INTRODUCTION AND CLASSIFICATION OF VIRUSES OF MEDICAL IMPORTANCE.

Question 1

What are viruses?

Answer 1

Viruses are obligate intracellular parasites
which means that they replicate (or multiply) only inside a living
host cell.

• These have very simple structural organization but are characterized by certain distinctive features.

Question 2

What are the Distinctive features of viruses?

Answer 2

1• Contain a single type of nucleic acid, either DNA or RNA but never both.

2• Contain a protein coat that surrounds the nucleic acid.

3• Multiply inside the living cells using the synthetic machinery of the host cell, and

4• Cause the synthesis of specialised elements that can transfer the viral nucleic acid to other cells.

Question 3

What are satellite viruses?

Answer 3

satellite viruses depend upon other
viruses for their survival and replication.

Question 4

What is the host range of a virus and examples?

Answer 4

The host range of a virus refers to the different kinds of organisms it can infect.

The polioviruses can cause infection only in humans. In contrast, the rabies virus attacks cells of the central nervous system in humans and a wide variety of warm-blooded animals.

Question 5

What is virus specificity?

Answer 5

•The specificity of a virus refers to the specific kinds of cells the virus can infect. Virus specificity is determined mainly by whether a virus can attach to a cell or not.

• This depends upon the presence of specific receptors on the
surface of the virus.

Question 6

What are some Diseases Caused by Viruses in Human Beings?

Answer 6

•The diseases caused by viruses in the human beings fall into a wide spectrum.
• Certain diseases carry invariably a fatal outcome.
• These include rabies and acquired immunodeficiency syndrome (AIDS).
• Some of the viral diseases such as hepatitis, encephalitis, ebola haemorrhagic fever and yellow fever frequently turn out to be fatal.
•Tremendous morbidity results from worldwide prevalent diseases such as measles, mumps, rubella, common cold, influenza and chickenpox.

•Some of these usually appear in epidemic forms (e.g. measles) and
•some may even acquire pandemicity (e.g. influenza).
•Rubella virus is well known to induce teratogenic effects and evidence is accumulating at a rapid pace to incriminate viruses as carcinogenic in human beings.

Question 7

How can viruses of medical importance be classified?

Answer 7

Viruses of medical importance can be classified based on various criteria,including their:
Structure, Genetic material, Mode of transmission, and the diseases they cause.

Question 8

What are Respiratory Viruses and examples?

Answer 8

Respiratory Viruses: These viruses primarily affect the respiratory system and can cause illnesses such as the ;-
•common cold, influenza, respiratory syncytial virus (RSV),
•and severe acute respiratory syndrome coronavirus (SARS-CoV)
•and Middle East respiratory syndrome coronavirus (MERS-CoV).

Question 9

Gastrointestinal Viruses:

Answer 9

Gastrointestinal Viruses: These viruses infect the gastrointestinal tract and can lead to diseases such as gastroenteritis, viral hepatitis (hepatitis A, B, C, etc.), and norovirus infections.

Question 10

What are Neurotropic Viruses and examples?

Answer 10

Neurotropic Viruses: These viruses target the nervous system and can cause diseases such as poliovirus (polio), herpes simplex virus (HSV), varicella-zoster virus (chickenpox and shingles), and rabies virus.

Question 11

What are Bloodborne Viruses?

Answer 11

Bloodborne Viruses: These viruses are transmitted through blood and other bodily fluids and include human immunodeficiency virus (HIV), hepatitis B virus (HBV), and hepatitis C virus (HCV).

Question 12

What are Vector-Borne Viruses?

Answer 12

Vector-Borne Viruses: These viruses are transmitted to humans through vectors such as mosquitoes, ticks, and other arthropods. Examples include Zika virus, dengue virus, West Nile virus, and chikungunya virus.

Question 13

What are Sexually Transmitted Viruses?

Answer 13

Sexually Transmitted Viruses: These viruses are primarily transmitted through sexual contact and include human papillomavirus (HPV), herpes simplex virus (HSV), and human immunodeficiency virus (HIV).

Question 14

What are Emerging and Reemerging Viruses?

