How Viruses Multiply?

Obligatory parasitism - Outside cells viruses are nonliving, inactive   particles but after entering into live cells these multiply fast by replication  like organism  thus these represent obligatory parasitism; these can be defined as inanimate obligatory parasites Obviously, these have their own hereditary blueprint for replication, but no machinery to use the genome. After the cellular metabolic machinery to obey their genome for their own replication thus their genome is the basic infectious material.

Host-cell Cycle Viruses

The different stages form the contact of a virus with its host cell to the release  of its copies form the host cell constitute the host cell cycle  of a virus, It may  also be called replicative or parasitic cycle of the virus, but not its like cycle, because growth never occurs in a virus, simply  the components o fits several copied are synthesized  and assembled in the host cell

Early studies on viral replication began with bacteriophages .The complete host cell cycle comprises following six phases-

(1)    Adsorption (attachment of viron with host cell)-As a virion comes in contact with suitable host cell, it become attached upon host cell surface due interaction its attachment proteins and specific receptor proteins of host cell membrane.

(2)   Penetration-As mentioned before, a  bacteriophage  virion  leaves its capsid outside and injects only nucleic  acid filament ds (DNA)  into the host bacterium by using its tail as a hypodermic syringe the tail fibres  bend, and the spring like tail compresses to inject the viral genome through  a puncture in the rigid wall of the bacterium.

Most plant viruses enter whole into the host cells at points of injuries' upon leaf surfaces, or these are inoculated into plant cells by arthropod   vectors,

Animal's viruses also enter whole into cells. Three types of penetration mechanisms have been described in their case---  

(1)   Direct passage --- No enveloped vir ions reach into host cell cytoplasm by simply pushing through the host cell membrane.

(2)   Fusion -The  envelope of some enveloped viruses fuses with host cell membrane,  becoming continuous with it and thus releasing the nucleocapsid into hast cell cytoplasm.

(3)   Endocytosis  - The virion, in this case is actively engulfed by the host cell by a process  resembling phagocytosis so that it is  enclosed in a vacuole or vesicle when it reaches into the cytoplasm of host cell .

(4)   Uncoating -Within the host cell, all of a virion except its genome and enzymes associated with  the genome, is digested by lysosmal  enzymes of the host cell

(5)   Biosynthesis - This phase includes replication of viral genome and synthesis of viral proteins, as well as the enzymatic proteins required for inactivation of host cell genome, and for initiation, regulation and control of viral synthesis, assembly and release. Replication   of viral genome of most DNA viruses is replicated in host cell cytoplasm. Viral proteins are always synthesized in host cell cytoplasm.

(6)   Maturation - This comprises assembly of viral components into progeny virions, It occurs in host cell nucleus or cytoplasm. In case of enveloped viruses, the envelope is respectively derived from nuclear and cell membranes.

(7)   Release-Usually, quite a large number of progeny viruses are formed in the host cell.  In case of bacteriophages,, progeny viruses are released by lysis of  host bacterium. In case of animal's viruses, progeny viruses are generally released by budding from host cell surface.

(8)   The   host cell cycle is completed in about 15 to 30 minutes in case of bactrio  phages, but in 15to30 hours in case  of animals viruses ,The progeny viruses, released from host cells attack fresh host cells in the infected  tissues.