Virus Morphology

Morphology of Viruses

• A virus consists of nucleic acids and proteins.
• The viral genome contains genetic information for multiplication and is surrounded by a protein coat called the capsid.
• The capsid is made up of polypeptides called capsomeres.
• The genome and capsid are collectively called the nucleocapsid.

Structure of Viral Genome

• The nucleic acid can be RNA or DNA, double-stranded or single-stranded, linear or circular, and plus or minus polarity.
• The nucleic acid can be segmented or unsegmented.
• Positive sense nucleic acid encodes the information for making viral protein.
• Negative sense nucleic acid is made up of complementary base to those of a positive sense nucleic acid.

Function of Capsid

• The capsid protects the viral genome from external harmful factors.
• It introduces the viral genome into the host cells.
• The capsid is antigenic and specific for each virus.

Symmetry of Viruses

• Three types of symmetry exist for viruses: icosahedral or cubical symmetry, helical symmetry, and complex symmetry.
• The arrangement of capsomeres around the nucleic acid determines the symmetry.

Enveloped and Non-Enveloped Viruses

• Viruses can be either enveloped or non-enveloped.
• The envelope surrounds the nucleocapsid and is made of lipoprotein.
• The lipid part is from the host cell membrane, and the protein part is virus-coded.

Characteristics of Envelope Viruses

• Envelope viruses are susceptible to heat and lipid solvents like ether.
• The envelope is made up of peplomers, which are antigenic and can bind to surface receptors facilitating entry.

Viral Proteins

• There are two types of viral proteins: structural and non-structural proteins.
• Structural proteins are present in the coat of the viruses.
• Non-structural proteins are viral enzymes and haemagglutins.

Types of Viral Enzymes

• Four groups of enzymes have been detected in viruses:
  • Neuraminidase or sialidase
  • RNA polymerase (copies DNA into RNA)
  • Reverse transcriptase (RNA-dependent DNA polymerase transcribes RNA into DNA)
  • Enzymes of cellular origin

Classification of Viruses

• Various methods exist for classification of viruses, including Holmes Classification, which is based on host.