Viruses: Origin, Structure, Types & Facts

Table of Contents

What Are Viruses?

In this article we are going to discuss biology and microbiology and viruses so let’s know first what are the Viruses and where they come from and how many types of they are really in microbiology lets know about first what are viruses so, replicate inside the host cell. They are non-cellular, microscopic infectious agents. They contain either an RNA or DNA genome that is surrounded by a protective virus coded protein coat. How viruses are transmitted, types of viruses, viral diseases, etc. Let us solve questions on viruses and their types.

We researched about and we found Viruses can infect animals, plants, fungi, and bacteria. The virus sometimes can cause a disease that may be fatal. Some viruses may also have one effect on one type of organism, but a different effect on each other. Viruses cannot replicate without a host so they are classified as parasitic as always. 

History of Virus OR Origin Of Viruses

Lets know that Who invented virus in biology? who is the father of virus? When was Virus Discovered? and What was the first virus in the world? D.J. Ivanowski (1892), a Russian botanist was the first to notice Viruses. He prepared an extract of a tobacco plant infected with Tobacco Mosaic Virus (TMV) and found that this extract could infect healthy plants even after it was passed through a filter that checks bacteria. M. W. Beijerink (1898) coined the term virus (Latin:virus = venom or poisonous fluid) for these infectious bacteria free plant extracts. and Martinus Beijerinck is often called the Father of Virology.

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More definite knowledge about the chemical nature of viruses was provided by American chemist W. M. Stanley in 1935. He isolated pure crystals of Tobacco Mosaic Virus and concluded that viruses are made of nucleoproteins, i.e., protein and nucleic acid. He shared 1946 Nobel

The physical structure of viruses can be Prize in chemistry for this fundamental work. studied only with an electron microscope. It showed that the crystals of viruses are actually composed of many individual complex units which were dned as virions. The virion is now described as the basic structural unit of a virus capable of infecting a specific host.


(1)Viruses are acellular, non-cytoplasmic, infectious agents.

(2) They are smaller than bacteria, and thus can pass through bacteriological filter

(3) Viruses are transmissible from diseased to healthy organisms.

(4) All viruses are obligate parasites and can multiply only within the living host cells.

🧾Some General Characteristic Questions and Answer OF VIRUSES !!😷

(5)Viruses contain only a single type of nucleic acid, either DNA or RNA.

(6)Viruses are host specific in that they infect only a single species and definite cells of the host organisms.

(7) Viruses are effective in very small doses. Most of them are highly resistant to extremes of physical germicides and conditions.


Shape and Size & whats the difference between a disease and a virus

So now one thing comes to mind: What is the structure of a virus?  What is a virus composed of?  What are the structural components of a virus? And what is the structure of viruses so let’s know about them.

The shape of different types of viruses varies considerably . They may be spherical or golf ball-like (poliovirus, herpes virus), rod-shaped (Tobacco Mosaic Virus; TMV), tadpole-like (bacteriophages), helical (influenza virus) or polyhedral (adenovirus).

Viruses are smaller than bacteria. They are so small that a hollow ‘shell’ of a Staphylococcus (bacterium) can contain thousands of smallest viruses. The size ranges from 10 nm (virus of foot and mouth disease of catties) to 300 nm (smallpox virus Variola). Plant viruses, in general, are smaller than bacterial or animal viruses.

Chemical Structure and Composition

Viruses have a very simple structure. A virus is made of a nucleic acid core and protein coat called capsid which surrounds the core. A fully assembled particle, i.e., a virion is capable of infecting the host.

