Everything you need to know about viruses. Some of the frequently asked questions are as follows:-
Q.1. Are enzymes found in virions? Give examples.
Ans. Some virions contain enzymes which help them in infecting their hosts and to carry on their metabolic activities inside their host, e.g., HIV a retrovirus is an RNA virus which replicates inside the cells of its hosts as DNA intermediate. The retroviruses possess RNA dependent DNA polymerase which is known as reverse transcriptase.
The enzyme reverse transcriptase transcribes information in incoming RNA into DNA intermediate. Some animal viruses contain enzymes known as neuraminidases that break down glycosidic bonds of glycoproteins and glycolipids of the connective tissue of animal cells which help in liberation of viruses. Some bacteriophages possess the enzyme called lysozyme which makes small holes in the bacterial cell walls thus permitting the entry of viral nucleic acids. This enzyme is produced in greater amount in one of the later stages of infection to bring about the lysis of the wall of the bacterial cell to release the virions out of the cell.
Q.2. It is easy to study bacterial viruses as bacteria can be cultured easily on media in Petri plates. How are animal viruses cultured?
Ans. Animal viruses can be cultured either on monolayer cultures → permanent cell lines → primary cell cultures or on organ cultures.
3. List the methods for quantification of viruses.
Ans. The methods used to quantify the number of virus particles are:
i. Plaque-assay:
The viral plaque is analogous to a bacterial colony. It is assumed that each plaque (clear zone) has been formed by activity of a single particle. But the concept of efficiency of plating should be kept in mind in terms of counts made of plaques that are always lower than the counts made by electron microscope.
ii. Animal infectivity method:
It is a sort of titration in infected animals. In serial dilution of unknown samples often ten times dilutions are prepared and the sample of each dilution is injected into sensitive animals. After a certain incubation period the fraction of dead and live animals at each dilution level is tabulated and the dilution at which half of the injected animals die is called end point dilution.
Q.4. Give the steps involved in viral reproduction.
Ans. The steps involved are:
(1) Attachment or absorption,
(2) Penetration or injection,
(3) Early steps in replication,
(4) Replication,
(5) Synthesis of proteins used as structural subunits of capsid or the viral coat,
(6) Assembly or packaging, and
(7) Release of the mature virus particles from the host cell.
Q.5. Give the receptors in different organisms that permit the attachment of viruses on them.
Ans. The receptors for bacteriophage in bacteria are pili or flagella besides cell envelope components and transport binding proteins. In case of influenza virus the receptor is a glycoprotein occurring on the surface of red blood cells and that of cells of mucous membranes of susceptible animals. The receptor site for poliovirus is the lipoprotein found on the surface of the cell.
Interestingly some animal and plant viruses have no receptors for the attachment of viruses. Instead, the virus enters passively inside the cells as a result of phagocytosis or because of some other endocytotic processes in which it is involved.
Q.6. How do host restriction enzymes destroy viruses?
Ans. The restriction enzymes of restriction modification system of the host recognize the viral DNA and destroy it.
Q.7. What is positive strand DNA virus?
Ans. The virus in which its genome is a single stranded DNA of plus configuration is known as a positive strand DNA virus.
Q.8. What is negative strand RNA virus?
Ans. A virus having an RNA genome with minus configuration is known as negative strand RNA virus.
Q.9. Give a brief account of taxonomy of viruses.
Ans. The efforts regarding viral taxonomy started in 1996 at the International Congress of Microbiology in Moscow. So far the International Committee on Taxonomy of Viruses (ICTV) has not raised higher taxa order of kingdom for viruses.
However, there are two exceptions. The complex bacteriophages will perhaps be placed in their own order and enveloped viruses with a single negative strand of RNA have been classified under the newly formed order Mono-negavirales. The single-stranded RNA viruses are categorized by their method of replication. Two thousand species of viruses so far known have been categorized under 73 families by ICTV.
The names of families end with viridae. The viral genera share certain characteristics and the suffix virus is applied to the generic name. A viral species, however, is defined as a group of viruses sharing the same genetic information and ecological niche.
The specific epithets for viruses have not been raised and therefore, the viral species are designated by their common names, e.g., human immunodeficiency virus (HIV) with subspecies if any, is named by a number (like HIV- 1). The common names of viruses are written in simple regular fond (e.g., Herpes simplex virus) while the genus names are written in italics (e.g., Simplex virus).
Q.10. How does one measure the concentration of viral suspensions?
Ans. It is measured by counting the number of plaques in terms of plaque forming unit (pfu).
Q.11. How are different viruses cultured?
Ans. The bacteriophages are grown in liquid media and bacterial cultures on solid media. The solid media have an advantage over liquid media that the plaque method can be used by detecting and counting the viruses. To culture bacteriophage a sample of bacteriophage is mixed with host bacteria and melted agar which subsequently is poured over a hardened layer of agar growth medium in a Petri plate. Each vision infects a bacterial cell, multiplies and releases several hundred new visions.
After several multiplication cycles all bacteria in the area surrounding the original virus are destroyed and result in formation of a number of clearings or clear zones called plaques, that can be seen against a lawn of bacterial growth. Animal viruses can be grown in living animals, embryonated eggs or in cell cultures.
Q.12. List the methods used in viral identification.
Ans. The following methods are employed for identification of viruses:
1. By taking electron-micrographs to study their morphology.
2. Western blotting.
3. Serological methods.
4. Studying effects of viruses on host cells.
5. Restriction fragment length polymorphism.
6. DNA fingerprints or PCR (PCR was employed to amplify RNA from autopsy specimens to identify Hantavirus outbreak in USA in 1991)
Q.13. When the relationship between cancer and viruses was first obvious?
Ans. It was demonstrated in the early 1990 when chicken leukemia and chicken sarcoma (cancer of the connective tissue) were seen to be transferred to healthy animals by cell-free filtrates.
Q.14. What are oncoviruses? What are oncogenes?
Ans. Viruses that can induce formation of tumours in animals are called oncoviruses. A gene which can cause malignant transformation is known as oncogene. Several DNA viruses and retroviruses are regarded oncogenic.
Q.15. What is a tumour? Name the two types of tumours.
Ans. A tumour is an excess of a tissue formed because of unusually rapid cell multiplication. Tumours are of two types (1) malignant (cancerous) and (2) benign (non-cancerous).
Q.16. Give examples of DNA oncogenic viruses.
Ans. They are Adenoviridae, Herpesviridae, Poxviridae and Papovaviridae. The EB virus which is a Herpesvirus is the causative agent of Burkitts lymphoma and nasopharangeal carcinoma; and Hepadnavirus is known to cause cancer of the liver.
Q.17. Give examples of RNA oncogenic viruses.
Ans. Among the RNA viruses only the retroviruses are oncogenic. HTLV 1 and HTLV 2 have been known to cause human leukemia and lymphoma. The examples of latent viral infections are cold sores and shingles.
Q.18. What are prions?
Ans. Unconventional infectious agents which are pure proteins without nucleic acids are known as prions, e.g., neurological diseases, scrapie, Creutzfeldt Jacob disease (mad cow disease).
Q.19. What are viroids?
Ans. The infectious pieces of RNA which cause some plant diseases are referred to as viroids. The examples of diseases caused by them are potato spindle tuber viroid disease.