Introduction:

Viruses are smallest living organisms. They do not have cellular structure. They can simply reproduce by occupying living cells. They can merely exist parasitically within new living organism. Due to this reason we can name them obligate parasites. As they are very tiny and cellular, have few but not every properties of life. Few people don't regard them as living organism. They are mainly disease carrying

Features:

Some features of viruses are given below:

i) They are obligate Endoparasites, causing disease to their host.

ii) They are cellular in shape

iii) They only reproduce by occupying living cells

iv) They are the smallest living organisms

v) They are easy in structure. Have only small piece of nucleic acid, either DNA or RNA enclosed by the protein or lipo-protein coat.

Formation of the Generalized Virus:  

Viruses are very small living organisms their size lie in range from 20-200mm. They are on average 50 times smaller than bacteria. Similar to bacteria they can't be observed with light microscope. They go by filters that hold bacteria. They don't encompass cellular structure. They are extremely plain in structure. They have the key part of generic material, either RNA or DNA both either in double or single strands.

There is the shielding coat or capsid around core. The two join to form nucleocapsid. Few may have the extra envelop round capsid. This envelop is composed of the extra lipoprotein layer. This is particularly true of one which causes H10 and influenza. The capsids are composed of indistinguishable recurring subunits known as capsomeres.

Specifications of virus infection and replication differ significantly with host kind, every virus share six essential steps in replication cycles. They are as follows: i) attachment; ii) penetration; iii) uncoating; iv) replication; v) assembly; vi)release. Virus should first connect itself to host cell. This is generally achieved by particular glycoprotiens on exterior of capsid, envelope or tail. Next, penetration happens, either of entire virus or just contents of capsid. If whole capsid enters, genetic material should be uncoated so that it is accessible to cell's replication machinery. Duplication of genetic material shows up, in addition to production of capsid and tail proteins. Once every essential part has been copied, individual virus particles are accumulated and released. Release frequently takes place in the destructive approach, bursting and killing host cell.

Number of viruses has somewhat more difficult duplication cycle comprising lytic and lysogenic stages. The lytic stage is comparable to that explained, with virus particles infecting and copied. In lysogenic stage, though, viral genetic material which has entered host cell happens to included in cell and lies inactive. It is forwarded to progeny of infected cells. Ultimately, lytic phase will begin again, and cells which were never infected themselves, but have viral genetic material will start to produce novel virus particles.

Significance of Viruses:

Viruses are significant to man due to their activities. They cause diseases not only to human being but also to the crops. They have been examined to have nucleoprotein that has strange molecular weight of up to two million.

The viruses that cause tomato and tobacco mosaic include molecular weight of given range 60 to 100 million. Those which cause fowl plague contain the molecular weight of given range 200 to 400 million. In spite of this they can't be known as living. They do not have their own life. They only show signs of living features when they go into hosts. No virus has any respiration of its own. It relies on enzymes of host to perform its work. The viruses which influence bacteria have only DNA as their fundamental type of nucleic acid. DNA is similar to genetic DNA and due to this many people believe that there is relationship in behavior of virus DNA and material DNA in nucleus of living things. Many studies are being performed by use of radioactive isotopes of many elements which are ingredients of proteins and nucleic acids. Viruses which influence plant that causes tobacco mosaic have RNA .

The virus that causes disease is known as human immunodeficiency virus. It belongs to group of RNA viruses called as retroviruses; as these viruses can alter their RNAs into DNA copy utilizing enzyme. Enzyme they utilize is known as reverse transcriptase. They perform this by replicating the section of DNA (gene) to create RNA in the process known as transcription. Enzyme in control in procedure is known as reverse transcriptase.

Reason why Viruses are Boundary-line Organisms:

Viruses are at times explained as boundary line organisms. They are observed as invading between living and non-living due to their features they could be explained as living and non-living. They are living as they contain genetic material and are capable to reproduce themselves. Though if we suppose that every living thing is compose of cells, in that case viruses are not living as they don't have the cellular construction. They too live in the inert state until they discover their correct host. When this occur genetic instructions they hold in DNA or RNA can now train host material to duplicate its kind and so they carry on producing. Until this occurs viruses are not apparent as they illustrate no life activity.

Just the once inside body of human, HIV virus communicates definite white blood cells known as T-helper lymphocytes. This yields in degeneration of immune system in human.

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