1. The nucleic acid. 

A virion always contains only a single kind of nucleic acid, i.e, either DNA or RNA. The nucleic acid may occur as single or double strands. Plant viruses contain only single or double stranded DNA or single or


Table 1. Viruses and Nucleic Acids

Virus                         Type of nucleic acid*

Herpes                      2 DNA

Chicken pox             2 DNA

HepatitisB               2 DNA

Cyanophages          2 DNA


T1.T3, T5. T7           2 DNA


M13. f1, fd, o x 174     1 DNA

Influenza virus            1 RNA

Rabies                         1 RNA

HIV                           1 RNA

Measles                            1 RNA

Mumps                             1 RNA

Polio                                 1 RNA

Hepatitis A                       1 RNA


f2, fr R17                         1 RNA

TMV                                1 RNA

Mycophages                   2 RNA

Reovirus                         2 RNA

Wound tumour virus       2 RNA

*1 Single stranded, 2 = Double stranded

double stranded RNA. Bacterial viruses (bacteriophages) contain single or double stranded

DNA or single stranded RNA.

The infectious property of a virion is due to its nucleic acid. A host cell can synthesize complete virion if only free viral nucleic acid is nested within the cytoplasm of a living host cell.

2. Capsid or the protein coat. 

The protein coat is called capsid. It is made of many identical subunits called The protein capsomeres. capsomeres are composed of either one or several types of proteins. Host specificity of viruses is due to the proteins of the capsid. In a virus particle, the capsomeres are arranged in a very symmetrical manner and give a specific shape to a particular virus. Some larger virus particles (i.e., virions) have an additional covering of lipids or lipoproteins outside the capsid. Such virions are called enveloped (e.g., influenza virus, mumps virus) and those without this additional covering are referred to as naked (e.g., TMV).


Various systems of classification were proposed from time and again. They were mostly based on their host range, and clinical, epidemiological and pathological symptoms. The classification proposed by Lwoff Home and Tournier (1962) was one that was used more commonly than others. It was based on the type of nucleic acid present in Viruses.

Baltimore (NP 1975) in conjunction with ICTV (International Committee on Taxonomy of Viruses) devised a classification and placed vi-brushes into seven groups

(1) ds DNA viruses (e.g.,Adenoviruses, Herpesviruses, Poxviruses)

(2) ssDNA viruses (+) DNA sense (e.g., Parvoviruses)

(3) dsRNA viruses (e.g., Reoviruses)

(4) (+)ssRNA viruses (+ sense RNA (e.g Picornaviruses,  Togaviruses)

Traditional Definition of Viruses  it May Have to be Reconsidered now in 2021

Viruses are infectious agents with no cell, nucleus, cytoplasm or organelles. They can replicate only inside a living host cell and are generally defined as obligate intracellular parasites. But in 1991, E.Wimmer, A. Molla and A. Pul successfully grew entire polioviruses in test tubes containing ground up human cells, but no live cells. They added RNA from Polioviruses into this cell free extract and complete new virus particles appeared in about 5 hours Although no other virus has been grown as yet, but if success is achieved in future, . the definition of viruses will have to be rewritten.

(5) (ssRNA viruses ) sense RNA (e.g Orthomyxoviruses, Rhabdoviruses) ssRNA-RT viruses (+) sense RNA with

(6) DNA intermediate in life-cycle e8, Retroviruses)

(7) dsRNA-RT viruses (e.g, Hepadnaviruses).of Committee

Virus International

Nomenclature has given a system of naming the virus. The system consists of two parts. The first part is the common name of the virus and the second part contains the coded information about the virus. The second part of the system, called cryptogram, is a descriptive code summarising the main properties of viruses. It contains following four pairs

(1) First pair

represents type of nucleic acid number of strands in nucleic acid.

(2) Second pair

represents molecular weight of nucleic acid / amount of nucleic acid expressed as percentage.

(3) Third pair

denotes the shape of virus/shape of nucleoprotein.

(4) Fourth pair

denotes the type of host carrier used in the transmission of virus.

Mosaic Virus (TMV) R/I: 2/5: E/E: S/A

It can be explained as under

– Nucleic acid RNA (R)

(1) First pair single stranded- (1).

(2) Second pair Molecular weight of nucleic acid

(2) hundred thousands/ Amount of nucleic acid -(5%).

(3) Third pair Shape of virus elongated(E shape of nucleoprotein-elongated (E).

(4) Fourth pair Host seed plants or spermat ophytes (S)/carrier of transmission-air (A).

