Kashmir University BA 1st Semester Botany Notes Download Pdf
I:-VIRUSES INTRODUCTION
Viruses are considered to be at the borderline of living and non-living world. If they are floating around in the air or sitting here and there, they’re inert. But if they come into contact with a suitable plant, animal or bacterial cell, they come alive.
The term virus was originated from a Latin word which means poison. The branch of study of viruses is called virology. The credit for the discovery of viruses goes to Beijerinck {1898}. The contributions of other pioneers like Pasteur, Mayer and Iwanowsky cannot be overlooked. Stanley successfully crystallized Tobacco mosaic virus. The concept of contagium vivum fluidum showed that viruses were fluid in their physical nature. notes app
| FIGURE: SMALL POX VIRUS |
Kashmir University BA Ist Semester Botany Notes
Q:-General Characteristics of Viruses.
(i) Viruses are a cellular, non-cytoplasmic infectious agents.
(ii) They are smaller than bacteria, and this can pass through bacteriological filter.
(iii) Viruses are transmissible from disease to healthy organisms.
(iv) All viruses are obligate parasites and can multiply only within the living host cells.
(v) Viruses contain only a single type of nucleic acid either DNA or RNA.
(vi) Viruses are host specific that they infect only a single species and definite cells of the host organisms.
(vii) Viruses are effective in very small doses. They are highly resistant to germicides and extremes of physical conditions.
Q: – History of discovery Viruses.
| Dmitri Iwanowsky (1864-1920) notes |
A: – Viruses were not known to biologists for a long time due to their ultramicroscopic structure though their presence was apparent by infectious diseases which were proved not due to bacteria. It attracted the attention of investigators only in the 19th century when a virus called tobacco mosaic virus (TMV) caused severe damage to commercially important tobacco crop.
| Martinus Beijernick(1851-1931) |
Kashmir University BA Ist Semester Botany Notes Download Pdf
Mayer demonstrated that the disease could be transmitted just by applying the sap of infected leaf to the leaf of healthy plant. He thought that the disease was due to a bacterium. It was then the Russian biologist Iwanowsky (1892) who demonstrated that the sap of infected leaves even after passing through bacterial filter remained infective, ruling out the bacterium as the causative agent. Dutch microbiologist Beijerinck (1898) confirmed the findings of Iwanowsky and called the fluid “contagium vivum fluidum” which means contagious living fluid. This was later on called virion (poison) and the disease causing agent as virus.
W.M. Stanley (1935), the American biochemist, isolated virus in crystalline form and demonstrated that even in that state it maintained the infectivity. This marked the beginning of a new branch of science called virology.
Q: – Biological position of viruses.
A: –Viruses lack a cytoplasmic membrane and they do not have the basic component of a cell.
They can only replicate inside the host cell. Outside the host cell, they are non-living.
Thus, viruses show characters of both living and non-living.
(I) Non-living Characters of Viruses:
Following characters of viruses assign them as non-living:
(a) They can be crystallized.
(b) Outside the cell, they behave like inert chemicals.
(c) They do not show growth, development, nutrition, reproduction, etc.
(d) They can be precipitated.
(II) Living characters of viruses:
(a) They multiply within host cells.
(b) They possess genetic material, either DNA or RNA.
(c) There are definite races or strains.
(d) They exhibit mutations.
Because of the above reasons, viruses form unique bridge between living and non-living things.
Q: – General Structure of Viruses.
A: – (i) Shape and size:-The shape varies considerable. They may be spherical or golf ball-like, rod-shaped, tadpole-like, helical or polyhedral. Plant viruses are smaller than bacteria.
(ii) Chemical structure and function:-Viruses have a very simple structure. The core of the viruses is made upon of nucleic acid, which is surrounded by a protein coat called capsid. The nucleic acid always contains only a single kind of nucleic acid i.e. either DNA or RNA. The infectious property of a virus is due to its nucleic acid.
Capsid or the protein coats:-It is made up of many identical protein sub-units called capsomeres. The capsomeres are composed of either one or several type of proteins. Capsomeres are arranged in a very symmetrical manner and give a specific shape to a particular virus. The host specificity of virus is due to proteins of the capsid.
Kashmir University BA Ist Semester Botany Notes Download Pdf
Q:-Structure of a virus.
A:-A virus is composed of two major parts 1.Capsid (the protein coat) 2.Nucleic acid.
The capsid is the outer protein coat. It is protective in function.
It is often composed of many identical subunits called capsomeres. Some of the viruses have an outer covering called envelope eg. HIV. They are called enveloped viruses.
Others are called naked viruses or non- enveloped viruses. The capsid is in close contact with the nucleic acid and hence known as nucleocapsid. The nucleic acid forms the central core. Unlike any living cell a virus contains either DNA or RNA, but never both. The infective nature of the virus is attributed to the nucleic acid while host specificity is attributed to the protein coat.
Replication
Viruses are obligate parasites. They can only function by using the energy of their hosts. In order to replicate themselves they must use the structures and energy of the host they have occupied.
The chart below shows the replication of a bacteriophage. This virus infects bacteria. It takes about 30 minutes to complete its replication cycle.
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