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.