Viruses (RNA and DNA types) With Special Reference to TMV

1. What is the Meaning of Virus?

The term "virus" is derived from the Latin word virus, which meaning "venom" or "poisonous fluid." Although plant diseases such as potato leaf roll and human illnesses such as yellow fever, small pox, and others have been recognized for a long time, the identity of the causal agent was discovered much later.

Adolph Meyer (1886), a Dutch agriculture scientist, saw a sick tobacco plant with mottling of the leaves and termed it mosaic. He was able to show the spreading nature of sick plant sap by grinding it, filtering it through double filter paper, and applying it to healthy plants.

He established that specific bacteria are the causal agent of tobacco mosaic after heating the fluid (sap) at 80°C and losing its infective potential. D. Iwanowski (1892), a Russian researcher, was the first to establish the transfer of tobacco mosaic virus infection from infected to healthy plants via sap, even after the sap had been filtered through a Chamberland filter candle, which was adequate to eliminate bacteria. W. M. Beijerinck (1898), a Dutch bacteriologist, showed that the transparent, filterable, and non-cultivable infectious sap could flow like a fluid through an agar gel.

Bacteria and other organisms are unable to do so. Based on the above, Beijerinck concluded that the fluid was alive and named it "contagium vivum fluidum," or contagious living fluid.

According to Loeffler and Frosch (1898), the agent of cow mouth and foot disease might pass through a bacteriofilter. The agent was not seen under a microscope or could not be produced in artificial culture media. Walter Reed and his colleagues (1900) identified the agent of yellow fever, the virus, which was the first viral illness known to man. Many viral illnesses in vertebrates are recognized to date. British scientist F. W. Twort (1915) was the first to detect certain viruses that kill bacteria. Felix d'Herelle (1917), a French scientist, separately saw and investigated the virus attacking bacteria two years later. He named the virus bacteriophage (bacteria-eater), also known as a phage.

Schlessinger (1933) was the first to use differential centrifugation to purify viruses. Later, in 1935, organic chemist Wendell M. Stanley discovered that the virus could be crystallized and was mostly made up of proteins.

It was later discovered that, in addition to protein, viruses also include a tiny quantity of RNA or DNA. As a result, a virus is a nucleoprotein organism in which the infective agent is nucleic acid rather than protein. A few years later, in 1946, W. M. Stanley was given the Nobel Prize for this discovery.

Viruses cannot be classified in either the animal or plant kingdoms due to their host nature. Virology is the branch of science that deals with viruses. Virologists are the scientists that work in this field.

2. Characters of Virus:

(a)They are self-replicating non-cellular agents.

(b) They can develop and replicate intracellularly as obligatory parasites (that is, they can only grow in a live host) or they can remain dormant outside the host.

(c) They are classified as cubical, helical, or complicated according to their symmetry.

(d) The viruses are made up of two parts: a nucleic acid in the center and a protein coat around it.

(e) The nucleic acid can be either DNA or RNA, but they do not coexist.

(f) Single-stranded or double-stranded nucleic acids are possible.

(g) Except for some animal viruses, which have extra polysaccharides, the outer coating, also known as the shell or capsid, is made up of protein units known as capsomeres.

(h) They lack their own machinery for protein synthesis, so they rely on host machinery to do so.

 

(i) During replication, their nucleic acid instructs the host cell to produce several viral components, which when assembled create a fully infectious particle known as a virion.

(j) They are extremely easily spread from one creature to another.

3. Viruses have both living and non-living characters.

(A). Living characters of viruses:

(a) They have nucleic acid (DNA or RNA), which is genetic material that can replicate itself.

(a) The availability of mutant forms in some viruses indicates mutation.

(c) Stimulants, such as radiation and chemical compounds, make them hypersensitive.

(d) They have the ability to replicate in the host's live cells.

(e) The viruses are antigenic in nature.

(f) They have the ability to attack specific hosts.

(B). Non-living characters of viruses:

(a) Outside of their host, viruses exist as inert substances.

(b) They are autocatalytic by nature.

(c) They don't have a cell membrane or a cell wall.

(d) The viruses do not have cellular organelles like ribosomes and mitochondria.

(f) The viruses have the ability to crystallize.

4. Structure of virus

1.    Viruses can only be seen using an electron microscope since they are ultramicroscopic.

2. Plant viruses are often smaller than animal or bacterial viruses.

3. The morphology of virions varies greatly amongst virus families. They might be rod-shaped, bullet-shaped, or any other form. Brick-shaped, oval, uneven and pleomorphic, or even coir rope-like

4. The virion of 'tailed' or T-bacteriophages consists of a complicated head and an attached tail.

5.      A virus particle's nucleic acid core, or virion, is encased in a protein covering, or capsid. A nucleocapsid is a nucleic acid-containing capsule. The capsid is made up of morphological components called capsomeres. Polypeptide molecules, which create an impermeable shell around the nucleic acid core, are the chemical units of a capsid.

