A virus is a microscopic infectious agent that is incapable of independent life and requires a host cell to replicate. Viruses are made up of genetic material (either DNA or RNA) enclosed in a protective protein coat called a capsid. Some viruses also have an outer lipid membrane called an envelope. Unlike other living organisms, viruses lack cellular structure and cannot carry out metabolic processes or energy production on their own. They are considered acellular (without cells) and are sub-microscopic, meaning they are too small to be seen with a light microscope.
General Characteristics of Viruses:
- Acellular Nature: Viruses lack cellular structure.
- Sub-microscopic and Intracellular: Viruses are too small to be seen with a light microscope and require host cells to replicate.
- Lack of Lipid Membrane and Energy Production: Viruses do not have a lipid membrane system and cannot produce energy on their own.
- Dependence on Host Machinery for Replication: Viruses use the host cell’s machinery to replicate and synthesize their components.
Structure of Viruses:
- Virion: The term “virion” refers to a complete virus particle, consisting of:
- Nucleic Acid (Nucleoid): Can be either RNA or DNA, but not both. The nucleoid contains the genetic material for the virus.
- Capsid: The protein coat surrounding the nucleoid, which protects the genetic material, facilitates host cell entry, and provides the virus with its shape. The capsid consists of subunits called capsomeres.
- Nucleocapsid: The combination of the nucleoid and capsid.
- Enveloped Viruses: Some viruses have an additional lipid layer, derived from the host, surrounding the protein coat.
- Virus Core: In some viruses, an additional protein layer is found between the capsid and nucleoid.
Nucleoid (Genetic Material):
- Contains either RNA or DNA, but not both.
- Nucleic acid content varies depending on the virus size and type.
- The nucleoid is the infectious part of the virus and holds the genetic instructions for protein synthesis and viral replication.
Capsid (Protein Coat):
- Protects the nucleoid from harmful environmental conditions.
- Facilitates the virus’s entry into the host cell.
- Shows antigenic properties.
- Contributes to the virus’s shape and structure.
- Interacts with the vector for virus transmission.
Morphology of Viruses:
- Shape and Size Variety:
- Elongated: Rigid rods or flexuous threads (1-10 nm in width, 480-2000 nm in length).
- Spherical (Polyhedral): Most viruses are polyhedral, with sizes ranging from 17 nm to 60 nm in diameter.
- Cylindrical (Bacillus-like rods): For example, rhabdoviruses (52-75 x 300-380 nm).
- Some viruses have more complex genomic structures, such as Bipartite and Multipartite viruses (e.g., Tobacco rattle virus and Alfalfa mosaic virus).
Composition and Structure of Viral Proteins:
- Amino Acids: Viral proteins are made up of amino acids. The sequence of these amino acids, encoded by the viral nucleic acid, determines the properties of the virus.
- Example: Tobacco mosaic virus has a protein subunit composed of 158 amino acids, with a mass of 17,600 daltons.
Composition and Structure of Viral Nucleic Acids:
- Viral Genome: Most plant viruses contain RNA, but some have DNA genomes.
- Nucleotides: The building blocks of RNA and DNA, consisting of:
- A base (adenine, guanine, cytosine, uracil for RNA; thymine replaces uracil in DNA).
- A sugar (ribose in RNA, deoxyribose in DNA).
- A phosphate group.
- Formation of Chains: Nucleotides link via the phosphate-sugar backbone to form long strands of RNA or DNA.
