engulfs the virus by endocytosis
• Viral contents are then released
• Viral RNA enters the nucleus which is replicated by the viral RNA polymerase
• Protien synthesis them protien maturation and viron assembly
• Release new viral particle into extracellular fluid not killed in the process

Question 18

Viral attachment

Answer 18

• Virus adheres to cell surface with no specificity
• Specific receptors on cell surface by envelope
  glycoprotein or protruding capsid protein

Question 19

Virus penetration

Answer 19

• By fusion of viral envolope and host cell membrane
  mediated by viral fusion proteins if envalope is present
• Coreceptor binding induces exposure of the gp41 fusion peptide, which inserts itself into the host cell membrane
• Tethers the viral and host membranes and promotes their fusion

Question 20

Naked virus penatration

Answer 20

• Enter cell via endocytosis
• Receptor binding triggers the process of endocytosis
• Cell membrane is pinched off
• The virus is engulfed
• A vesicle is formed

Question 21

Virion uncoating

Answer 21

Disassembly of the virion to release the genetic
material in the nucleus of the cell

Question 22

Virus genome replication and protein synthesis

Answer 22

• Viruses hijack the host cell synthesis machinery to:
• Replicate their genome
• Produce their proteins

Question 23

Assembly of NEW virions

Answer 23

• Many capsid subunits are synthesised via endomembranous system, envelope glycoproteins are synthesised by the cell and sent to the plasma membrane
• Capsid subunits assembled together to form a capsid

Question 24

Release of new virions

Exocytosis/Budding

Answer 24

• Completed viral particles exit the cell and can infect other cells
• Enveloped viruses new capsid particles bind the membrane
• The membrane pinches off and viruses acquire the envelope and there are released. Cells are not destroyed

Question 25

Release of new virions

Lysis – Naked viruses

Answer 25

Viruses are released only when the cell dies

Question 26

Prions

Answer 26

• Don’t contain nucleic acids causes transmissible spongiform encephalophaties
• May cause fatal neurological diseases

Question 27

Spongiform encephalopathies

Answer 27

• Long incubation periods
• Impairment of brain function
• Mental and physical abilities rapidly deteriorate
• Always fatal
• Many tiny holes appear in the cortex causing it to appear like a sponge

Question 28

Defence against pirons

Answer 28

• Extremely resistant to standard sterilization techniques cannot autoclave at 121
• Requirement to autoclave at 134 degrees