6. The viral envelope is lipoproteinaceous in nature and is generated from the host-cell membrane. Recent studies have revealed that the lipoprotein's lipid comes from the host cell while the protein comes from the virus.

7. A hollow core is encircled by a contractile sheath in the tail.

8. An end-plate with associated tail fibres is present at the tail's terminal end.

9. A narrow disc or collar joins the tail to the head at the head end.

10.  T-The nucleic acid in bacteriophages' heads is double-stranded DNA.

11.  The hexagonal end-plate It contains a pin at each corner and is held together by six extremely long tail fibres. These tail fibres keep bacteriophages connected to the host cell.

 

(A) The virus consists of two parts:

(1)   Nucleic acid (located in the middle) and (2) Protein coat, which may include an extra envelope.

1.     1. Nucleic acid

Viruses only have one kind of nucleic acid, either DNA or RNA. Viruses that include DNA are known as Deoxyviruses, whereas those that carry RNA are known as Riboviruses. The structure of their nucleic acids differs.

Except for a few, most plant viruses contain RNA that is either single-stranded (TMV) or double-stranded (Rice ragged stunt viruses), and a few have DNA that is either single-stranded (Gemini viruses) or double-stranded (Rice ragged stunt viruses) (Dahlia mosaic virus).

Animal viruses typically have double stranded DNA or single (Polio virus) or double (Reo virus) stranded RNA, whereas bacteriophages mostly have double stranded DNA but also have single stranded RNA (f2, R17, fr) or single stranded DNA (f2, R17, fr) (f1, fd, M13). Each virion has only one molecule of nucleic acid, termed the genome, which is made up of nucleotide pairs ranging from 1000 to 250,000. A virion's nucleic acid content is usually proportional to its size. Small viruses have 4–8 genes per virion, while big viruses have 100–200 genes per virion.

2.     Protein coat

Capsid refers to the protein coat that surrounds the genome, while nucleocapsid refers to the capsid and the nucleic acid it encloses. The capsid is made up of several protein components known as capsomeres (Fig. 2.38A).

Many mammalian viruses have a bilayered lipoprotein envelope that covers the nucleocapsid and is mostly of host cell origin (Fig. 2.38B).

5. Symmetry:

Around the core nucleic acid, the capsid is symmetrically organized.

The viruses are divided into three types based on their capsid symmetry:

 (a) Cubical (icosahedral),

(b) Helical, and

(c) Complex.

(a) Cubical (icosahedral) Capsids:

They have a polygon that has 12 vertices (corners), 20 facets (sides), and 30 edges. Each side of the triangle is an equilateral triangle. Pentons (pentagonal capsomeres at the corners) and Hexons (hexagonal capsomeres at the corners), for example, enveloped herpes and toga viruses and naked papova and adenoviruses.

(b) Helical Capsids:

Capsomeres and nucleic acid are wrapped together to create a spiral or helical tube. Tobacco mosaic virus (TMV), Influenza virus, and other helical kinds are all RNA viruses with the majority of them being enclosed.

(c) Complex Capsids:

Complex capsids are viruses that do not fit into any of the above two categories owing to their structure's complexity, such as pox virus and bacteriophages like T2, T4, and T6. (Fig. 2.38C)

 

6. Classification of virus on the basis of nucleic acid

1.      DNA virus 2. RNA virus

1.      1.  DNA Virus:

The viral genome is made up of DNA.

                                i.            Adenovirus, Herpesvirus are examples of double-stranded DNA viruses.

                              ii.            Single-stranded DNA virus, such as the Parvovirus and the 174 virus.

2.      2. RNA Virus

RNA is the genome.

                                i.             Reovirus, a double-stranded RNA virus.

                              ii.             Single-stranded RNA virus: there are two types of single-stranded RNA viruses.

Ø  Poliovirus and Hepatitis A both have positive sense RNA (+RNA).

Ø  Rabies virus, Influenza virus have negative sense RNA (-RNA).

7. Tobacco mosaic virus (TMV)

Tobacco Mosaic Virus (TMV): An Overview

This is the most well-known viral illness. The tobacco mosaic virus infects all dicotyledonous plants, with tobacco and tomato being the most common. It has no effect on monocotyledonous plants, though.

Although Adolph Mayer initially noticed the mosaic pattern on afflicted tobacco plant leaves in 1886, it was not until 1898 that Beijerinck provided the first scientific confirmation of the existence of a virus.

He was able to show that the juice from a sick tobacco plant may cause mosaic disease in healthy tobacco plants.

The tobacco mosaic virus, on the other hand, is made up of sixteen amino acids, according to C. A. Knight.

