Viruses

 

What is Virus?

¨Viruses are obligate intracellular parasites.

¨Not cells, do not have nuclei or other cellular components.

¨Viruses replicate or multiply, do not grow.

¨ Replicate or multiply only within living cells.

qIts genome, which is composed of either DNA or RNA, is enclosed in a protein coat.

 

Discovery of Virus

qBeijerinck (1897) coined the Latin name “virus” meaning poison.

q He studied filtered plant juices of Tobacco & found that it infects healthy plants also (Transmissible).

q Wendell Stanley (1935) crystallized filtrate from sick tobacco plants.

q He discovered that viruses are made of nucleic acid and protein.

 

Size of Virus

¨Virus is smallest particle, Measured in nanometers

¨Size range – 5 to 300 nanometers (nm)

¨E.g.  Rabies virus 125nm

¨HIV, 110nm

¨Influenza, 100nm

¨Poliomyelitis, 30nm

 

 

Characteristics

qNonliving structure

qNon cellular or Subcellular particle

qContain a protein coat called the capsid

qHave a nucleic acid core containing DNA or RNA

qCapable of reproducing only when inside a HOST cell.

qSome viruses are enclosed in a protective envelope

qSome viruses may have spikes to help attach to the host cell

qMost viruses infect only SPECIFIC host cells

 

 

q    Naked viruses

qWithout envelope.

qContain only a protein coat (Capsid).

qCapsid made up of identical subunits called capsomers

q The capsid together with the nucleic acid are nucleocapsid.

 

 

q    Enveloped Viruses

qSome viruses contain an outer envelope with capsid

qEnvelope made up of Lipid bilayer, derived from host plasma membrane and viral glycoproteins.

 



Capsid Types

¡ Helical - continuous helix of capsomers forming a cylindrical nucleocapsid

¡ Icosahedral - 20-sided with 12 corners


 

Nucleic Acids

¨DNA viruses 

¡ usually double stranded (ds) but may be single stranded (ss)

¡ circular or linear

¨RNA viruses

¡ usually single stranded, may be double stranded, may be segmented into separate RNA pieces

¡ ssRNA genomes ready for immediate translation are positive-sense RNA.

¡ ssRNA genomes that must be converted into proper form are negative-sense RNA.

 

 

Baltimore Classification

Ø Classification (first defined in 1971), divided viruses into seven groups, On the basis of

Ø Nucleic acids (DNA or RNA)

Ø Strandedness (Single stranded or double)

Ø Sense (Positive or Negative sense)

Ø Method of Replication

Ø Viruses can be placed in one of the seven following groups:

Ø I: dsDNA viruses (e.g., Adenoviruses, Herpesviruses, Poxviruses)

Ø II: ssDNA viruses (+) sense DNA (e.g., Parvoviruses)

Ø III: dsRNA viruses (e.g., Reoviruses)

 

¨IV: (+) ssRNA viruses (+) sense RNA (e.g., Picornaviruses, Togaviruses)

¨V: (−) ssRNA viruses (−) sense RNA (e.g., Orthomyxoviruses, Rhabdoviruses)

¨VI: ssRNA-RT viruses (+) sense RNA with DNA intermediate in life-cycle (e.g., Retroviruses)

¨VII: dsDNA-RT viruses (e.g., Hepadnaviruses)