Question

What are the three main symmetries of viruses?

Answer 1

Hint: Virus is an obligate intracellular parasite. It may have either RNA or DNA as genetic material. They are classified depending upon the mode of replication, chemical composition, the structure of genome, size, shape and structure.

Complete step-by-step answer:
Viruses differ in their structures. The DNA or RNA particles are present within the protective coat called a capsid. The capsid made up from the proteins and proteins is encoded by a viral gene within the genome. The shape of the capsid may differ from one to another virus. According to the shape of capsid the virus can be classified as-
A) Helical viruses:
Made up of a single type of protein subunit stacked and arranged around the central axis to form a helical shape.
Helix is hollow from inside and looks like a hollow tube and results in filamentous virions or rod-shaped.
Virion may vary from long to short and rigid to flexible.
RNA is the genetic material which wound in a spiral and positioned towards the capsid.
For example – Tobacco mosaic virus
B) Icosahedral viruses:
Proteins are arranged in a regular geometrical pattern similar to a soccer ball.
Only a few gene codes for protein to form a capsid.
Capsids are constructed from a knob-shaped or ring-shaped unit called capsomers. Each capsomer is made from 5-6 promoters which are joined by non-covalent bonding.
For example – cowpea chlorotic mottle virus, poliovirus
C) Complex symmetry
Most of the viruses neither have helical capsid nor icosahedral capsid. They are not fit into either of the categories.
Have extra structure such as protein tail or complex outer wall.
Some bacteriophages for example enterobacteria have icosahedral head bound to helical tail that has a hexagonal base plate with protruding protein tail fibre. This structure acts as a syringe and helps to enter the bacterial cell.
Examples of complex symmetry viruses are bacteriophages, poxviruses, etc.

Note: The complex symmetry viruses are difficult to understand because they either fall in any of the categories. The viruses under this category have different complex structures. For example, poxvirus is ranging from ovoid to brick shape, whereas the bacteriophages have icosahedral head and helical tail. This complexity can be understood by studying the structure of each virus.