Answer 14

Emerging and Reemerging Viruses: These are viruses that have recently appeared or reappeared in a population, causing outbreaks or epidemics. Examples include Ebola virus, Zika virus, and coronaviruses like SARS-CoV-2 (responsible for COVID-19).

Question 15

Classification Based on Physicochemical Criteria.

Answer 15

The viruses have been divided into families which are split into genera and finally species.
•The primary criteria employed are :

•a. Kind of nucleic acid and strategy of viral replication

•b. Morphology of virion which includes size, shape, symmetry of nucleocapsid and presence of envelope.

Question 16

Discuss Nucleic Acid and Classification.
All DNA viruses are double stranded apart from?
What are positive and negative sense nucleic acid?

Answer 16

•Apart from the structure and shape, the classification is primarily based upon type, polarity and shape of the nucleic acid.

•The DNA can be double stranded or single stranded and either linear or circular in appearance.

• All DNA viruses are double stranded except parvoviruses which are single stranded.

•RNA of the viruses may be single stranded or double stranded; in influenza viruses and reoviruses this is segmented and some may have positive polarity and some negative.

•A nucleic acid that encodes the information for making proteins needed for the virus is called a positive sense nucleic acid.

• A nucleic acid made up of bases complementary to those of positive sense nucleic acid is called a negative sense nucleic acid

• Host cell ribosomes reading the base sequence of a negative sense nucleic acid also contain an enzyme called transcriptase.

• Inside the host cell this enzyme uses the negative sense nucleic acid as a template and makes a complementary positive sense nucleic acid.

Question 17

What is the Nomenclature of Viruses?

Answer 17

•The nomenclature of viruses, like their classification,is decided by the International Committee on Taxonomy of Viruses.
•The viruses are grouped into families which are named with a suffix-viridae. Subfamilies, if any, shall end with suffix-virinae and the genera will have suffix of -virus.
• The prefix may be another latin word or a sigla, i.e. an abbreviation derived from some initial letters.
• The latinized names are written in italics whereas vernacular names are written in roman letters.

Question 18

What is a virino?

Answer 18

pathogenic nucleic acid coated with host protein and called as virino.

Question 19

What is a virino?

Answer 19

pathogenic nucleic acid coated with host protein and called as virino.

Question 20

What are VIROIDS?

Answer 20

Some diseases of the plants have been found to be caused by a new class of subviral agents which do not exhibit any extracellular dormant phase and carry very small nucleic acids.
•These have been designated as viroid.
•These are RNA molecules which are resistant to heat but destroyed by nuclease.
• Till date, no viroid has been shown to cause human disease, but there is no reason to suppose they cannot infect animals.

Question 21

What are the Features distinguishing viroids from viruses.?

Answer 21

1• Each viroid consists of a single specific RNA
2• Viroids exist inside cells as particles of RNA without capsids
3• Viroid particles are not apparent in infected tissues without using special techniques to identify nucleotide sequences in RNA
4• Compared to viral nucleic acids, the RNA of viroids is a low-molecular weight material.

Question 22

What are PRIONS?

Answer 22

•The name prion has been derived from proteinaceous infectious particles.
• These are sensitive to proteases but resistant to nucleases and UV rays.
• Strong evidence is now available to incriminate these in causing transmissible spongiform encephalopathies (TSE) which is a group of neurological degenerative diseases of man.
•These proteins are self-replicating and when come in contact with normal proteins convert them into infectious elements.

The Nobel Prize for 1997 was given to Prussner for his work on prions and their possible association with transmissible spongiform encephalopathies in animals (mad cow disease) and humans.

Question 23

Properties of prions.

Answer 23

1• Very resistant to chemicals, radiations, and heat
2• Do not exhibit virus morphology in electron microscopy
3• Not associated with foreign nucleic acid isolated from
infected host cells
4• Proteinaceous, filterable
5• Multiply in cell culture but no CPE
6• Do not elicit inflammatory reaction
7• Do not elicit antibody formation in host
8• Responsible for plaques and abnormal fibres forming in brain of host
9• Very difficult to transmit