What is the Structure of a Virus? Class 11 Biology – Viruses
Viruses Class 11 Biology – Viruses are infective agents that characteristically consists of a nucleic acid molecule in a protein coat. It is a very small particle that is capable of infecting a cell (known as host cell) and multiply in the host cell to cause disease. Most viruses are too small to be seen with a normal microscope but they can be visualized using an electron microscope. The infected (by virus) host cell is forced to produce thousands of identical copies of the original virus at an extraordinary rate.
Viruses occupy a special taxonomic position: they are neither plants, animals, nor prokaryotic bacteria(single-cell organisms without defined nuclei). In fact, the viruses should not be even considered as living organisms, because they are not free-living; i.e., they cannot reproduce and carry out their metabolic processes without a host cell.
Viruses are mere strands of DNA or RNA surrounded by a protective protein coat called a capsid. They are living, only, when they have invaded some living cell. Outside of a host cell, viruses are completely inert. Unlike most living things, the cells of viruses do not divide on their own, they use the machinery of the host cells which they have infected. The unique characteristic feature of viruses is that unlike still simpler infectious agents, viruses contain their own genes, which gives them the ability to mutate and evolve. Over 5,000 species of viruses have been discovered.
STRUCTURE OF A VIRUS
Viruses contain nucleic acid—either DNA (deoxyribonucleic acid) or RNA (ribonucleic acid) and protein. The nucleic acid encodes the genetic information which is unique for each virus. The infectious, external covering (core) of a virus is called the virion. The virion capsid has three functions:
(1) Protection of the viral nucleic acid from digestion by certain enzymes (nucleases).
(2) To furnish sites on its surface that help to recognize and attach (adsorb) the virion to receptors on the surface of the host cell, and, in some viruses.
(3) It provides proteins that enable the virion to penetrate through the cell surface membrane inject the infectious nucleic acid into the interior of the host cell.
It contains at least one unique protein which is synthesized by specific genes in the nucleic acid of that virus. In virtually all viruses, at least one of these proteins forms a shell (called a capsid) around the nucleic acid either DNA or RNA. The capsid consists of numerous capsomers, each having a few monomers or structural units. Each structural unit is made up of one or more polypeptide chains. The different arrangement of capsomeres in the capsid determines the shape of a virion. Viruses have three different types of symmetry as explained below:
i. Icosahedral symmetry: Most viruses have a spherical, cubical or polygonal shape which is basically icosahedral or 20 sided. In this type of symmetry, the assembly of capsomeres causes the capsid of the virus to be at the state of minimum energy. The icosahedral capsid comprises of both pentameres (capsomeres containing five structural subunits), and hexameres (capsomeres having six units). The minimum number of capsomeres in this type of symmetry is 12 or more as 32, 42, 72, 92, etc. For example, in Bacteriophage ɸ X174 has 12 pentameres, Poliovirus has 32 capsomeres, Herpes virus has 162 capsomeres etc.
ii. Helical or Cylindrical symmetry: The capsomere and nucleic acid are woven together to form a helical or spiral tube-like structure. Most of the helical viruses are enveloped and contain ribonucleic acid (RNA). The typical example with helical symmetry is the tobacco mosaic virus (TMV), which is an RNA virus with 2130 identical capsomeres arranged in a helix.
Helical Capsid is rod-like structure with the RNA in the center of the helix. A helix is made by stacking repeating units in a spiral.
iii. Complex symmetry: Most viruses have symmetrical shapes, but some viruses have symmetrical structures and are referred to as“complex”. Some bacteriophages, such as Enterobacteria phage T4, have a complex structure consisting of an icosahedral head bound to a helical tail, which may have a hexagonal base plate with protruding protein tail fibers.
For Class 11 Biology Notes and virtual lectures on Viruses, click CBSE Class 11th Biology.
Notes of Class 11 Biology Online click below
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