Exam Questions and Answers on Virus. In this article we will discuss about:- 1. Definition of Viruses 2. Morphology of Virus 3. Reaction to Physical and Chemical Agents 4. Multiplication of Virus inside the Host Cell 5. Cultivation 6. Classification 7. Major Groups of DNA and RNA Viruses.
Contents:
- Definition of Viruses
- Morphology of Virus
- Reaction to Physical and Chemical Agents of Virus
- Multiplication of Virus inside the Host Cell
- Cultivation of Viruses
- Classification of Viruses
- Major Groups of DNA and RNA Viruses
Exam Question and Answer # 1. Definition of Viruses:
Viruses are unicellular, ultramicroscopic particles containing either RNA or DNA, which reproduce inside living cells, pass through filters that retain bacteria and are covered by a protein coat.
The general properties of viruses are:
1. Do not possess cellular organization.
2. Contain one type of nucleic acid, either DNA or RNA but never both.
3. Lack enzymes necessary for protein and nucleic acid synthesis and so depend upon synthetic machinery of host cells.
4. They multiply by complex process and not by binary fission.
5. They are unaffected by antibiotics they are sensitive to interferon.
Rivers’ Postulates:
At the time Koch’s postulates were formulated true viral pathogens were unknown. In 1937, TM River created a similar group of rules to establish causative role of viruses in disease River’s postulates are as follow –
1. The viral agent must be found in the host’s body fluid at the time of the disease or in the cells showing lesions.
2. The viral agent obtained from the infected host must produce specific disease in a suitable healthy animals or plant or provide evidence of infection in the form of antibodies against the viral agent.
3. Similar material from such newly infected animals or plants must in turn be capable of transmitting the disease.
Exam Question and Answer 2. Morphology
of Virus:
1. Size:
Viruses are very widely in size. The largest agony them is pox virus measuring about 30nm. The smallest viruses are parvovirus (foot and mouth disease) measuring about 20 nm.
2. Shapes:
Some viruses have characteristics shape, e.g. – rabies virus has bullet shape, pox viruses are brick-shaped, tobacco mosaic virus is rod- shaped, bacteriophage has head and tail, influenza or polio viruses are spheroidal shape.
Structure and Symmetry:
Viruses have central core of nucleic acid which is either RNA or DNA but never both. This central core of nucleic acid is covered by protein coat called capsid. The capsid itself is composed of number of subunits called capsomere.
The capsomere may be arranged as follows:
1. Around coiled nucleic acid which is known as helical arrangement.
2. As cube around spheroidal nucleic acid known as icosahedral arrangement.
3. Some viruses do not fit either helical or icosahedral symmetry.eg-pox virus, bacteriophage etc.
The infectious virus particles are called virion (A complete virus particle consist of nucleic acid surrounded by a protective coat of protein called capside). Virion may be enveloped or non-enveloped. The envelope is derived from host cell membrane when virus is released by budding. Envelope is Lipo-Protein in nature.
Protein subunits may be seen as projecting spikes on the surface of the envelope. These are called peplomers. A virus may have more than one type of peplomer. E.g.-influenza virus has two peplomers.
1. Triangular Spike, Hemagglutinin.
2. Mushroom shaped, Neuraminidase.
Exam Question and Answer 3. Reaction to Physical and Chemical Agents
of Virus:
Heat and Cold:
Viruses are mostly destroyed by heating at 60° C for 30 minutes except hepatitis virus, adeno – associated virus, and scrapie virus. Viruses may be preserved by storage at 20 to 70°C.
pH – 5 to 9
Radiation:
Uv light-rays and heavy particles in activate viruses.
Vital Dyes:
Toluidine blue, neutral red and acridine orange penetrate virus particles. These dyes unite with nucleic acid making viruses susceptible to inactivation by visible light.
Glycerol:
Virus remains viable in 50 percent glycerol.
Disinfectants:
Lysol, Dettol, is ineffective against viruses. Higher concentration of chlorine, iodine may kill viruses. Dilute formaldehydes and beta propiolactone, hydrochloric acid, KMno4, H202 are most useful disinfectants against virus.