The Cryptogram of Influenza Virus R/I (2-3)/10 S/E: VIA It can be explained as under:

Nucleic acid-


(i) First pair single stranded (1)

(2) Second pair – Molecular weight of nucle: acid (2 or 3) hundred thousands/ Amount of nucleic acid (10%). Shape of virus spherical

(3) Third pair (S shape of nucleoprotein- elongated (E). vertebrate (Vy Host

(4) Fourth pair Transmission by- air (A).



Tobacco Mosaic Virus (TMV) is the best known plant virus. It was discovered by D. Ivanowski (1892) and later crystallised by W. M. Stanley (1935). TMV is rod-shaped, measuring approximately 3000 A in length and 150 A in diameter. It is made of RNA and proteins. The RNA is single stranded and is helically coiled around the central hollow axis of the rod  I

extends throughout the length of the Virus particle. The protein coat or capsid is made of approximately 2100 identical subunits, the capsomeres. These are arranged in a company helical manner around the central axis. In fact, helical RNA runs in grooves in capsomeres. The TMV is of naked type, i.e., the protein at (capsid) is not surrounded by an additional covering.


Viruses which grow as intracellular parasites of bacteria are called bacteriophages (Greek: phagin = to eat) which literally mean “eaters of bacteria”, 1These were discovered independently by Frederick W. Twort in England in 1915 and Felix d’Herelle in Paris in 1917. Bacteriophages can be found in all types of bacteria. Structurally, bacteriophages are almost similar to other types of viruses. They are also composed of protein and a nucleic acid, but differ from other viruses in having bacteria as their host cells. Of all the bacteriophages, T-series

(characterised by the presence of tail) is the most extensively studied. It infects non-motile strain B of Escherichia coli. It has a tadpole -like shape with a hexagonal head and tail. The length of head and tail is almost equal (approximately 950A each). The head is 650A in diameter. It has a protein coat in which a double 50 long) is tightly stranded DNA (about packed. The cylindrical tail is made entirely of proteins. It has a core tube (80A in diameter) filled with lysozyme type of enzymes (these Enzymes probably help in the penetration of the tail in the susceptible cell). Basal end of the tail has a hexagonal spiked end plate. At each corner of the hexagonal plate there is a thin tail fibre,measuring 1300 Å in length. The tail fibres and the end plate help in the attachment of phage particles to the bacterial cell. The upper end of The tail is attached to the head by a narrow neck. The core tube of the tail is surrounded by a contractile sheath. It is attached to the neck by a collar.

A complete phage particle has almost equal amounts of DNA (40- 50%) and proteins (50-60%). 

Influenza Virus: 

Animal Virus Influenza viruses are spherical in shape measuring 800-1200 Å in diameter. The protein capsid encloses a single stranded RNA. Proteins constitute

Viral Specificity

Viral specificity refers to the specific kinds of cells a virus can infect. It is determined mainly by whether a virus can attach to a cell. Attachment of virus depends on the presence of Specific receptor sites on the surface of host cells and on specific attachment structures on Viral capsids or envelopes. Specificity is also affected by whether appropriate host enzymes and other proteins the virus needs in order to replicate are available  in the cell.

 Functions of the Various Structural Components of Bacteriophage.

Functions                                    Component

1. Carries Nucleic acid –   the genetic information necessary for replication of new phage particles.

When bacteriophage is

2. Tail sheath   adsorbed on the surface of  the host cell, the tail sheath retracts so that the nucleic acid can move from the head into the host cell’s cytoplasm. Attaches the phage to specific

3. Plate and tail fibres receptor sites on the cell wall of a susceptible host bacterium. approximately 90% of the virus particle while the rest (10%6) is RNA.

The nucleocapsid (nucleic acid + protein is enclosed in an envelope of lipoproteins, a coat envelope of influenza virus is characterized by the presence of projections, called spikes surface. They are made of hemagglutinin protein that agglutinates the red blood corpuscle (RBC) of the host cell.

Reproduction of virus 

Let’s know How do viruses reproduce? and Do you know really that viruses reproduce asexually or sexually ? if you know put a comment on below.

