Viruses are extremely small infectious agents, which can only be seen under an electron microscope. The range in size from 250 nanometer (nm) of poxviruses to the 20 nm of parvoviruses. They are 10 to 1000 times smaller than most bacteria, so they can pass through the pores of filter from which bacteria cannot pass. Viruses cannot be grown on artificial media. They can reproduce only in animal and plant cells or in microorganisms, where they reproduce by replication (a process by which many copies or replicas of virus are formed). Thus the viruses are obligate intracellular parasites.
Viruses lack metabolic machinery for the synthesis of their own nucleic acid and protein. They depend on the host cell to carry out these vital functions. During reproduction in the host cells, viruses may cause disease. All viruses are generally resistant to broad range of available antibiotics such as penicillin, streptomycin and others.
The complete, mature and infectious particle is known as virion. The virions are composed of a central core of nucleic acid, either DNA or RNA, which is also known as the genome and is surrounded by a protein coat, the capsid. Capsid gives definite shape to virion. Capsid is made up of protein subunits known as capsomeres. The number of capsomeres is characteristics of a particular virus. For example 162 capsomeres are present in the capsid of herpes virus and 252 in the capsid of adenovirus which cause some common colds. In some animal viruses the nucleocapsid (nucleic acid and capsid) is covered by another membrance derived from the host cell, the envelope. Non enveloped viruses are known as naked virions. Animals and plant viruses may be polyhedron (having many sides), helical (spiral), enveloped or complex.
The most recently discovered (1983) and least understood micro organisms are the prions, which their nature is very controversial. They are composed of protein only that contains the information that codes for contains the information that codes for their own replication. All other organisms contain their genetic information in nucleic acid (DNA or RNA). Prions are responsible for mad cow infection and mysterious brain infection in man.
Bacterio-phages occur in two structural forms having cubical or helical symmetry. In general appearance cubical phages are regular solid or icosahedral (having 20 faces), and helical phages are rod shaped. Many phages consist of head and tail. In those cases heads are polyhedral but tails are rod shaped. Morphology of some viruses and bacteriophages has been shown in fig 5.3.
Life Cycle Of Bacteriopages
Earlier researches on bacteriphages were mainly on limited number of phages that infect Escherichia coli. Of these the best known phages are T phages (T for type).
Among T phages, the T2 and T4 phages are mainly used in phage studies. The overall structure of T4, studied with electron microscopy, resembles that of tadpole, consisting of head and tail (fig 5.4). the head is an elongated pyramidal (having two triangular structures with common base), hexagonal, prism – shaped structure, to which straight tail is attached. Within the head double stranded DNA molecule is present. The structure of phage tail is more complex than head. A layer of distinct protein forms the inner tube of core, which is enclosed in sheath made up of another type of protein. On one side of sheath is collar and on other side is end plate. To end plate six tail fibers are attached, which are the structures for attachment. The volume of the phage is about 1/1000 of the host.
The bacteriophage replicates only inside the bacterial cell. The first step in the replication of a bacteriophage is its attachment (adsorption) to host cell at receptor site on the cell wall of bacterium. During attachment, week chemical union between virion and receptor site takes place. In the next step, Penetration, the tail releases the enzyme lysozyme to dissolve a portion of the bacterial cell wall. The tail sheath contracts and tail core is forced into the cell through cell wall and cell membrane. The virus injects its DNA into the cell just as the syringe is used to inject the vaccine. The protein coat, which forms the phage head and tail structure of virus remains outside the cell (fig 5.5). many animal viruses, however enter the host cell as a whole.