Virology ( Introduction )

                                                VIROOGY

Viruses are the smallest known as infective agents . The genome of a virus is wither DNA or RNA . That is the only content one kind of nucleic-acid . Viruses are metabolically inert or infective or enter into the cell . The viruses undergoes replication and that increases in number . It does not divided by Binary fission as in bacteria .

Structure of Virus

A virus basically consist of a core of nucleic acid , surrounded by a protein coat which protects the virus genome from environmental factors . The structures which make-up a virus particle are known as (i) Virion (ii) Capsid – which is the protein coat .

   The capsid is composed of capsomeres  which is the unit structure may surrounded by an  envelope .

Cultivation of Viruses

As viruses replicate within living cells . Special methods are used to culture viruses like

i)                    Tissue culture – cells are obtained from man or animals are grown in artificial cultures in the laboratory and these cultures support viral replication .

ii)                   Chick embryo – Some viruses grown in cells in the chick embryo

iii)                 Laboratory animals – Different laboratory animals are used for the replication  of viruses .

Viruses can’t be grown in artificial cultures as bacteria .

Effects of virus on cells 

It affects on cells in three way

i)                    Cell death – It causes cytopathogenic effect which kill the cell .

ii)                   Cell transformation -  Here cell is not killed but changed from a normal cell to malignant or cancerous cell .

iii)                 Latent infection – The viruses remains in the cell in an active state but produces minimum effects on its function .

Classification of Viruses

            DNA viruses

i)                    POX viruses – It causes small-pox

ii)                   Herpes viruses – It causes Herpes , Chicken-pox .

iii)                 Adeno viruses – It causes respiratory infections and conjunctivitis .

iv)                 Hepadna viruses – Hepatitis B virus causes Hepatitis .

            RNA viruses

i)                    Rhabdo viruses – Rabbies virus

ii)                   Picorna viruses – Polio virus which causes polio . Hepatitis viruses .

iii)                 Flevi virus – It causes Encephalitis .

iv)                 Reo Viruses – Rota virus which causes gastroentrities , specially in children .

v)                  Retro viruses – HIV virus which causes AIDS .

Properties of Virus

i)                    Heat – Viruses are usually inactivated at 56*c for 30minutes and 100*c for few seconds .

ii)                   Cold – Viruses are stable at veryt low temperature even at -70*c .

iii)                 Ultra violet rays – It inactive viruses .

Viruses are usually killed by hydrochloride solution and glutaraldehyde .

Entry of Viruses

Viruses enter the body via respiratory tract by inhalation . some viruses however entire by ingestion or by inoculation or by the bite of a insect . some viruses may gain entrance by sexual contact .

Viruses replication

Virus replication takes place in 7-stages

i)                    Adsorption – To specific receptors on the cell/plasma membrane .

ii)                   Entry – Probably by invagination of cell membrane .

iii)                 Un-coating – That is release of the virus nucleic acid .

iv)                  Transcription – That is production of virus messenger RNA to form the viral genome – this is carried out by the host cell or virus specified enzyme .

v)                  Synthesis of virus component

vi)                 Assemble – New virus genomes and proteins are assembled to form  new virus particles .

vii)               Release – Either by sudden rupture or by gradual budding of the viruses through cell membrane .