What is a Virus ?
  • a noncellular particle made up of genetic material and protein that can invade living cells

Structure of a Virus
  • Typical virus is composed of a core of nucleic acid surrounded by a protein coat called a capsid
  • The capsid protects the nucleic acid core
  • Depending on the virus, the nucleic acid core is either DNA or RNA but never both
  • The core may contain several genes to several hundred genes
  • Bacteriophages - a more complex structure occurs in certain viruses
  • It has a head region, composed of a capsid(protein coat), a nucleic acid core, and a tail
  • Multiply quickly
  • Viruses come in a variety of shapes
  • Vary in size from 20 to 400 nanometers (one nanometer is a billionth of a meter)


Specificity of a Virus
  • Usually, specific viruses will infect specific organisms
  • Viruses are capable of infecting every kind of organism, including mammals, birds, insects, and plants

Life Cycle of a Lytic Virus
  • In order to reproduce, viruses must infect a living host cell
  • Not all viruses invade living cells in the same way
  • Infection - virus is activated by chance contact with the right kind of host cell
  • The virus then injects its DNA into the cell, but the complete virus particle itself never enters the cell
  • Growth - the host cell cannot tell the difference between its own DNA and the DNA of the virus
  • The same enzyme RNA polymerase from the cell’s own DNA begins to make messenger RNA from the genes of the virus
  • This viral messenger RNA now acts like a molecular wrecking crew, shutting down and taking over the infected host cell
  • One viral gene actually produces an enzyme that destroys the host cell’s own DNA but does not harm the viral DNA
  • Replication - the virus uses the materials of the host cell to make thousands of copies of its own protein coat and DNA. Soon the host cell becomes filled with hundreds of viral DNA molecules
  • The DNA molecules help assemble new virus particles
  • The infected cell will burst and release hundreds of virus particles that may now infect other cells
  • Lytic Infection - causes cell membrane breakdown
  • Lysogenic Infection - the virus does not reproduce and lyse its host cell
  • Prophage - virus’s DNA inserted into the DNA of the host cell (viral DNA)

Prophage Activity
  • Presence of the phrophage can block the entry of other viruses into the cell and may add useful DNA to the host cell’s DNA
  • A virus may not stay in the phrophage form indefinitely
  • Factors such as change of temperature and availability of nutrients can turn on these genes and activate the virus

  • Contains RNA as their genetic information and are an important class of viruses
  • Their genetic information is copied backwards, from RNA to DNA instead of from DNA to RNA
  • Responsible for some types of cancer in animals and humans (ex. AIDS)

Viruses and Living Cells
  • Viruses depend on their hosts for respiration, nutrition, and all of the other functions that occur in living things, this viruses are parasites
  • Parasite - an organism that depends entirely upon another living organism for its existence in a way that it harms the organism

Origin of Viruses
  • Viruses are completely dependent upon living cells for growth and reproduction, and cannot live outside their host cells
  • First viruses may have evolved from the genetic material of living cells and have continued to evolve, along with cells they infect, over billions of years

Section Review

  1. What is a virus ? A non cellular particle that is made up of genetic materials and proteins and are able to invade living cells
  2. List and describe the parts of a bacteriophage. The bacteriophage consists of a head region, composed of a capsid (protein coat), a nucleic acid core, and a tail.
  3. Describe two methods of viral infection. -Lytic infection - the process of the cell membrane breakdown. -Lysogenic Infection - a dormant lytic infection, the virus’s DNA is inserted into the host cell but does not become active and does not lyse the cell
  4. How can a virus be helpful to its host ? The phrophage virus can block the entry of other viruses into the cell and may add useful DNA to the host cell’s DNA

Possess all the other characteristics of living things
(replicate, require energy, adapt, respond, display heredity)
Not composed of cells
Has 6 out of the 7 characteristics of living things
Acellular, do not contain no cytoplasm or cellular organelles
They can mutate
Viruses don’t grow and divide
Can reproduce quickly but only in living host cells
Majority of viruses either have DNA or RNA but not both