VMC 211: General Veterinary Microbiology (1+1)

SYMMETRY

• Viruses show basically four types of symmetry, icosahedral, helical.complex and binal symmetry.  

Icosahedral

• An icosahedran is a polyhedran made of 20 triangular faces with 5 each at the top and bottom and 10 at the middle. There are 12 vertices and 30 edges. Each triangle is symmetrical and can be inserted in any orientation which ever way it is inserted. The vertices have five fold symmetry. This means that rotation of the icosahedran by one fifth of a revolution presents a position which is indistinguishable from the starting orientation. Each of the 20 faces has a three fold axis of symmetry and each of the 30 edges has a two fold axis of symmetry.

Helical

• In this type of symmetry the capsomeres are set in a helical manner around the genome nucleic acid.

http://pathmicro.med.sc.edu/int6.jpg

Complex

• Certain viruses have complex symmetry. Eg: pox viruses have the most complex virion structure containing different proteins and lipoproteins.

http://pathmicro.med.sc.edu/int7.jpg

Binal symmetry

• Certain viruses have binal symmetry. Eg: bacteriophages - viruses which infect bacteria. They have an icosahedral head with  a helical body.  

http://www.ucmp.berkeley.edu/alllife/virus.gif, http://pathmicro.med.sc.edu/int8.jpg