The tobacco mosaic virus causes distortion, blistering, and necrosis in the host's photosynthetic tissue. It also causes the afflicted plants to become dwarfed. It is one of the most harmful plant viruses, causing massive losses in the tobacco crop by lowering production and quality.

(A). Tobacco Mosaic Virus Symptoms:

Systemic mosaic type is the symptom. On immature leaves, the primary sign is the appearance of mild circular chlorotic lesions that gradually disappear as the veins clear.

One strain of tobacco mosaic virus, for example, may create yellow mottling on the leaves, while another causes necrosis simply, and yet another causes a severe deformity. Another variation is the type of plant that is harmed. Petal mosaic symptoms can be seen in flowers. Striking of the stem is a symptom of severe stresses. The host is seldom killed by the sickness.

(B). Tobacco Mosaic Virus Causal Organism:

Tobacco mosaic virus 1, Marmor tabaci Holmes, is the most common tobacco mosaic virus.

Even after 25 years, the virus is still active in the juice taken from the host plant. It's a hardy virus that can survive desiccation for up to 25 years. It may be found in extremely high concentrations in plants, with a dilution end point of 10-6. The virus's thermal inactivation point is 90°C.

The virus particles are rod-shaped and measure 280 by 15 millimeters in length and breadth. Between the protein subunits, the ribonucleic acid thread intertwines more or less centrally.

The presence of particular cell inclusions distinguishes tobacco plant cells infected with tobacco mosaic virus. There are two types of intracellular inclusions:   

(1)   intracellular inclusions and (2) intra-nuclear inclusions.

The intracellular inclusions are (a) X-bodies and (b) crystalline plate striate material.

The X-bodies are amorphous protoplasmic inclusions that are more or less vacuolate. Protein reaction occurs in the striate material of crystalline plates. The pure virus-protein crystals look similar to these crystals. A yellow-mottling strain of tobacco mosaic virus produces intra-nuclear fibrous and crystalline inclusions.

(C). Tobacco Mosaic Virus Disease Cycle

Infected tobacco plant waste, tobacco rubbish from warehouses, cigarettes, cigars, pipe and chewing tobacco, and perennating hosts that represent the source of primary inoculum are all sources of the virus.

One of the most contagious plant viruses is this one. Mechanical transmission, handling tobacco plants during transplanting, other field activities, and contact by man and cultivation instruments are all ways the virus spreads from plant to plant. The virus infects the host tissue and quickly multiplies, causing illness symptoms.

(D). Tobacco Mosaic Virus Control

Some of the recommended control measures are as follows:

(I) Seed beds should be far away from tobacco warehouses.

(ii) Tobacco waste should be removed from seed beds.

(iii) Steam sterilization of seed bed soil is recommended.

(iv) Hands and agricultural instruments should be kept clean to avoid infection.

(v) Because pipe tobacco, cigarettes, and chewing tobacco are all sources of primary inoculum, no tobacco should be smoked or chewed.

(vi) Susceptible hosts, such as weeds or other plants, should be killed to prevent the virus from spreading.

(vii) Plant detritus from the previous year should be burned.

(viii) To prevent the illness from spreading further, diseased plants should be removed and burned.

(ix) Planting resistant cultivars yields positive outcomes.

8. Virus's Economic Importance

•        Antidotes/vaccinations: Pox, mumps, polio, jaundice, and other illnesses can be managed by infiltrating the human body with live or dead viral vaccines.

•        In the management of dangerous animals and insects: A particular virus can be used to control some animals and insects that are detrimental to people.

•        Disease control: The T2 bacteriophage virus protects people against dysentery by destroying dangerous bacteria such as e-coli. Viruses are used as vectors in virotherapy to treat a variety of ailments because they can specifically target cells and DNA. It has the potential to be used in cancer treatment and gene therapy.

•        In the lab: Virus is utilized as the simplest living model in the lab. The virus is commonly utilized in genetics studies. It's a crucial topic in genetic engineering.

•        As proof of evolution: Because viruses have both living and non-living properties, they serve an important role in gaining knowledge about the evolution trend and the process of production of living species.

•        Viruses are classified as organic nanoparticles in nanotechnology. They've been employed as a pattern for arranging nanoscale materials due to their size, shape, and topologies.

•        In Seawater: A teaspoon of seawater contains around one million viruses, making it the most abundant natural component in aquatic environments. They are beneficial to both saltwater and freshwater habitats in terms of waste disposal.

•        Viruses enhance the number of photosynthesises in the oceans and are efficient in reducing carbon dioxide levels in the atmosphere by around 3 gigatonnes per year. Viruses are a fraction of the size of bacteria. Virion is the name for a viral particle. The virions come in a variety of sizes.

The tiniest virus has a diameter of roughly 10 mm (e.g., foot-and-mouth disease virus). The biggest virus (e.g., poxvirus) is around 250 nm in diameter, which is roughly the same size as the smallest bacterium or mycoplasma.