Viruses were first discovered in 1898 by Dutch scientist Martinus Beijerinck. Other Characteristic Features
Does not increase in shape and sizeContainsnucleic acid, DNA or RNA as their genetic materialIts genetic material is surrounded by a protein envelope called a capsidReproduces at a much faster rate than bacteria Passes through membrane filters
Parts and their Functions
A typical virus consists of two basic parts: 1) nucleic acid genome and 2) protein capsid – together called the nucleocapsid. Besides these, many animal viruses also contain 3) lipid envelope along with some additional parts such as the neck, tail sheath, tail fibers, pins, and endplate to form a complete virion.
Nucleic Acid Genome
The genetic material of most viruses is double-stranded DNA (Deoxyribonucleic acid), with some exceptions where the genetic material is either single-stranded DNA or RNA (Ribonucleic acid). Functions
Providing genetic information to make it possible to synthesize all viral proteinsHelping in the synthesis of new viruses within the host organism
Protein Capsid
It is the protein shell that encloses the viral genome. Capsids are made up of individual subunits called capsomeres, which form the basic structure of a virus. Based on structure, capsid can be of following types: a) Icosahedral: Capsid with a hollow, quasi-spherical structure. Twenty equilateral triangles arranged around the surface of a sphere forms an icosahedron. A particular type of icosahedral shape called prolate has an elongated symmetry found in bacteriophages, as shown in the diagram. b) Filamentous: Capsid with a linear, thin, thread-like appearance. They may also be called rod-shaped or helical. c) Complex:Capsid having a combination of icosahedral and helical shape and may have a complex outer wall or head-tail morphology. Functions
Enabling the virion to penetrate the host cell membraneProtecting the nucleic acid from digestion by host enzymesContaining unique sites on its surface that allow the virion to attach to a host cell
Lipid Envelope
In some viruses found in animals, the nucleocapsid is surrounded by a membrane, also called an envelope. The envelope is composed of lipid-bilayer with proteins attached to them. Many viruses also develop special structures called spikes made of glycoprotein on their envelopes. Functions
Helping the entry of viruses into the host organismHelping the virion to attach to specific host-cell surfaces using spikesProtecting the genetic material of virus from destruction by host enzymesEscaping identification by the host immune system
