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