Virus

Viruses

Viruses are not cells, therefore, they are neither prokaryotes nor eukaryotes. They may be considered midway between the living and nonliving systems. In the free (extracellular) state, they are totally inert (inactive) and do not show any activity of life such as movement, growth, respiration, nutrition and reproduction. They may even be purified and crystallized much like the chemical substances as salt, sugar. In crystal form, they can be stored indefinitely without any change or damage. The crystals can be dissolved, and the vital particles are fully capable of infecting cells. When they get into a living host cell, they become active and multiply much like the living systems and may mutate also. Thus, the viruses resemble the living organisms in the intracellular state and the nonliving chemicals in the extracellular state. They have been defined as the living chemicals.

Since the viruses reproduce like living organisms, it is necessary to learn about them. In fact, their knowledge is essential as they are causative agents of many important plant and animal diseases.

The study of viruses is called virology. A specialist in virology is termed virologist.

Morphology: - the viruses are a heterogeneous group, showing a good deal of variation.

Size:- the viruses are too small to be seen with a light microscope, they can be photographed only with an electron microscope. They are scarcely larger than some very large single molecules of protein or nucleic acid. They vary in size from 50 Å (bactriophagus) to 2750 Å (psittacosis virus). Although individual virus particles are not visible under a light microscope, the latter often shows inclusion bodies in the virus-infected cells. These seem to be large colonies of virus particles.

Shape:- viruses vary in form too. They may be spherical, cuboidal, polyhedral, rod-shaped, and comma-shaped.

Structure: - a virus is far simpler than prokaryotes or eukaryotes cells. It exists in two very different states, one within a host cell and the other outside a cell. Outside the cell, the virus is a minute nucleoprotein particle, on virion, composed of a core of a single nucleic acid molecule, called viral chromosome, surrounded by a protein sheath termed capsid. It is also called nucleocapsid.

i.            Nucleic acid: the nucleic acid acts as the genetic material. It is DNA in some viruses, RNA in other. These viruses are respectively called DNA viruses and RNA viruses. The DNA is double-stranded but may be single-stranded as in ɸ viruses. It may be linear or circular. The RNA is single-stranded but may be double-stranded. The viral genetic material contains information for little more than production of more virus particles of the same kind. It is active in this function only when inside the host cell.

                                                           

forms of virus paticles

structure of a T-even bacteriophage

ii.            Capsid: The capsid is symmetrical or quasi-symmetrical. It is formed of a number of subunit or molecules termed capsomers. The latter vary in form, number and arrangement. The TMV virus has 2,200 capsomers in its capsid. The capsid may have molecules of one to over 50 different proteins. Certain highly specialized viruses, herpes virus, have around the capsid a membranous envelope derived from the plasma membrane of the host cell. The capsid protects the viral chromosomes during the extracellular state or its virus. It also helps in the recognition of the genetic material into it by contraction. The capsid contains certain ‘’ INTERNAL VIRUS PROTEIN’’ beside the genome. Viruses lack energy-yielding and biosynthetic enzymes. hence, they must necessarily be intracellular parasites. Viruses may have just 3 to as many as 500 genes. The fewer the genes, the more the virus depends on the host cell for materials it needs for  its multiplication.

                                                             

linear DNA molecule released from the ruptured head of a T2 bacteriophage.

Classification: - the virus are classified on the basis of the host they infect, the organ and tissue infected, the mode if transmission, and the disease symptoms. They are often divided into three main groups: bacterial viruses, plant viruses and animal viruses,

i.            Bacterial viruses: these viruses grow only within bacterial cells, which swell up and die. They are called bacteriophages or simply phages. They were discovered in 1917 by the French scientist d herelle. They are the most complex viruses. They may contains RNA or DNA in linear or circular form. The bacteriophagus occur in nature wherever bacteria are found, and are specially or strain of bacteria. Since the bacteriophagus kill bacteria, an attempt was made to treat patients suffering from bacterial disease such as dysentery and staphyloccus infections by using bacteriophagus. However, no bacteriophagus preparation proved successful to any significant extent. Bacterial viruses have played an important role in the development of molecular biology. They are widely used to investigate biochemical and genetic events.

I.            ‘’T-even’’ E.coli Bacteriophage. This is the most widely studied bacteriophage. It infacts the colon bacillus,Escherichia coli. It is called colliphage. It has the form of a lollipop and Consists of a polyhedral head, a short neck with a collar, and a straight tail ending in a base plate. The head measures 90*60 nm. It consists of a linear, double-stranded, greatly coiled DNA molecule surrounded by a protein capsid. The latter is composed of about 2000 protein molecules or capsomers. 

                                                           

chromosomes of influenza virus(A), poliomycelitis virus(B), and tobacco virus (C)

    The neck connects the head with the tail. It is surrounded by a narrow collar at its middle. The tail has a hollow core enclosed by cylindrical sheath of several different proteins. The phage DNA passes into the host cell through the core of the tail. The base plate bears spikers, each carrying a long, hair-like tail. The tail, base plate and tail fibres together make the phage look like a landing module for the moon.

II.            Plant viruses: -  these viruses attack the plant cells, disturb their metabolism and cause severe disease. They usually have linear RNA as the genetic material. The tobacco mosaic virus  is a common plant virus. Other important examples are southern beet mosaic virus and turnip yellow virus.                                                                             Tobacco mosaic virus(TMV):  it a  much studied virus. It is rod-shaped measuring 300*15 nm. Its genetic material is a single-stranded , linear RNA molecule coiled into a regular spiral extending through the axis of the rod. The nucleic acid is surrounded by a protein capsid of about 2200 elliptical, spirally arranged capsomeres. TMV wasisolated and crystallized by W.M.Stanley in 1935. Since then many other viruses have been obtained as crystals. Plant viruses have revealed that RNA can act as a genetic material.