Viruses do not have their own metabolic activity and, hence, use cellular AlP ribosomes, t-RN and certain biosynthetic products of the host cell Thus viruses multiply only inside the host cell The process of multiplication is best understood Phages can in the life-cycles of  bacteriophages. Phages can multiply by two alternative mechanisms-the lytic cycle or the lysogenic cycle. The lytic cycle ends with the lysis and death of the host cell whereas in lysogenic cycle the host cell remains alive. 

In the following description the lytic and lysogenic cycles have been described under separate headings.

Lytic Cycle

Lytic cycle is seen during the multiplication of T-even bacteriophages (T2, T4, the host Escherichia coli. The multiplication cycle involves following five steps .

1. Attachment or adsorption.

If phage particles collide in the correct orientation with host cells, the phages will attach to or adsorb onto the host cell surface. The attachment is a chemical interaction in between the receptor site on host cell and phage tail fibres that bind to these

3. Biosynthesis. 

The viral genome is too small to contain all the genetic information to replicate itself. Therefore, it utilizes the biosynthetic machinery of the host cell. Once the phage DNA enters the host cell, the bacterial DNA is hydrolysed into nucleotides which are used as The phage building blocks of new phages. infection in fact directs the host cell to make only viral products, viz., viral DNA and viral proteins.

4. Maturation. 

In this process the phage proteins (capsid) and DNA synthesized in the host cell following infection, are assembled into First the capsid proteins complete virion. synthesize the head and then a viral DNA molecule is packed into each head. The tails are assembled separately from newly formed bases and collars. These are later plates, sheaths attached to the nucleic acid filled heads.

5. Release. 

This is the last stage of viral multiplication in which the virions are released from the host cells. In this process enzyme lysozyme coded for by a phage gene, breaks down the host cell wall, allowing Viruses to escape. Since the virions are released due to lysis of the host cell wall, hence T4 phages are called virulent or lytic phages.

The released phages infect now can more susceptible bacteria. infection Virus Such by virulent phages represent a lytic cycle of infection.

Lysogenic Cycle

Some phages do not destroy their host cell when they multiply. In such phage infections, the viral DNA gets incorporated into the host cell’s DNA and is replicated with the host chromosome. 

This is called lysogeny and such phages are çalled temperate phages. The most widely studied lysogenic cycle is the multiplication of lambda 

(A) phage in Escherichia coli. It can be summarised into following points.

(1) The phage attaches itself to the receptor site on the host surface just as in lytic cycle.

(2) Phage DNA is injected into the host cell. Initially the DNA appears to be linear but soon forms a circle.

(3) This circular phage DNA integrates into the circular bacterial chromosome at a specific location (compare with lytic cycle in which the phage DNA multiplies and forms new phage components within the host cell).

(4) The viral DNA within the bacterial chromosome is called a prophage and the combination of a bacterium and a temperate phage is called a lysogen.

(5) Once established as a prophage, the virus can remain dormant for a long time. Each time a bacterium divides, the prophage is copied and is passed on to the progeny a part of the bacterial bacteria as chromosomes.

A lysogenic phage, either spontaneously or in response to some outside stimulation, can become active and initiate a typical lytic cycle.

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Gonorrhea, AIDS, hepatitis, polio and typhoid fever are examples of diseases that are spread by human carriers.

Breast milk is a significant source of HIV infection in infants born to infected mothers. At least a quarter of the 2.5 million units of blood administered in Africa are not screened for the AlIDS virus. Treatment with AZT (azidothymidine) during the last few weeks of pregnancy reduces mother-to fetus HIV transmission by half. Generally, serum antibodies to HIV can be detected by indirect ELISA (enzyme-linked immunosorbent assay) within 6 weeks of infection.

specific receptor site. The fibres bend and allow the pins to touch the cell surface.

Alter attachment, the phage injects its nucleic acid into the bacterium. This acess is facilitated by an enzyme, lysozyme released by the phage tail. This enzyme breaks down a portion of the bacterial cell wall. 