Exam Question and Answer 4. Multiplication
of Virus inside the Host Cell:
Virus depends on the synthetic machinery of host cell for replication because it lacks biosynthetic enzyme sequence of events as follows.
1. Absorption Attachment:
The virus is absorbed at the particular site of host cell which is called receptor.
In case of polio virus the receptor is lipoprotein, in case of influenza virus receptor is glycoprotein attachment is specific and is mediated by binding of virion surface structure known as legends to receptor on cell surface.
2. Penetration:
Virus particles may be engulfed by animal cell by the mechanism called viropexis. Viropexia is like phagocytosis. In case of enveloped virus, viral envelope may fuse with plasma membrane and release nucleocapsid into the cytoplasm.
3. Uncoating:
This is a process by which the virus loses its outer layer and capsid. In some cases, uncoating is effected by lysosomal enzyme of host cell. For example, in pox virus, uncoating occurs in two steps. Outer coating is removed by lysozyme present in phagocytic vacuole of host cell. This is the first step. In second step, internal core of virus is released into cytoplasm and is affected by viral uncoating enzyme.
4. Biosynthesis:
There is synthesis of viral nucleic and capsid protein. There is synthesis of regulator protein which shuts down the normal cellular metabolism and direct sequential production of viral components in host cell nucleus except pox virus. RNA virus synthesize their components in cytoplasm of host cell except orthomyxo viruse, paramyxovirus.
Biosynthesis Consist of Following Steps:
i. Transcription of m-RNA from viral nucleic acid.
ii. Translation of m-RNA into protein.
iii. Replication of viral nucleic acid, synthesis of late proteins, which are the components of daughter virion capsids.
5. Maturation:
Assembly of daughter virion follows synthesis of viral nucleic acid and proteins; it may take place in nucleus or cytoplasm. Enveloped virus gets envelope from the cell membrane of the host during a process of budding.
6. Release:
In bacterial viruses release take place by lysis of infected bacterium. In animal, viruses release occurs without lysis. Some viruses like polio may cause cell lysis during their release.
Exam Question and Answer 5. Cultivation of Viruses
:
1. 15-30 hours in animal virus.
2. 15-30 minutes in bacterial phage.
Viruses Cultivation:
There are three methods for cultivation of virus:
1. Animal inoculation
2. Chick embryo
3. Tissue culture
1. Animal Inoculation:
It is one of the oldest methods for the cultivation of viruses. The polio virus after intraspinal inoculation in monkeys causes typical paralytic disease and so isolation of viruses. Growth of virus in animal maybe known by the disease, visible lesions or death. Sometime immunity in experimental animal may interfere with the growth of viruses in that animal. It is not out of place to mention the animal inoculation.
2. Chick Embryo:
This method is better than animal inoculation because of following reasons:
1. They are clean and bacteriologically sterile.
2. They do not have immune mechanism like animals to interfere in virus cultivation.
3. They do not need feeding and caging.
4. Chick embryo provides several sites for cultivation of viruses.
5. Chorioallantoic membrane (CAM) – variola or vaccinia and herpes virus.
6. Allantoic cavity – Influenza and paramyxoviruses.
7. Amniotic sac – Isolation of influenza.
8. Yolk sac – cultivation of chlamydiae and some other virus.
Allantoic cavity may be used for growing influenza virus for vaccine purposes.
Disadvantages of Chick Embryo Method:
i. Eggs may be contaminated with mycoplasma which may interfere with the growth of other viruses.
ii. The susceptibility of chick embryo is limited to a few viruses only.
iii. Even small amount of bacterial contamination in the inoculums may kill the embryo.
3. Tissue Culture:
Tissue cultures of human or animal cells are frequently used for the cultivation of viruses.
There are Mainly Three Types of Tissue Culture:
i. Organ Culture:
E.g. tracheal ring organ culture is employed for the isolation of corona virus.
ii. Explant Culture:
This is not useful in virology. In the past adenoid tissue explant culture were used for adenovirus.
iii. Cell Culture:
This is very popular method for cultivation of viruses. From tissue, fragments cells are dispersed by proteolytic enzymes like trypsin and mechanical shake. After washing the cells, they are suspended in growth medium and distributed in petridishes, test tubes or bottles. The cells adhere to glass surface and grow out to form a monolayer sheet.
