Viruses [structure mainly]

What are characteristics of living organisms? (7)

• Have cellular organisation

• Show metabolic activity

• Reproduce

• Grow and develop

• Have a common hereditary molecule

• Respond to stimuli

• Adapt to the environment

Why could viruses be regarded as living organisms? (5)

• Possess genetic material capable of propagating genetic information

• Inside host cell

  • Directs host enzymes to carry out metabolic processes

  • Reproduce by creating multiple copies of self

  • Undergo mutation and reassortment of genetic material during replication → New viral strains

  • React and adapt to environment

Why could viruses be regarded as non-living organisms? (6)

• Acellular and lack cellular organelles

• Without a host cell

  • Do not carry out metabolism

  • Lack the ability to reproduce on their own

  • Do not grow

  • Do not respond to stimuli

  • Do not by natural selection

What can viruses be described as?

Obligate parasites: Require a living host to support many functions

What do viruses mainly consist of?

• Genome

• Glycoprotein

• Capsid

• Envelope

What is the structure of the genome?

• Single or segmented, circular or linear molecules of nucleic acid that function as the virus genetic material

• DNA or RNA (never both), single-stranded or double-stranded

What is the function of a genome?

Code for the synthesis of viral components and enzymes for replication and assembly of a virion

What is the structure of the capsid?

• The protein coat surrounding the viral nucleic acid

• Capsid + nucleic acid = nucleocapsid

• Composed of capsomeres (protein subunits)

What is the function of a capsid?

Protect, attach and introduce genome into host cells

What is the structure of the envelope

• Surround the nucleocapsid

• Phospholipids and glycoproteins arranged to form a lipid bilayer

• Derived from the host cell membrane via budding

  • Virus incorporates glycoprotein spikes of its own into the envelope

What are animal viruses that only consist of the nucleocapsid?

Naked viruses / non-enveloped viruses

What are the 4 main morphologies of viruses?

• Helical

• Icosahedral

• Enveloped

• Complex

How does a virus replicate?

1. Infects a specific host

2. Uses host metabolism and machinery to synthesize own nucleic acid

   1. Hydrolyses DNA of host cell and uses DNA polymerase of host to replicate own DNA

   2. Uses host’s RNA polymerase for transcription and ribosomes for translation

   3. Host’s tRNA, nucleotides, amino acids, ATP

3. New viral particles formed through self-assembly

4. Viral particles exit through budding, exocytosis or lysis of host

What are antigens?

Specific molecular structures that antibodies in our immune systems recognise when they bind to immune receptors

• Vaccines work by enabling the immune system to recognize the antigens

What is antigenic drift?

Accumulation of mutations in the genes encoding surface glycoproteins of the virus → Surface antigens or glycoproteins that have a different conformation from the previous virus strain → Cannot be recognized by antibodies

How does antigenic drift occur in influenza?

• Small mutations on haemagglutinin and neuraminidase genes that make the protein unrecognizable to the pre-existing host immunity

• Results from the lack of proof-reading ability of RNA-dependent RNA polymerase + fast rate of viral replication

What is antigenic shift?

Sudden and major change in the surface antigens of the virus ← Genetic reassortment that confers a phenotypic change

How does antigenic shift occur?

2 or more different strains of a virus or strains of 2 or more different viruses combine to form a new subtype that has a mixture of the surface antigens of the two or more original strains

How does antigenic shift come about in influenza?

Genome is segmented → Major genetic changes by reassortment of segmented RNA genome

1. 2 different strains of influenza infect the same cell simultaneously

2. Their protein capsids and lipid envelopes are removed, exposing their RNA which is then transcribed into mRNA

3. Host cell forms new viruses that combine their antigens (e.g. H3N2 + H5N1 → H5N2)