Morphology and Structure of Viruses

Morphology and Structure of Viruses

Definition of Viruses

Virus is a smallest infectious particle, Unicellular, Obligate Intracellular parasite. Lets see Morphology and structure of viruses in detail.

Characters of Viruses

  1. Size of Viruses
  2. Shape of Viruses
  3. Structure of viruses – It consist of Capsid , nucleic acid and envelop.

1. Size of Viruses

Viruses are smaller than bacteria and they are filterable.

Size of virus is measured by using

  • Ultrafiltration
  • Ultracentrifugation
  • Electron Microscopy

Size of viruses ranges from 20 nm to 300 nm.


  • Size of Parvovirus is 20 nm and it is the smallest virus.
  • Size of Pox virus is 300 nm and it is the largest virus.

2. Shape of Viruses

There are different shapes of viruses, mostly all animal viruses are roughly spherical but there are also different shapes of viruses.

Different shapes of viruses and their examples

  • TMV Virus is Rod Shape
  • Pox Virus is Brick Shape
  • Rabies Virus is Bullet Shape
  • Corona virus is Petal shape
  • Rota Virus is Wheel Shape
  • Astrovirus is Star Shape
  • Ebola Virus is Filamentous in Shape

3. Structure of Virus

  • Virus is mainly made up of two components and that is Nucleic acid and Capsid But some viruses may also posses envelop.
  • Nucleic acid is enclosed with a protein coat called as capsid and this capsid may be surrounded by a lipoprotein layer called as a envelop.
  • Envelop is derived from host cell membrane so chemical structure of envelop of viruses is similar to the cell membrane of a host cell.

Lets see the structural components in detail .

1. Capsid
Capsid is made up of a protein coat called as capsomere. The chemical unit of capsomeres are polypeptide molecule.
Capsid shows two types of symmetry and that are

a) Icosahedral Symmetry

b) Helical symmetry

a) Icosahedral Symmetry 

  • It is made up of 12 vertices and 20 sides.
  • It shows two types of capsomeres.
  • The 12 vertices made up of pentagonal capsomeres. The number of pentons are fixed that is 12.
  • The 20 sides are made up of hexagonal capsomeres. The number of hexons may vary.

b) Helical symmetry

Here Nucleic acid and capsomeres are bound together to form helical structure. It may get coiled on itself or the helical structure may be rigid.

Some viruses may show complex symmetry.

2. Nucleic acid

Viral cell have only one type of Genetic material that is it may contain either DNA or RNA as a genetic material. Viruses do not contain the enzymes that are important for replication and multiplication of viruses. So virus cells are dependent on host cell for its multiplication and replication.

DNA viruses – All DNA viruses are double stranded except parvovirus. Parvovirus has single stranded DNA.

  • Examples of DNA Virus – Vaccinia virus, Human papilloma virus, Parvovirus, Herpes simplex, Hepatitis B virus, Adenovirus.

RNA virus – All RNA viruses have a single copy of single stranded, unsegmented RNA.

  • Examples of RNA Virus – Mumps virus, Measles virus, Influenza Virus A,B, C, Rotavirus, Reovirus, Human T Lymphotropic Virus, HIV etc.

3. Envelop

  • Envelop is the outermost covering of viruses.
  • The Viruses that contains envelop are called as envelop viruses.
  • The viruses in which envelop is absent are called as non – envelop viruses.
  • Envelop of viruses is made up of lipoprotein molecules and this envelop is derived from host cell membrane during lysis of cell.
  • The chemical composition of envelop is similar to host cell membrane as the envelop is derived from host cell.

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