During the process of penetration the tail sheath contracts, the hollow tube or core is driven through cell wall and it eventually comes in contact of the

bacterial cell membrane. Now the nucleic acid moves from the head through the tube into the bacterial cell. The outer protein coat or capsid of the phage particle remains outside the host cell.


The viruses are transmitted through various agencies, the chiet being seeds, grafting, vegetative propagation, insects, mechanical means, nematodes, etc.

[I] Seed transmission

Virus particles sometimes enter reproductive arts of the plant such as ovary wall, ovules, emles, embr etc. These transmit the disease to the next generation. The viruses found in the floral and immature fruits, generally become inactivated as the seeds mature. Hence, seed transmission is not a very important method.

[I] Transmission by grafting and vegetative propagation 

The viruses are transmitted from one cell to another and hence can pass through graft unione Similarly, if a part of a virus infected plant is used for vegetative propagation, the new plant would also be virus infected.

[II] Transmission by insects

Insects which transmit Viruses are commonly known as insect vectors. These are provided with mouthparts adapted for biting or sucking. The common insect vectors are plant bugs, leafhoppers, white flies, aphids, etc.

[IV] Transmission by mechanical methods

The viruses are transferred through the sap which comes out due to injury from agricultural implements or gardening tools, etc.

[V] Transmission by nematodes

Nematodes like Xiphinema, Longidorus, etc., also spread viruses by root infection.


The plant and animal Viruses are economically important because of the diseases they cause Plant diseases

1. Symptoms.

 A variety of symptoms are caused by viral infections. These include local lesions clearing of veins, mosaic formation, ring spotting chlorosis, distortion, necrosis, breaking of blossoms, stunting and premature defoliation Some virus diseases of plants, along with symptoms are given in below.

[I] Animal diseases

Diseases caused in mammals include small pox chicken pox, poliomyelitis, influenza, measles yellow fever, encephalitis, infectious hepatitis, viral bronchitis. Viral enteritis, common cold, mumps. AIDS, etc. Some very common diseases are given below in 


So are you curious abou virods  so let’s know What is the difference between virion and viroid? and know everything abouts Viroids and symptoms of viroids.

👉🧾Some Questions and Answers of VIROIDS 📚👈

Because of their simplified structures, viroids (also prions) are sometimes called subviral particles. Viroids are a type of infectious agent, smaller than a virus. It was first described by O. T. Diener, a plant pathologist in 1971, while studying potato spindle tuber disease. The viroids are composed solely of RNA and have so far been identified in plants only.

Each viroid consists of a single circular strand of RNA molecules of low molecular weight. It is made up of 270 to 380 nucleotides. This amount of RNA would be about 1/10 the genetic material present in the smallest of the viruses. The circular RNA strand is collapsed into a rod by intrastrand base pairing. Viroids are not surrounded by a protein capsid.

Viroids appear in the nucleus of the plant cells. Unlike virus RNA, which may be copied In the host cell’s cytoplasm or nucleus, Viroid RNA is always copied in the host cell nucleus.


Production of tiny yellow leaves does not bear fruits. Yellowing of leaf veins. Upward rolling of leaves, tissues rigid and leathery, necrosis of phloem, tubers small. Leaves rolled up, deformed, growth stunted, fruit bearing reduced. 

Dark green streaks on petioles and along veins, shortening of crown, stunted growth. 

Profuse tillering. sprouting of lateral buds, poor conformation. Mottling of leaves, dark and light green streaks. The precise mechanism by which viroids disrupt host cell metabolism is not known. They may interfere with the cell’s ability to process mRNA molecules. Without mature m RNA molecules, proteins cannot be synthesized and as such the cell’s metabolism would be disrupted that could lead to cell death. Viroids are the causative agents of potato spindle tuber disease (PTSD), chrysanthemum stunt disease, cucumber pale fruit disease and tomato apical stunt disease.

