A2.3

• Relatively few features are shared by all viruses: small, fixed size; nucleic acid (DNA or RNA) as genetic material; a capsid made of protein; no cytoplasm; and few or no enzymes.
• Viruses


  • Highly diverse in their shape and structure
  • Genetic material may be RNA or DNA, which can be either single- or double-stranded
  • Some viruses are enveloped in host cell membrane and others are not enveloped
• Virus examples


  • bacteriophage lambda
  • coronaviruses
  • HIV
• Lytic cycle of a virus

  1. Attachment to host cell

  2. Entry

  3. Replication of viral genome

  4. Transcription of virus mRNA and protein synthesis

  5. Maturation and assembly of viruses

  6. Release from host cell
• Lysogenic cycle of a virus

  1. Phage attachment to host cell

  2. Phage entry

  3. Integration of phage DNA into host genome

  4. DNA replication

  5. Cell division
• The diversity of viruses suggests several possible origins. Viruses share an extreme form of obligate parasitism as a mode of existence, so the structural features that they have in common could be regarded as convergent evolution. The genetic code is shared between viruses and living organisms.
• Rapid evolution in viruses

  • Reasons for very rapid rates of evolution in some viruses
  • Evolution of influenza viruses
  • Evolution of HIV
• Viruses are involved in one of two lifecycles: Lytic cycle - a cycle in which the virus attaches to the host cell and then lyse the cell on exiting using an enzyme call lysozyme. Lysogenic cycle - the virus becomes a resident in the host cell where its maintained in a dormant state.
• Viruses are Dependent on host for: Energy, Nutrients, Replication, transport
• Obligate parasite


  Need a host cell to reproduce
• Capsid


  Protein coat that surrounds the nucleic acid
• Enveloped Viruses


  • Viruses are classified as either enveloped or non-enveloped depending on the presence of a lipid bilayer membrane on the outer part of the virus
  • The bilipid bilayer usually comes from the host cells and is embedded with glycoprotein spikes
  • The glycoproteins bind to receptor molecules on a host cell and help the virus enter the cell
  • The envelope protects the virus from enzymes and other chemicals
• Non-enveloped Viruses


  • Since non-enveloped viruses don't have the lipid membrane, they destroy their host cell when existing in a process called cell lysis
  • They are more resistant to extreme pH, heat, dryness and simple disinfectants
• Examples of enveloped and non-enveloped viruses

  • Enveloped: HIV, Influenza, Coronavirus
  • Non-enveloped: Bacteriophage, poliovirus, Adenovirus
• Retrovirus


  Has an enzyme (reverse transcriptase) capable of making a complementary DNA copy of the viral RNA, which is then inserted into a host cells DNA
• HIV


  • Host: Primate T-cells with a CD4 receptor protein (a type of white blood cell)
  • Genetic material: Two copies of a single stranded RNA
  • Structure: Icosahedral capsid with an envelope
  • Life cycle: Lytic
  • Associated human disease: Acquired Immunodeficiency Syndrome (AIDS)
• Coronaviruses - SARS-CoV-2


  • Host: Mammalian epithelium cells with an ACE2 receptor protein
  • Genetic material: Single stranded, linear, positive sense RNA
  • Structure: Complex capsid composed of a helical nucleocapsid (proteins around the RNA) surrounded by an icosahedral (spherical) structure that has distinctive club shaped spikes
  • Enveloped
  • Life cycle: lytic
  • Associated human disease: Coronavirus Disease of 2019 (COVID-19)
• Bacteriophage lambda


  • Host: E. coli bacteria
  • Genetic material: Double stranded linear DNA
  • Structure: Complex capsid structure with an icosahedral head bound to a helical tail
  • Tail sheath consists of contractile proteins that contract to drive the tail tube through the host cells outer membrane
  • Tail fibres that attach to the host cell
  • Non enveloped
  • Life cycle: Alternates between lytic and lysogenic
  • Associated human disease: none
• Viruses are involved in one of two lifecycles: Lytic cycle or Lysogenic cycle
• Lytic Life Cycle of a Bacteriophage lambda

  1. Phage Attachment

  2. Entry of DNA

  3. DNA Replication

  4. Protein Synthesis

  5. Assembly of new phage Viruses

  6. Lysis

  7. Spread
• Lysogenic Cycle of Bacteriophage lambda

  1. Phage attachment to host cell

  2. Phage Entry

  3. Integration of phage DNA into host genome

  4. DNA replication

  5. Cell division
• If the bacterium is exposed to stressful conditions ie UV light, low nutrients or chemicals the prophage may spontaneously extract itself from the host genome and enter the lytic cycle in a process called induction.
• There are many different types of viruses. It seems unlikely they all share a common ancestor. It's hypothesized that they have arisen multiple times in the past. They are therefore considered polyphyletic in origin.
• Virus-First hypothesis


  The virus-first hypothesis proposes that viruses existed in a precellular world as self-replicating units. Perhaps, simple replicating virus molecules, existing before the first cell formed, developed the ability to infect the first cells.
• The virus first hypothesis is not supported by current evidence. Evidence against the virus-first hypothesis: All Viruses are intracellular parasites, requiring a host cell to replicate.
• Polyphyletic


  Having multiple origins, not sharing a common ancestor
• Hypotheses for the origin of viruses

  • Virus-first hypothesis
  • Virus evolved from cells
• Virus-first hypothesis


  Proposes that viruses existed in a precellular world as self-replicating units
• The virus-first hypothesis is not supported by current evidence
• All viruses are intracellular parasites, requiring a cell to replicate
• Viruses use the same genetic code as all cells
• Viruses and cells share a common ancestor - a fully functioning, self replicating cell that lived around 3.4 billion years ago
• Hypotheses for how viruses evolved from cells

  • Progressive hypothesis
  • Regressive hypothesis
• Convergent evolution


  Different lineages evolve similar features because of selection for function rather than inheritance from a common ancestor
• All viruses use nucleic acid as their genetic material
• Generation time


  The average time between two consecutive generations in the lineages of a population
• Viruses have a very short generation time
• Eclipse period


  The time between the entry of the virus genetic material into the host and the appearance of new mature virus in the host cell
• The eclipse period of SARS-CoV2 is 10 hours
• Virion


  Single virus particle