m2- unit 3

• Viruses


  • They are the smallest microorganisms, ranging in size from 10 to 400 nm in diameter
  • They are acellular, i.e. not cellular and non living
  • They only reproduce when present within living cells
  • They are infectious agents
  • A complex virus particle or virion consists of one or more molecules of DNA or RNA enclosed in a coat of protein
  • Viruses can exist in two phases: extracellular and intracellular
  • The extracellular phase known as virion possesses few if any enzymes and cannot reproduce independent of living cells, it is metabolically inert and does not carry out respiration
  • In the intracellular phase, viruses exist primarily as replicating nucleic acids in the host cells that induce host metabolism to synthesise virion components which are later released
• Virus structure


  A virus is made up of a central genetic nucleic acid molecule surrounded by a protein coat called a capsid. The combination of both is called the nucleocapsid. The capsid surrounds and protects the viral nucleic acid. The capsid also gives the virus a characteristic shape and help to establish the specificity of the virus for a particular host cells. Capsids are large macromolecular structures that self assemble from many copies of one or a few types of proteins. The protein used to build the capsids is called protomers. The simplest virus is a naked virus (nucleocapsid) consisting of a geometric capsid assembled around a nucleic acid. On the other hand, we can have a virus made up of a nucleocapsid surrounded by a flexible membrane called an envelope. This type of virus is called an envelope virus.
• Viral genomes


  Viruses have either DNA or RNA genomes (one group of viruses does use both DNA and RNA as their genetic material but at different stages of the replication cycle). Virus genomes can be classified based on whether the nucleic acid in the virion is DNA or RNA and further subdivided to whether the nucleic acid is single or double stranded. Linear or circular, some viral genomes are circular but most are linear. We can have single stranded DNA, double stranded DNA, single stranded RNA and double stranded RNA. All four types are found in animal viruses. Most plant viruses have single stranded RNA genomes and most bacteria viruses contain double stranded RNA.
• Virus reproduction


  1. Attachment of the virion to a susceptible host

  2. Penetration or entry of the virion or its nucleic acid into the host

  3. Synthesis of virus nucleic acid and protein by cell metabolism as directed by the virus

  4. Assembly of capsids and packaging of viral genomes into new virions

  5. Release of mature virions from the cell
• Virus cultivation


  Animal viruses are cultivated by inoculating suitable host animals or embryonated egg – fertilised chicken eggs incubated about 6 to 8 days after laying. More recently, animal viruses have been grown in tissue (cell) culture on monolayers of animal cells.
• Virus purification


  1. Differential and density gradient centrifugation

  2. Precipitation of viruses' particles

  3. Denaturation of contaminants

  4. Enzymatic digestion of host cells constituents