by Microbiology Doctor dr (doctor_dr)(doctor-dr) plant diversity
VIRAL SYMMETRYNegative staining or negative contrast technique is used to observe viral symmetry. A beam of heavy metal atoms is allowed to fall on the virus particle at an. angle, the particle is then put in a vacuum chamber and metal atoms are evaporated, the particles are shadowed or stained and become visible under electron microscope. Now the detailed structure of viruses can be observed because the stain can penetrate between, and show up.
Negative staining revealed that viruses fall under three symmetry groups:
i. Icoshedral Symmetry
The viruses exhibiting iscoshedral symmetry are formed of almost identical Atriangular icoshedron units arranged in clusters of fives or sixes. An icosahedron has 20 triangular faces with 12 corners and 30 edges. Each of 20 faces are made up of a number of capsomeres. Most spherical viruses exhibit this symmetry. The examples are polioviruses, adenoviruses and herpesviruses which cause poliomyelitis, respiratory infections and herpes.
ii. Helical Symmetry
The viruses with helical symmetry contain RNA core covered by capsid formed of closely packed cpasomeres arranged in a regular helix. Most plant viruses and few animal viruses, exhibit helical symmetry. Most of these viruses are rod shaped. The best studied viruses exhibiting this type of symmetry are tobacco mosaic virus (TMV). Its nucleic acid core consists of RNA, which is covered by a capsid consisting of closely packed capsomeres arranged in a regular helix Animal viruses with capsid displaying helical symmetry include measles, mumps, influenza and rabies. In these viruses, the nucleocapsid is a flexible structure packed within a lipoprotein envelope.
iii. Complex-Structured Viruses
These are viruses with complex or uncertain symmetries. For example, the smallpox virus, have the most complex virion structure. They consist of many different proteins and lipoproteins:
Figure: different symmetry groups found in viruses
