Lecture: An Intro to Viruses

Microbiology Lecture 10-17-19

The search for viruses, Louis Pasteur postulated that rabies was caused by a “living thing” smaller than bacteria(1884), he developed the first vaccine for rabies.

Ivanovski and Beijerinkck showed a disease in tobacco was caused by a virus (1890s).

1950’s virology was multifaceted discipline, viruses are non-cellular particles with a defintie size, shape , and chemical composition.

Position of Viruses in the Biological Spectrum

There is no universal agreement on how and when viruses originated?

Viruses are considered the most abundant microbes on earth, and play a role in the evolution of Bacteria, Archaea, and Eukarya.

Are obligate intracellular parasites.

General Size of Viruses – ranges most < 0.2 um, requires electron microscope.

Viral Structure – bear no resemblance to cells and lack protein synthesizing machinery.
Viruses contain only the parts needed to invade and control a host cell.

General Structure of Viruses

Caspids – all viruses have capsids, protein coats that enclose and protect their nucleic acid. The capsid together with the nucleic acid is the nucleocapsid .

Two types of Capsid: Helical – continuous helix of capsomers forming a cylidrical nucleocapsid, examples include the Tobacco mosaic viruse, influenza, measles, and rabies .

Second type – Isocahedra – 20 sides with 12 corners, vary in the number of capsomers, some are enveloped, examoles – Papiloma virus.

Influenza virus is an enveloped helical RNA virus that causes the flu.

Ratoviruses, a type of naked RNA virus that causes diarrheal disease, and can be fatal.

Herpes simplex virus, a type of enveloped DNA virus that causes cold sores.

Viral Envelope – mostly animal viruses, and are acquired when the virus leaves the host cell. Exposes protein on the outside of the envelope, called spikes, are essential for attachment of the virus to the host cell.

Function of Capsid/Envelope

  • Protects the nucleic acid when the virus i soutside of the host cell
  • Helps the virus bind to a cell surgace
  • Assists the penetration of the viral DNA or RNA into a suitable host cell.
  • Parts of viral capsids and envelops stimulate the immune systenm to protect the host cells from future infections.

Atypical Viruses

  • Poxvirues lack a typical capsid and are covered by a dense layer of lipoproteins
  • Some bacteriophages have a polyhedral nucleocapsid along with a helical tail and attachment fibers

Nucleic Acids contain the Viral genome of either DNA or RNA but never both.

  • Carrises genes necessart to invade host cell and redirect cells activity to make new viruses. Number of genes varies.

DNA viruses, usualy double stranded but may be single stranded, circular or linear.

RNA viruses usually single stranded, may be double stranded, may be segmented into separte RNA pieces.

  • ssRNA genomes ready for immediate transaltion are positive-sense RNA.
  • ssRNA genomes that must be converted into proper form are negative sense RNA, not used immediately.

Matrix Protein Enzymes

Pre formed enzymes may be present

  • Polymerases – DNA or RNA
  • Replicases – copy RNA
  • Reverse transcriptase – synthesis of DNA from RNA (AIDS virus)

Informal Classification of Viruses

On the basis of:

  • Host infected – animal, plant or bacteria viruses
  • General structure – naked or enveloped
  • Chemical composition – DNA or RNA
  • Capsid arrangement – helical or icosahedral viruses
  • Disease produced – poliovirus, rabies virus, etc.

Newer Systems of Classification

  • Main criteria preesently used are structure, chemical composition, and genetic makeup.
  • Currently recognized; 7 orders, 96 familes, and 350 genera of viruses
  • Family name ends in -viridae,
  • Genus name ends in -virus, Simplexvirus
  • Herpes simples virus I (HSV-I)

Animal Virus Multiplication

  • Absorption – binds to the surface of the host cell.
  • Penetration – Genome enters the host cell
  • Uncoating – the viral nucleic acid is release from the capsid
  • Synthesis – viral components are produced.
  • Assembly – new viral particles are constructed.
  • Release – assembled viruses are release by budding (exocytosis) or cell lysis

Host Range of Viruses

  • Specturm of cells a virus can infect is called host range.
  • May infect one species or many, HIV infects only humans vs Rabies can infect many animals.
  • May infect one tissue or many within a host , Hepatitis (liver) vs Poliovirus (primate intestinal and nerve cells)

Replication and Protein Production

  • Varies depending on whether the virus is a DNA or RNA virus.
  • DNA viruses generally are replicated and assembled in the nucleus.
  • RNA viruses general replicated and assembled in the cytoplasm.
  • Positive-sense RNA contain the message for translation.
  • Negative-sense RNA must be converted into positive-sense message.


  • Budding – exocytosis, nucleocapsid binds to membrane which pinches off and sheds the viruses gradually, cell is not immediately destroyed.
  • Lysis – non-envolped and complex viruses release when cell dies and ruptures.

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