Viroids are presumed to have entered crop plants from unknown wild plants. Common practice of growing plants in close association and use of machinery for harvesting, may help in the spread of disease. In fruit trees they may spread by grafting


Is prion virus or bacteria and if you know about prions do comment in below

🧾Some Questions and Answers of PRIONS 📚

Prions are infectious protein particles which lack nucleic acid. These are resistant to heat (can

tolerate 90°C for three minutes), UV rays and enzyme nuclease, but are sensitive to proteases.

American neurobiologist Stanley Prusiner (1982) coined the term prion for these proteinaceous particles. Prusiner was later awarded infectious

Some Common Viral Diseases of Plants Disease Host

1. Little leaf of Brinjal     Brinjal, (Baingan,) (Solanum melongena)

2. Yellow vein mosaic Lady’s finger, (Bhindi)

(Abelmoschus esculentus)

3. Potato leaf roll Potato, (Alu,) (Solanum tuberosum)

4. Leaf curl of Papaya  Papaya. (Papita) (Carica papaya)

5. Bunchy top  Banana, (Kela) (Musa sp.)

6. Grassy shoot Sugarcane, (Ganna,) (Saccharum officinarum)

7. Tobacco mosaic   Tobacco, (Tambaku) (Nicotiana sp.)

Nobel Prize in 1997 for purification of hypothetical inflections perion. The proteins constituting the prions are designated as PrP (prion protein). The genes for these proteins are found in the normal host DNA.

In human beings the PrP gene is located in chromosome 20. An abnormal form of PrP, called

PrP, is known to cause a neurological disease bovine spongiform encephalopathy (BSE) or mad cow disease in at least nine animals. In 1996, | The United Kingdom had to kill hundreds of thousands of cattle because of an epidemic of this disease. In human beings the prions are known to cause diseases like kuru, Creutzfeldt-Jakob 

disease (CJD), Gerstmann-Straussleur-Scheinker syndrome and familial insomnia, a fatal disease. These diseases are genetic and the infection is also transmitted through organ or tissue surgical transplantation, and contaminated instruments. also transmitted through organ or tissue surgical transplantation, and contaminated instruments.

Eat, or Not to Eat ? Treatment

🧾Some Questions and Answers of VIRUSES 📚

The 2004 avian flu outbreak is the fifth since 1997 to infect humans. This raised red alert fearing that the strain could mutate and initiate flu pandemic, a worldwide epidemic such as the so-called spanish flu of 1918 that killed at least 40 million people. Avian flu is caused by Type A influenza virus. The viral subtype responsible for the current outbreak is called H5N1. It originates in water fowl, primarily ducks. Ducks can withstand virus, making them ideal carriers, but chickens have little resistance. When one chicken is infected, the entire flock receives that virus and would soon kill nearly all the birds. The bird to human transmission took place in Hong Kong. The virus is not passed on the meat or eggs and hence there is no danger from cooked chicken. The chicken eating has suddenly stopped the world over and the eaters have kept the anxious chicken meat shopkeepers guessing.

The virus is passed on through bird feces; the breathing of fecal dust transmits the virus to man. Humans can not infect other humans with H5N1. Experts fear that the next pandemic will start as avian virus moves from a chicken to a person already suffering from human flu. The two viruses might then combine into a mutant strain to which humans would have no resistance.

Significance of Prophage

Insertion of phage DNA into a bacterial chromosome alters the genetic characteristics of bacteria.

(1) Genes present in the prophage produce proteins that inhibit virus replication.

(2) Prophages also contain genes that provide immunity to the lysogenic cells to reinfection by another phage of the same type. However, these genes do not protect the lysogenic cells against infection by a different type of temperate or virulent phage.

(i) Some lysogenic cells show properties which are of medical significance. For example, the bacterium Corynebacterium diphtheriae causes diphtheria. The disease causing property of this bacterium is related to the synthesis of a toxin. The bacterium can produce the toxin only when it carries a temperate phage because the genes coding for this toxin are present on the phage DNA.


So we the team live learns described the all Viruses are biological entities that are present in all living beings. Some are harmless, while others can cause a range of diseases, from the common cold to Ebola.

Seeking protection from potentially hazardous viruses — for example, through vaccinations — can help prevent serious illness and others

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