How did the viruses originated?

 

An infective agent that typically consist of a nucleic acid molecule in a protein coat, is too small to be seen by light microscopy, and is able to multiply only within the living cells of a host.

u  Latin word

u  Meaning poison

u  Smaller than bacteria

u  Smaller than the smallest cell

Can you find the size of viruses?

u  The name virus was given by Beijernick in 1898

u  He studied filtered  plant juices and

   found that  they caused  healthy plants to

   become sick .

1.      Viruses have non- living structures

2.      Non cellular structure

3.      Contain a protein coat called the capsid

4.      Viruses have nucleic acid core containing DNA or RNA

5.      Capable of reproducing only when inside a host cell

6.      Most viruses infect only specific host cells

7.      Viruses are inactive outside of the host cells

8.      Viruses cause many common illnesses/diseases

9.      Some viruses may cause some cancers like leukemia

Edward Jenner introduced the term virus in microbiology in 1798.

Edward Jenner noticed that milk maids who infected with cowpox develop immunity against smallpox.

Mechanisms of viral evolution:

Evolution:

u  The constant change of viral population  in

    the face of selective pressures.

u  Mutation

u  Recombination

u  Reassortment

u  Selection

u  Although viruses are smaller and simpler than the smallest cells, it is unlikely that they could have been the first living things.

u  Since viruses depend on living things. It is most likely that they developed after living cells.

u  The first viruses may have evolved from genetic material on living cells and continue to evolve , along with the cells they infect, over billon of years.

u  Regressive hypothesis(degeneracy  hypothesis)

u  Viruses are remnants of cellular organism.

u  Progressive hypothesis or cellular origin hypothesis

u  Some viruses may have evolved from  genetic elements

u  Coevolution hypothesis

u  Viruses may have evolved from complex molecules of protein

u   and nucleic  acid

Regressive hypothesis(degeneracy  hypothesis)

u  Reduction of a parasite

u  Facultative parasite become dependent upon  the host

u  Parasite looses gene and become more dependent on the host

u  Viral ancestors were cellular organisms similar to bacteria

Progressive hypothesis or cellular origin hypothesis

u  Viruses are derived from subcellular components and macromolecules that escaped from cell walls ad replicated inside the hosts

u  The progressive hypothesis states that viruses arose from the genetic material that gained the ability to move between cells

 

Coevolution hypothesis

This is also called the virus –first hypothesis and

proposes that viruses may have evolved from

complex molecules of protein and nucleic acid

 at the same time as cells first appeared on earth

 and would have been dependent on cellular

 life for billions of years. 

How did viruses originate

u  Tracing the origins of viruses is difficult because they do not leave fossils and because of the tricks they use to make copies of themselves within the cells they have invaded . Some viruses even have the ability to stitch their own genes into those of the cells they infect, which means studying their ancestry requires untangling it from the history of their hosts and other organisms.

Role of viruses

Viruses s have played key roles in shaping the history of life on our planet by shuffling and redistributing genes in and among organisms and by causing disease in animals and plants. Viruses have been the culprits in many human diseases, including smallpox ,flue ,AIDS and certain types of cancers.