III.            Animal viruses: - these viruses attack animal cells, and may cause fatal diseases. They have DNA or RNA molecule of linear or circular form. Some animal viruses, e.g., influenze and herpes viruses have around the capsid a membrane derived from the plasma membrane of the host cells. The envelope consists mainly of lipids are similar to those in the plasma membrane of the infected host cell.

                                                           

structure of tobacco mosaic virus (TMV)

Poliomyelitis virus: it is spherical in form. It consists a single-stranded RNA molecule surrounded by a protein capsid of 60 capsomers. A few viruses can infect animals as well as plant cells. For examples, potato yellow dwarf virus can grow in leafhoppers and in plants.

Mode of infection

Life cycle of T-even bacteriophage illustrates the general pattern by which the virus particles infect their host cells. Free bacteriophage particles come in contact with bacterial cells by random collisions. When a phage collides with its specific bacterial cell host, a protein on the surface of the tail fibres binds or adsorbs to a specific receptor protein on the bacterial cell wall. This interaction determines the host range of a virus. An enzyme from the tail core digests part of the bacterial cell wall. The head and tail sheath then contract and inject the DNA molecule into the host cell. The protein sheath remains outside as empty shell. The internal viral proteins, if any, may also enter the host cell.

The animal viruses are often taken up by the host cells by phagocytosis and their protein coat is digested away.

Reproduction

There are two modes of the reproduction in viruses: lytic cycle and lysogenic cycle.

1.    Lytic cycle: - on entering a host cell, the virus immediately starts reproduction and exploits the biosynthetic machinery, raw materials and catalysts of the host cell. First the viral nucleic acid is replicated nucleic acids then directs the synthesis of proteins for their coats. They form viral mRNA, on which viral proteins are synthesized, using host cell’s ribosomes, amino acid, tRNAs and other substances. The first formed RNAs formed later. 

                                                       

(A) virus attaches to a bacterium. (B) viral DNA passes into the host cell through a hole made by an enzyme from the tail core. (C) new viral DNA and protein molecules are synthesized in the host cell. (D) host cell wall is dissolved by a viral enzyme lysozyme to let the new virus paricles escape

    These viral proteins are of two types: some proteins act as inhibitory factors which stop cell metabolism, majority of proteins are used in constructing new capsid components. As the head and tail portions accumulate in the host cell, the replicated nucleic acid molecules get into the heads. Then the heads and tails join to form complete viral particles. After completion of viral particles, a final viral protein (lysozyme enzyme) cause breakdown of the bacterial cell wall. This releases the newly formed viral particles into the surrounding medium. The above sequence of events is called lytic cycle. The entire cycles takes about half an hour and produces about 1000 progeny phages. The new viral particles about 1000 progeny phages. The new viral particles remain inert unless they contact and infect fresh host cells, when another cycle starts.

    Up to 10,000 viral particles may be produced in a single human cell           infected with the polo virus, this shows that the viruses multiply very       rapidly. In their power of reproduction, the viruses resemble the living   systems.

                                                

maturity of an enveloped virus.

In many plants and animal virus infections, the host cells are not lysed, the dead host cell releases the virions as it gradually disintegrates.

Viruses with an envelope bud from the host cell and thereby acquire and envelope on their outside. The viral encoded protein pass through the host-cell membrane and project from its surface. The virion containing nucleic acid and internal viral proteins escapes from the cell by budding through the plasma membranes, acquiring a phospholipid bilayer envelope having viral proteins on the surface. It may be added that the viruses do not really reproduce, but are reproduced by the biosynthetic machinery of their host cells. This is what the viruses do not grow on cell-free culture media.

Growth

There is never any kind of growth stage in viruses. They are assembled from the components directly into the mature-sized virion.

2.    Lysogenic cycle

The DNA of phage, on entering  and E.coli cell, may behave  in one of the two ways. It may undergo the lytic cycle and   produce more phages as described above. Alternatively, it may integrate with the help of the enzyme integrase, formation a prophage. The viral DNA does not exploit the host’s machinery to form more virus particles, but replicates along with the host’s DNA. the replicated prophages pass from the parent host cell into daughter cells during cell division. Existence of the phage DNA as a part of the host’s DNA is called lysogeny.

                                                            

lysogenic cycle of temperate bacteriophase such as lamda phage

 Such viruses, called  lysogenic or temperate phage, do not produce any visible effect on the host cell. Many animal viruses also show lysogenic cycle. The most important of these are retroviruses of eukaryotes.

Inheritance

Inheritance in viruses occurs by genes, and the nature and behavior of viral genes are the same as those of cellular genes. The phenotype of a virus is represented not only by the structure of the virus particle itself, but also by the effect it produces on the infected host cell.

Genetic crosses and recombination

If two or more different viruses simultaneously infect the same cell, recombination can occur between their DNA molecules. At some point during the formation of new virus particles in the infected cell, viral DNA molecules from the different parental types may pair and cross over by breakage and exchanges.The recombinant viral chromosomes are then packed in protein coats, and when the host cell ruptures, are released to the medium to infect fresh host cell. The viral recombinants are detected through biochemical’s changes, such as in the proteins of viral coats or the phenotype developed by the infected host cell. Like the cells or organisms, virus strains may be characteristics as wild type or mutant there is inherited by the progeny, and one type may be converted into the other by mutation. Mutation involving gene exchange enables viruses to undergo evolutionary process. This is another resemblance between viruses and living organisms.