VIRUSES
The word virus is derived from Latin word venome which means poisonous fluid.
Virus can be defined as:
Non-cellular, submicroscopic infectious agents which contain either RNA or DNA, enclosed by proteinaceous coat, and reproduce only in living cells (obligate intracellular: parasites)
Viruses use biosynthetic machinery of the host to make their materials and then transfer too their cells. Study of virus is known as virology
DISCOVERY OF VIRUS
Some viral diseases have been known from centuries.The first infectious disease against which presentation was developed was a viral disease.
1. Work of Edward Jenner:
In 1796. Edward Jenner discovered an effective method for the prevention of a viral disease small pox. He removed material from cowpox lesion on the hand of milkmaid and injected into an 8 years old boy (James Phipps). After six weeks the boy was injected with pus from a small pox victim. He did not develop the disease. Jenner used material for vaccination from cowpox lesions and successfully vaccinated 23 persons. As the material was obtained from cow (called vacca in Latin), this method was named as vaccination by Louis Pasteur.
2. Work of Charles Chamberland:
Charles Chamberland (1884) found that bacteria cannot pass through porcelain filters. However agents responsible for rabies can pass through these filters. Any toxic substance that caused disease was called virus. These unseen filterable agents of rabies were called as filterable viruses.
Rabies is a disease which is transferred to human by bites of rabid dogs, foxes, cats, bats and other animals.
3. Work of Ivanowski:
In, 1892, lvanowski discovered that the agent which caused tobacco mosaic disease was filterable.He obtained bacteria free filtrate from infected plants and placed it on healthy leaves of tobacco.The filtrate caused the disease in healthy plants. Later these ultramicroscopic agents were also observed in victims of many diseases including foot and mouth disease (1898) and yellow fever (1901).
The Dutchman Beijerink formed the name 'virus' (Latin for poison) to describe the infectious nature of certain filtered plant fluids. Although progress was made in isolating highly purified samples of viruses and in identifying them chemically as nucleoproteins (nucleic acids combined with proteins). the particles still proved mysterious because they were too small to be seen with the light microscope. They were among the first biological structures to be studied when the electric, microscope was developed in the 1930s. Stanley (1935) crystallized the tobacco mosaic virus.
CHARACTERISTICS OF VIRUSES
Viruses are small infectious agents and can be seen under electron microscope. They have following characteristics:
1. Size:
They range in size from 250 nm of Pox viruses to 20 nm of Parvoviruses.
2. Filterable:
They are 10 to 1000 times smaller than bacteria. So they can pass through the pores of filter from which bacteria cannot pass.
3. Obligate Intracellular Parasites:
Viruses cannot grow on artificial media. They can reproduce in animal cells, plant cells or in microorganisms. Here they reproduce by replication (a process by which many copies or replicas of virus are formed). Therefore the viruses are obligate intracellular parasites.
4. No Metabolic Machinery:
Viruses have no metabolic machinery for the synthesis of their nucleic acid and protein. They depend on the host cell to complete vital functions.
5. Disease Production:
They can cause disease in the host during reproduction.
6. Resistant to Antibiotics:
They are generally resistant to many antibiotics such as penicillin, streptomycin and others. Each type of virus will recognize and infect only certain types of cell. In other words, viruses are highly specific to their hosts.
STRUCTURE OF VIRUS
VIRION:
The complete, mature and infectious particle is known as virion. It has following parts:
1. Genome: The virions are composed of a central core of nucleic acid (DNA or RNA) which is also called genome.
2. Capsid:
The genome is surrounded by a protein coat, the capsid.
Nucleic acid and capsid is collectively called nucleocapsid.
Capsid gives definite shape to virion.Capsid is made up of protein subunits known as capsomeres.
The number of caspomeres varies in a particular virus.
Examples:
• 162 capsomeres in the capsid of herpes virus.
• 252 capsomeres in the capsid of adenovirus which cause some common colds.
Enveloped or Naked Vireons:
(i) In some animal viruses the nucleocapsid is covered by a membrane called envelope. This membrane is derived from the host cell.
(ii) The viruses which are not enveloped are known as naked vireons.
SHAPES OF ANIMAL AND PLANT VIRUSES
Animal and plant viruses may be:
• polyhedron (having many sides)
• helical (spiral)
• enveloped or
• complex
SHAPES OF BACTERIOPHAGES:
These have two shapes (structural forms):
Cubical: These arc regular solid or icosahedral (having 20 triangular faces).
Helical Symmetry: They are rod shaped. Many phages consist of head and tail. The heads are polyhedral but tails are rod shaped.
STRUCTURE OF BACTERIOPHAGE
Bacteriophage meaning bacteria eater, earlier workers studied few phages that infect Escherichia coli. Of these the best phages are T phages (T for type).Two main types of T phages are T2 and T4.
Structure of T4:
The structure under electron microscope looks like a tadpole and consists of head and tail.
Head: The head is elongated having Pyramidal (having two triangular structures with common. base), hexagonal or prism shaped structure to it straight tail is attached. Head contains double stranded DNA.
Tail: The structure of tail is more complex than head. Tail has a core of protein which is surrounded by a sheath of another protein. On one side of the sheath is collar and on the other side is end plate (Base plate). Six tail fibers are attached to the end plate. These fibers are for attachment.
It is about 111000 of the host.
CLASSIFICATION OF VIRUSES
Different types of plant, animal and bacterial viruses are classified on the basis of morphology, cover and nucleic acid properties.
Genetic Material: The genetic material may be DNA or RNA
Cover: The genetic material may be naked, enveloped or complex.
