11 Viruses

Question 1

Viruses

Definition

Answer 1

Obligate intracellular parasites

Question 2

Viruses

Reasons behind definition

Answer 2

• Can only survive in a living host cell
• Need to hijack host cell machinery to carry out metabolic processes eg. respiration
  
  • Do not have raw materials eg. nucleic acids for DNA/ RNA replication; amino acids, ribosomes for protein synthesis & ATP

ie. Metabolically inert virions in extracellular conditions; metabolically active & can reproduce in intracellular conditions

Question 3

Viruses

Types

Answer 3

• Bacteriophages (typically contains DNA genome)
  
  • Virulent, lytic: T4
  • Temperate, lysogenic: Lambda
• Animal (typically contains RNA genome)
  
  • Enveloped: Influenza
  • Retrovirus: HIV

Question 4

Viruses

General Structure + Purpose

Answer 4

• Genome: codes for both structural & regulatory proteins [ALL]
  
  • DNA or RNA
  • ss- or ds-
  • linear or circular
• Capsid = protein coat of capsomeres: protects Genome [ALL]
  
  • Helical or icosahedral etc.
• Envelope = phospholipid bilayer: helps with Attachment & Adsorption during infection of host cell
  
  • contains Glycoproteins
  • made of host cell cell surface membrane
• Enzymes: aid entry & exit of host cell/ replication & transcription of viral genome
  
  • Lysozymes: breakdown of bacterial peptidoglycan cell wall
  • Neuraminadase: aids release of viral progeny
  • RNA-dependent RNA polymerases (eg. viral replicase within Influenza)
  • RNA-dependent DNA polymerases (eg. reverse transcriptase within HIV)

Question 5

Viruses

General Mechanism

Answer 5

1. Adsorption/ Attachment
   → Recognision & binding of viral glycoproteins to host cell receptors on the cell surface based on complementary 3D conformation
2. Penetration/ Entry
   → Engulfing of virus (genome) into a vesicle
   → Release of viral genome
3. Replication & Synthesis
   → Take over of host cell machinery & raw materials for replication & transcription of viral genome, viral protein synthesis
   → Synthesis of capsomeres, glycoproteins, enzymes etc. occur
4. Assembly
   → Formation of viral nucleocapsides from viral nucleic acids & capsomeres
   → Embedment of glycoprotines into host cell membranes
5. Release
   → Lysis of host cell in case of phages → death of host cell
   → Budding off from host cell cell surface membrance in case of animal viruses → With ↑ budding, lysis can occur

Question 6

Bacteriophages

T4 Phage → Stucture

Question 7

Variation within Viruses

Antigenic Drift + Cause

Answer 7

1. Lack of proof-reading mechanism in polymerases eg. replicase/ RNA dependent RNA polymerase & reverse transcriptase
2. Single-stranded RNA genome → Lack of backup complementary strand which serves as template for repair
3. ↑ error rates during viral genome replication → spontanous mutations in genomes
4. Δ in genome → Δ in mRNA → Δ in amino acid sequence → Δ in 3D conformation of glycoprotein → Δ in surface antigens
5. Accumulation of mutations within genes coding for surface glycoproteins
6. Surface glycoproteins ie. antigens have a different conformation from previous strains
7. New strain cannot be recognised by antibodies produced during previous infection
8. ↑ ease of infection & spread throughout partially immune population

Question 8

Variation within Viruses

Antigenic Shift + Cause

Typically applied to Influenza

Answer 8

1. Segmented genome of virus enables for major genetic changes of type through reassortment of genome
2. ≥ 2 different strands of a virus/ strains of different viruses infect the same cell simultaneously
3. Viral protein capsids & lipid envelopes removed
4. Genome is exposed & is transcribed into mRNA
5. Host cell packages the 2 different strains such that antigens undergo recombination
6. New combination of viral glycoproteins with different specific 3D conformation
7. Immune system cannot recognise new recombinant strand of virus & do not have antibody protection
8. Further evolution of virus can cause ↑ spread of virus within host cells or virus to jump to a different house