There are Three Type of Cell Culture:
1. Primary Cell Cultures:
When normal cells freshly taken from body grown for the first time, they are called primary cell culture, they can be maintained in serial culture. They are useful for isolation and cultivation of viruses for vaccine production. E.g. rhesus monkey kidney cell culture human amnion cell culture, chick embryo fibroblast culture.
2. Diploid Cell Stains:
They are capable of 100 divisions in culture. They are useful for the isolation of pathogens and for the production of viral vaccine. E.g. Human embryonic lung cell strain and rhesus embryo cell strain.
3. Continuous Cell Lines:
They are single type of cells mainly derived from cancer cells. These also can be grown in successive generation by transferring them from one test tube to another without change in character of cells. These are used only for the isolation of virus. Vaccine preparation on these cells is not safe for human use, e.g. HeLa.
Exam Question and Answer 6. Classification of Viruses
:
Till 1950 lettle was known about the viruses, Viruses may affect animals, insects, plants and bacteria. In starting classify the viruses on the basis of their affinity to different systems or organs of the body.
i. Those producing skin lesion (small pox, chicken pox, measles).
ii. Those affecting nervous system (polio, rabies).
iii. Respiratory tract-involving viruses (influenza, common cold).
iv. Viruses causing lesions (hepatitis). It was also suggested that viruses should be classified based on epidemiological criteria.
Some of the examples are as follows:
a. Enteric fever – adeno virus, hepatitis virus, reo virus, picorna virus.
b. Respiratory – Orthomyxovirus, Para-myxovirus, adenovirus. Now a days viruses are classified into two groups depending on the type of nucleic acid they possess, those containing RNA are called ribovirus and those containing DNA are called deoxyriboviruses.
Exam Question and Answer 7. M
ajor Groups of DNA and RNA Viruses:
1. Pox Virus:
They are large brick shaped particle. They may cause small pox, vaccinia, avian pox etc.
2. Herpes Virus:
They are enveloped and icosahedral. They may cause encephalitis, chicken pox etc. Example are herpes simplex virus, cytomegalovirus etc.
3. Adeno Virus:
They multiply in nucleus they may cause infection of lymphoid tissue, mild respiratory disease, conjunctivitis, e.g. Adeno virus etc.
4. Papova Virus:
They are icosahedral, multiply in nucleus. They may cause human waste. They may be oncogenic. Examples are papilloma virus, SV40 etc.
5. Parvo Virus:
They are very small icosahedral symmetry non envelope. e.g. – adeno satellite virus.
1. Orthomyxo Viruses:
They are spherical or filamentous, enveloped with lipoprotein, studded with neuraminidase and hemagglutinin. They may cause epidemics of influenza.
2. Paramyxo Virus:
They are larger than orthomyxovirus they may cause respiratory infection bad cold, measles, mumps.
3. Rhabdo Virus:
They are large enveloped, bullet-shaped. They may cause rabies in mammal’s e.g. Rabies virus and Stomatitis virus.
4. Toga Virus:
They are icosahedral and enveloped by lipid. They require arthropod vectors and may cause meningoencephalitis, e.g. yellow fever, dengue viruses.
5. Arena Virus:
They are spherical enveloped and causing meningitis e.g. lymphocytic chorio meningitis virus etc.
6. Reo Virus:
They are spherical non-enveloped icosahedral double stranded RNA causing mild respiratory.
7. Picorna Virus:
They are small icosahedral, causing neuronal damage with paralysis, aseptic meningitis etc. e.g. Polio virus.
8. Leukovirus:
They induce malignant transformation of cells with formation of new antigens and enzymes with loss of contact inhibition. E.g. – leukemia, sarcoma, Rous sarcoma, murine leukemia, murine mammary tumour, virus etc.
9. Corona Virus:
They are spherical causing cold and acute respiratory infection, mouse hepatitis etc.eg are human and avian virus.