Viruses are particles composed of genetic material ( DNA or RNA, but not both), surrounded by a protective protein coat. Outside a host cell, they are intert; inside, they enter a dynamic phase in which they replicate, pirating the mpst cell's enzymes,  nucleic and amino acids 5, and machinery to accomplish what they are not equipped to do alone. Viral replication is often carried out at the expense of the host.

1): Study of virus is called Virology.

2): Virus was discovered by Russian Scientists lvanovsky in the year 1892. ( During the tests of Mosaic disease in tobacco).

3): In nature, there are ultra microscopic particle known as viruses.

4): It had both the characters of living and non living , so it  is a connecting link between living & non living

5): Dr. Stanley first isolated the virus causing mosaic disease in tobacco in the form of crystals.

Discovery  of Virus:

In 1852,D.I.Ivanovsky by Russian botanist passed an infectious extract from tobacco plants(  which were suffering from the 'mosaic disease ') through a filter to prevent the passage  of bacteria. In 1898, Beijerinck,a Dutchman, described the infectious nature of certain plant fluids as ' virus' for the first time. Later, it was found that viruses are the smallest living organisms, ranging from sizes of 20 to 300 nm; on an average, they are about 50 times smaller than bacteria.

Structure of virus:

Viruses have a very simple structure, consisting components discussed below .

■ Core: The genetic material, either  DNA or RNA. The DNA or RNA may be single stranded or double stranded.

■ Capsid: A protective coat of protein surrounding the core.

■ Nucleocapsid: The combined structure formed by the core and capsid.

■ Envelope:A few  viruses, such as HIV and influenza viruses, have an additional lipoproteins layer around the capsid, derived from the cell surface membrane of the host .

■ Capsomers: Capsids are often built up of identical repeating subunits called capsomers.

Viruses that attack bacteria form a  group called bacteriophages, or simply ' phages'some of these have a distinct icosahedral head, with a tail showing helical symmetry.


Diseases such as herpes, rabies, influenza, some cancers, poliomyelitis and yellow fever are of viral origin. Of the estimated 1000_ 1500 types of viruses, approximately 250 cause diseases to humans ( over 100 of which cause the common cold) and 1up infect animals. 

Functions of viruses:

Viruses are microscopic infectious agents consisting of genetic material (DNA or RNA) enclosed in a protein coat. They are considered non-living as they cannot reproduce or carry out any metabolic processes without a host cell. Viruses infect different types of organisms, including humans, animals, plants, and even bacteria.

The main function of a virus is to infect a host organism and replicate inside its cells. To accomplish this, viruses have several specific functions:

1): Attachmentand Entry: Viruses have proteins or other structures on their outer surface that enable them to attach to specific receptors on host cells. Once attached, they enter the cell either by direct fusion with the host cell membrane or by being engulfed by the host cell.

2): Replication:Once inside the host cell, the virus hijacks the cellular machinery to replicate its genetic material and produce multiple copies of viral components. This process varies depending on the type of virus and the host cell it infects.

3): Assembly:After replication, the viral components are assembled to form new virus particles, also known as virions, within the host cell.

 4): Release:Finally, the newly formed virions are released from the infected host cell either by cell lysis (rupture) or by budding (where the virus acquires a piece of the host cell membrane as it is released).

It's important to note that viruses can cause various diseases, including the common cold, influenza, HIV/AIDS, COVID-19, and even certain cancers. However, not all viruses are harmful to their hosts, as some viruses can establish a symbiotic relationship with their host or play a role in regulating ecosystems.

Additionally, virusesviruses have been extensively studied and utilized in scientific research, such as in gene therapy, vaccination development, and understanding cellular processes.

Characters of virus:

1): They became active inside a living cells or host cell.

2): Nucleic acids replicate themselves and they reproduce rapidly. 

3): They cause disease like bacteria & fungi.

According to parasitic nature, virus is of three types__

1): Plant virus__ RNA is present as nucleic acid in 75% plant virus.

2):  Animal virus __ DNA or sometimes RNA is found  in it.

3):  Bacteriophages __ A virus that infect and replicates within bacteria. They kill the bacteria.  Example _ T_ 2 phage. Bacteriophages are composed of nucleic acid molecule surrounded by a protein sheath.

● In man virus cause disease like mumps, chicken pox, hepatitis,  polio, AIDs, Herpes etc.

● HIV often change its structure due to the presence of an enzyme reverse transcriptase. 

● The enzymes integrase produced by virus allow the integration of HIV_ DNA into the host cell DNA.

● EBOLA is a virus, causes Ebola fever. Fruit Bats are the natural host of this virus, first outbreak in West Africa. 

Note: Those viruses in which RNA is found as genetic material are called Retrovirus.

● Viruses are parasite which need living cell to reproduce. Outside host it is similar to chemical substance.

● Virus have no enzyme of their own.

Lifecycle of virus:

In principle, the lifestyle is the same for all phages.Some complete their lifecycle without a break. Such lifecycle are called lytic cycles. However,  some phages, like lambda phage, insert their DNA into the host DNA and remain dormant for many generations. Each time the host cell divides, the phage DNA is copied with the  host  cell DNA. This dormant stage of the phage is called the prophage .Eventually, it is activated again and completes its lifecycle, causing death of the  host cell in the usual way. Such phages are described as lysogenic.

AIDS Virus:

The human immunodeficiency virus( HIV), which causes acquired immunodeficiency syndrome ( ADIS), principally attacks T4A Lymphocytes, a vital part of the human immune system. As a result, the body's ability to resist opportunistic bacterial, viral, fungal, protozoal an other infection is greatly weakened.  Pneumocystis carinii pneumonia is a leading cause of death among people with HIV infection, but the  incidence of certain types of cancers such as B__ cell  lymphomas and Kaposi's sarcoma is also increased. Neurological complications and dramatic weight loss or wasting are the characteristics of end _ stage HIV disease ( AIDS). HIV can be transmitted sexually through contact with contaminated blood,tissue or needles; and form mother  to child during pregnancy or breast feeding. Full _ blown symptoms of AIDS may not develop even for more than 10 years after infection.


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