All Materials © Cmassengale

Discovery of Viruses

  • Beijerinck (1897) coined the Latin name “virus”  meaning poison for  the substance infecting tobacco plants
  • Wendell Stanley (1935) crystallized sap from tobacco leaves infected with Tobacco Mosaic Virus (TMV) & found virus was made of nucleic acid & protein


Wendell StanleyTobacco Leaf with Virus


  • Edward Jenner developed smallpox vaccine using milder cowpox viruses
  • Virology – study of viruses
  • Deadly viruses are said to be virulent
  • Viruses couldn’t be seen until electron microscope invented

Viral Characteristics

  • Not living organisms
  • Noncellular
  • Consist of a nucleic acid core (DNA or RNA) and a protein coat called the capsid
  • Capsid made of protein subunits called capsomeres
  • Cannot grow or replicate on their own (inactive particles)



  • Can only reproduce inside of a living host cell using its raw materials & enzymes
  • Lack ribosomes & enzymes needed for protein synthesis or metabolism
  • Are extremely small particles ranging from 20 – 400 nanometers on average
  • Largest virus is 1000 nanometers in dimension
  • Some can cause disease (smallpox, measles, mononucleosis, influenza, colds, AIDS, Ebola

Ebola Picture
Ebola Virus

  • Some may also cause cancers such as leukemias
  • Virus free cells are rare
  • Highly host specific (only infect certain cells)
  • Referred to as phages
  • Viruses are classified into 2 main groups by their nucleic acid — DNA or RNA Viruses
  •  DNA & RNA viruses are subdivided by capsid shape & whether they do or don’t have an envelope

Viral Structure

  • DNA or RNA core surrounded by protein sheath called capsid
  • Nucleocapsid  includes the viral nucleic acid & its capsid
  • Some form lipid rich covering around capsid called the envelope
  • Envelope usually formed from host cell membrane
  • Envelope may have spikes to help chemically recognize & attach to the host cell
  • Shaped determined by the arrangement of proteins making up the capsid
  • TMV is rod shaped

  • Adenovirus & polio viruses are icosohedral (20 sided)

Virus Structure

  • Measles & rabies viruses are helical
  • T -phages have a head & tail

Bacteriophage Structure

Bacteriophages or T-Phages

  • Among the most complex viruses
  • Attack bacterial cells
  • Composed of a icosohedral head, tail, base plate, & tail fibers
  • Long DNA molecule is inside the head 
  • Tail helps inject the viral DNA into host cell
  • Tail fibers used to attach to host


  • Contain RNA
  • Have an enzyme called reverse transcriptase which helps use the RNA to make DNA
  • Use the host cell’s ribosomes & raw materials to make viral proteins
  • Cause some cancers & AIDS

HIV Virus


  • Smallest particle able to replicate
  • Made of a short, single strand of RNA with no capsid
  • Cause disease in plants

Viroid Attack on Potatoes


  • No nucleic acid or capsids
  • Made of protein particles that have folded incorrectly
  • Attacks the central nervous system
  • Cause animal diseases in cows (Mad Cow disease), sheep, & humans

Lytic Cycle

  • Viral replication that rapidly kills the host cell causing it to lyse or burst
  • Involves 5 steps —– Adsorption, Injection, Replication, Assembly, & Lysis
  • Adsorption — phage attaches to cell membrane of host
  • Injection — nucleic acid (DNA) of virus injected into host cell
  • Replication — viral DNA inactivates host cell’s DNA & uses host’s raw materials & ribosomes to make viral DNA, capsids, tails, etc.
  • Assembly — new viral parts are combined to make new phages
  • Lysis — enzymes weaken & destroy the cell membrane causing it to lyse releasing new viruses that infect other cells


Phases of the Lytic Cycle of a Virulent Virus:

  • Absorption:
    1. Virus attaches itself to the cell.
  • Entry:
    1. Enzymes weaken the cell wall and nucleic acid is injected into the cell, leaving the empty caspid outside the cell. Many viruses actually enter the host cell intact.
  • Replication:
    1. Viral DNA takes control of cell activity.
  • Assembly:
    1. All metabolic activity of the cell is directed to assemble new viruses.
  • Release:
    1. Enzymes disintegrate the cell in a process called


    , releasing the new




Lysogenic Cycle

  • Replication in which the virus stays inactive inside of the host cell & doesn’t immediately kill it
  • Viruses are called temperate phages
  • Lysogenic steps include adsorption, injection, recombination, cell reproduction, activation, replication, assembly, & lysis
  • Recombination —Viral DNA joins with host cell DNA forming an inactive prophage
  • Host cell reproduces  normally until activated by an external stimuli 
  • External stimuli unknown, but could be ultraviolet radiation, carcinogens, etc.
  • Once activated, prophage forms new viruses & destroys host cell
  • HIV is an example of a temperate phage


The Lysogenic Cycle of a Temperate Virus:

  • The virus attaches itself and injects its DNA into the cell.
  • The viral DNA attaches itself to the host DNA, becoming a new set of cell genes called a prophage.
  • When the host cell divides, this new gene is replicated and passed to new cells. This causes no harm to the cell, but may alter its traits.
  • Now there are two possibilities:
    • The prophage survives as a permanent part of the DNA of the host organism.
    • Some external stimuli can cause the prophage to become active, using the cell to produce new viruses.





Viral Control

  • Interferon are proteins made by cells to fight viruses
  • Two types of viral vaccines exist — inactivated & attenuated
  • Inactivated virus vaccines don’t replicate in the host’s system
  • Attenuated viral vaccines have been genetically altered so they can’t cause disease
  • Antiviral drugs (AZT, acyclovir, & azidothymidine) interfere with viral DNA synthesis
  • Protease Inhibitors interfere with viral capsid production
  • New viruses emerge as rain forests are cleared (Ebola virus)