Introduction to Virology

Viruses

obligatory intracellular infectious agents of sizes ranging from 20 to 300 nanometere with an absolute dependence on living cells for their replication.

Dmitri Ivanovsky

Russian scientist, reported that tobacco mosaic disease could be transmitted from a diseased plant to a healthy one using filtered leaf extract from the diseased plant as inoculums.

Dmitri Ivanovsky

used Chamberland filter (designed in Pasteur laboratory by Charles

Chamberland) to filter leaf extract from tobacco plant infected with tobacco mosaic

2. Organelles

Viruses do not possess cellular organization and do not have organelles.

iii. Complex Symmetry

• Large viruses with large genome have complicated symmetry

• For example, poxviruses have complex symmetry which is neither icosahedral nor helical

ii. Helical Symmetry

• Single stranded RNA viruses such as paramyxoviruses, orthomyxoviruses and rhabdoviruses have helical symmetry

• The capsid is in form of a helix

• Helical viruses resemble long rods that may be rigid or flexible

i. Cubic/Icosahedral Symmetry

• Highly structured capsid in which capsomeres are arranged in form of an icosahedrons

• In the icosahedrons, there are 20 triangular faces and 12 apices/corners

• The capsomeres of each face form an equilateral triangle

capsomeres

contribute to the rigidity of the capsid and help protect the nucleic acid genome

• Icosahedral capsids

generally assembled in the host cell prior to incorporation of the viral nucleic acid

1. Cubic/Icosahedral

2. Helical

two basic symmetrical patterns which could be:

envelope

• Present only in some viruses

• It is made up of lipids

• It is derived from the plasma membrane of the host cell during the release of the virus from the cell by budding

ii. Capsid

• It is made up of proteins arranged in multiple almost identical units called capsomere

• It offers protection for the nucleic acid against adverse conditions

• It facilitates attachment and entry of the virus into host cell

ii. Capsid

• It possesses antigens used for virus identification in serological tests • It determines the symmetry of the virus

i. Viral Nucleic Acid

• This can either be RNA or DNA

• It contains the information necessary for directing the infected cells to synthesis virus specific proteins

i. Viral Nucleic Acid

• It may be single stranded or double stranded

• It may be linear or circular

i. Viral Nucleic Acid

• It may be positive sense or negative sense ( a positive sense nucleic acid possesses the same polarity as the mRNA and so can be translated directly into protein without first being transcribed)

i. Viral Nucleic Acid

• It may be a single piece or segmented

• It is haploid except in retroviruses in which it is diploid

• Helical viruses

resemble long rods that may be rigid or flexible

capsid of helical viruses

formed by the insertion of protein units between each turn of the nucleic acid helix.

membrane/matrix protein

stabilizing protein membrane beneath the envelope lipid bilayer. This is referred to as the

• Enveloped viruses

usually susceptible to detergent and are rendered non-infectiuos following damage to the envelope

flexuous helical nucleocapsid

always contained within a lipoprotein envelope

M-protein

may be rigid as in the case of bullet shaped rhabdoviruses or readily distorted as in orthomyxoviruses and paramyxoviruses