Morphology: On the basis of morphology viruses are classified into rod shaped (T.M.V), spherical (poliovirus), tadpole like (bacteriophage) and many more figure.
RETROVIRUSES (ON COVIRUSES)
RNA tumor viruses are widely distributed in nature: These produces tumor in the fowls, rodents, cats and many other animals. The most familiar virus is the human immune deficiency virus (HIV) which causes acquired immune deficiency syndrome (AIDS).
Structure: They are single stranded RNA viruses which are spherical in shape and about 100 nm in diameter. They are enveloped by host plasma membrane.
Characteristics:
Retroviruses have following characteristics:
(i) Host Specificity:
A few retroviruses are non-specific and can infect any cell. Most of them infect host cell that have required receptors.
In the case of AIDS virus, the host ceil has a receptor that allows viral adsorption and penetration of many types of leukocytes (white blood cells) and tissue cells.
(ii) Reverse Transcriptase:
The retroviruses have a special enzyme called reverse transcriptase. It can convert a single stranded RNA genome into double stranded viral DNA.
(iii) Mode of Action:
The DNA of the retrovirus infects the host cells. They also unite with host genome as a provirus that can be passed to progeny cells. In this way some of the retroviruses can converts normal cells into cancer cells.
DISEASES CAUSED BY VIRUSES
1. Small Pox:
(i) Virus: It is caused by pox viruses: This virus is DNA enveloped.
(ii) History: This disease occurred as epidemic in China in 12th century B.C. Until the early 20th century, it was common.
(iii) Symptoms/Effects: This disease results in the formation of raised fluid-filled vesicles on the body. These become pustules later on and form pitted scars, the pocks.
(iv) Prevention: By 1950’s immunization and other control measures had decreased the disease. However it is still present in the third world countries where many people are affected.
2. Herpes Simplex:
(i) Virus:
It is caused by Herpes virus (DNA virus). It is naturally occurring disease of
mankind.
(ii) Symptoms / Effects: This disease most occurs in the mouth. on the lips, and at other skin sites. In this disease vascular lesions are formed in the epithelial layers of ectodermal tissues.
3. Influenza:
(i) Virus: Influenza viruses are enveloped RNA viruses.
(ii) Epidemic Disease: It is very common in man and occurs in epidemic form.
4. MUMPS AND MEASLES:
(i) Virus: Mumps and Measles viruses belong to group paramyxoviruses. They are large, enveloped, RNA viruses.
(ii) Occurrence: It is highly contagious (communicable) wide spread, but rarely fatal. About 60 of adults are immune to it.
Measles is a common disease among children and adults all over the world. This disease develops immunity in its victim.
5. Polio:
(i) Virus: Poliomyelitis is caused by polio virus. It is the smallest known virus. It
contains RNA in spherical capsid.
(ii) Occurrence: It is present all over the world. It occurs mostly in children. The age at which primary infection occurs depends upon social and economic factors.
6. Acquired Immuno Deficiency Syndrome (AIDS)
(i) Discovery: The AIDS was reported by some physicians in early 1980's in the young males. All these young patients were homosexuals.
This disease was also discovered in non-homosexual patients who were given blood (blood transfusion) or blood products.
In 1984 the agent causing the disease was identified by research teams from Pasteur Institute in France and National Institute of health in USA.
In 1986 the virus was named as human immunodeficiency virus (HIV).
(ii) Symptoms:The patient has complex symptoms such as severe pneumonia, a rare vascular cancer, sudden weight loss, swollen lymph nodes and general loss of immune system.
(iii) Pathogenicity:The major cell infected by HIV is the helper T -lymphocyte. It is the major part of the immune system.
The decrease of helper T-lymphocytes results in failure of the immune system.
Now the infected person is susceptible to other diseases. Cells of central nervous system can alsobe infected by HIV.
(iv) Host Specificity:It has been found recently that HIV infect and multiply in monkey but do not cause disease in them. It means that HIV is host specific.
(v) Transmission:
The HIV is transmitted by:
(a) Sexual contact
(b) Contact with blood
(c) Breast feeding and
(d) Healthcare workers can also acquire HIV during professional activities.
(vi) Prevention: Important preventive measure is avoiding the direct contact with HIV. Intravenous injection if drugs with common syringes must be prevented. Sterile needles/syringes and utensils must be used.
(vii) Vaccination: Now the vaccine against HIV has been formed and its experimental supply to humans started in early 200 I in South Africa
(7) hepatitis8). PRIONS
These are infectious proteins discovered in 1983. They contain the information that codes for their own replication. All other organisms contain their genetic information in nucleic acid (DNA or RNA).
EFFECTS: Prions are responsible for: mad cow infection and mysterious brain infection in man.responsible for: mad cow infection and mysterious brain infection in man. The bacteriophage replicates only inside the bacterial cell.There are many steps in replication. Attachment (Adsorption) of Phage to the Host Cell: First of all the bacteriophage attaches to the bacterial cell at receptor site. The receptor sites are present on the cell wall of bacterium. During attachment week chemical union occurs between virion and the receptor site.
Penetration:
o In this step the tail releases the enzymelysozyme. This enzyme dissolves a portion of the bacterial cell wall.
o The tail sheath contracts and tail core is forced into the cell through cell wall and cell membrane.
o The virus injects its DNA into the cell (just as the syringe is used to inject the vaccine).
o The protein coat, consisting of head and tail, remains outside the cell.
o Many animal viruses enter the host cell as a whole.
o After penetration one of the following cycles takes place:
(i) Lytic cycle
(ii) Lysogenic cycle
Lytic Cycle: