viruses, history and origin of viruses, types of viruses, semester 3, botany notes,

  In nature, there are many ultramicroscopic particles known as viruses. A virus is a small particle composed of two types of substances, protein and nucleic acid; they are seen only with the help of electron microscope. Viruses have an important position in comparative study of living and nonliving, because they are entities sharing the characters of both they cause wide range of disease in plant and animals. The study of viruses has become so important and detailed that their study is treated in a new branch known as Virology.

  VIRUSES  

Definition: -

Various definition of viruses has been given, some of them are as follow: -

"A virus particle is in effect, a minute package of DNA or RNA surrounded by a protein coat or capsid." 

"A genetic element enclosed in a protein coat". 

"A core of nucleic acid either DNA or RNA surrounded by a protein coat shell".

 Green (1935) define viruses as "the smallest unit showing reproductive properties considered typical of life".

Bawden (1949) defined viruses as "obligate parasite too small to be seen".

According to Luria (1953) viruses are "submicroscopic entities capable of being introduced into a specific living cell and reproducing inside such cells only."

HISTORY: -

In 1886 Mayer described Tobacco Mosaic Disease but he could not find any casual agent. Credit for the discovery of viruses goes to the Russian botanist Iwanowski (1892) who found the causes of tobacco. He reported that the sap of infected plant filtered through bacteria proof filter was equally potent in causing the disease and conclude that the cause of disease was not bacterium but some other a smaller particle known as Virus.

Beijerinck (1898) called them "Contagium vivum fluidum " (i.e, living fluid infectant).

• In the following year the other filterable viruses" were discovered that were responsible for infection in plant animals and bacteria Bacteriophages (Virus infecting bacteria) were discovered by Twort (1915) and d. Herelle (1917). Shafferman and Moris (1963) discover Cyanophage that cause infection in blue green algae. In 1935 W.M. stanley crystallized TMV and showed that crystals retained their infectivity even when stored for indefinite period in a bottle. That the viruses could be crystallized showed that were not cells, but much simpler chemical entities. 
• Bawden and Pirie (1937) studies the chemical nature of TMV particles and reported that they are nucleoproteins.

CHARACTER OF VIRUSES: -

• Viruses are ultramicroscopic infections particles ranging from 20 to 35 nanometer in diameter.
• They're nucleoproteins.
• They contain DNA or RNA but not both which is either single standard or double standard.
• They DNA or RNA is surrounded by a protein coat that may be attached to more complicated structures.
• They are highly pathogenic cause disease in plants, animals, and bacteria.
• They are without protoplasm.
• They grow multiply and undergo mutation only within living host cells.
• They cause infection in host by nucleic acids.
• The genome has four gene, one for the core protein, two for the replicase enzyme and the fourth for a protein that probably enable the virus to spread from cell to cell in the plant.
• They are easily transmitted from one organism to another.
• They are not affected by antibodies.
• They can be crystallized.
• They are perfected obligate intracellular parasites that is they depend upon a specific host for their production and development the cells of animals, plants, and bacteria can serve as host to virus. Outside the host cell viruses exit as individual particles called Virions.

👉On the basis of the characters, given above it is not easy to say that viruses are living or nonliving. They have characters of both.

NOTE: -with few exceptions plant viruses have only RNA and a single coat made of one or a few kinds of proteins.

(a)character of non-livings: -

1. They do not have protoplasm. 
2.  They do not have enzyme system.
3.  They do not have respire.
4. They can be crystallized.
5.  Attempts to culture viruses in different types of culture media (outside the cells) have failed.

(b)Characters of living beings: -

1. They replicate, although inside the living cells.
2.  Nucleic acid present in their body, are capable of synthesizing protein for their coat, although they use ribosomes of the host for the propose.
3. Nucleic acid shows similar gene mutation as chromosome of the living organism.
4. They cause disease lack bacteria and fungi.

 However, on the basis of the above given characters many scientists considered a virus as bridge between living and nonliving or in other words they represent the transitional form of life, lying on the border line between the living and nonliving because viruses lack many features associated with life, they are referred to as particles.

CLASSIFICATION: -

According to Holmes (1948), viruses are classified on the basis of their host into the following kinds: -

(a)Animal viruses ( Zoophagineae): -They infect, fowl, pigeon, parrot, dog, crow and arthropods insect, etc. They've usually DNA or many also have RNA.

(b)Plant viruses (Phytophaginae): - They impact angiosperms, like potato, tobacco, sugarcane, cucurbits, beans and any other higher plants. They have RNA.

(c)Bacterial viruses (Phagineae): - They are commonly called bacteriophages or phases i.e, eaters of bacteria they have DNA.

 

I)TOBACCO MOSAIC VIRUS: - It is the most thoroughly studied rod shaped virus. It was isolated in the crystalline form by W.M.stanley(1935). Nobel prize was awarded to him for this discovery. Electron microscope a study have related that the TMV is a road shaped structure, 18 nanometer is diameter and 300 nanometer long. The protein coat (capsid or shell) consists of 2,130 identical subunits (capsomeres), which are arrange in a helix around a central hole of 4 nanometers in diameter. The protein constitutes 94.5% of virus. TMV contains a single chain of RNA of about 330nm in length and runs the entire length of the TMV rod. It contains about 7300 nucleotides. RNA is not present in the center of the hole but in terms with the protein subunit. It protrudes from one end of the virus. It is 5.5% of the virus. Frankel Conart and Schramm (1955) found that RNA alone is capable of causing infection. 

A part of highly magnified tobacco mosaic virus: -

II)INFLUNZA VIRUS: -

The particles are rounded or been shaped structure, measuring from 70 to 125 millimicrons. The particle can withstand at low temperature. It is killed at 65-degree Celsius. Infection cause fever, sneezing and cough. The particle usually multiplies in respiratory track. The nucleic acid is the center in RNA, enclosed by the protein coat.

III) BACTERIOPHAGES: - In 1915 F.W. Twort  and in 1917, Felix de herelle  discovered a virus which was parasitic on the intestinal bacteria, Escherichia coli. The shape of the virus, called bacteriophage, is similar to tadpole having a head, like a hexagonal prism 950 angstrom long and 650 angstroms wide which contains the nucleic acid double standard DNA molecule of about 50 micrometers in length and a cylindrical tail about 950 angstrom long consisting of a core of diameter 80 angstrom and a sheath. Tail ends on a base plate, having six spikes, each provided with 1500 angstrom long tail fiber. This is an order protein structure.

REPRODUCTION: -

Bacteriophage can be divided into two groups according to the manner in which they interact with their cells. 

(a)Virulent phages (Lytic bacteriophages): - Injected bacteria quickly multiply and then lyse the host cell.

(b)Temperate phages (Lysogenic phages): - The lambda phages.

(A)Lytic life cycle- The life cycle is completed in forest edge: -

1)INFECTION STAGE: - The page particle attached itself by absorption on a specific receptor side on the wall off bacterium. The attachment is affected by the aid of tail fibers which unwind and are removed. The host cell Wall dissolve and the tail sheath contracts, and the core forces its way through cell wall, like a hypodermic syringe.

The DNA of the phage, which is coiled up inside the head, is then forced into the bacterium through the hollow tail. The empty protein coat and fibers are left outside and are called ghost or doughnut. That only the DNA of phase enters the bacterial cell to cause infection was demonstrated by Hershey and Chase 1952 by labelling DNA with P32 and proteins with S35 in two separate experiments.

2). Latent stage: - (Also called period of eclipse)

After infection the face DNA attacks the DNA of bacterium, which disintegrates. The page DNA starts in the synthesizing a series of enzymes, essential for the replication of phase DNA.  

3) Vegetative stage: - After about 4 minutes of infection, the viral DNA replicates into about 100-150 DNA molecules which tail fibers are added.

4)Lysis or stage of liberation: - After vegetative stage, the lysis of host cell take place and very large, numbers of infective phase particle are released, each of which is capable of infecting a fresh host.

B) LYSOGENIC LIFE CYCLE: - These phages are called temperate viruses and after infection they can behave in other of the following two ways: - 

I)Behave like virulent phages: -

II)Follow lysogenic cycle: - In this cycle the bacterial DNA is not destroyed but phase DNA is incomplete into it that is the phage DNA gates attached to the bacterial DNA in a specific manner and replicates along with this, In this condition, it is transmitted to the progeny of bacteria. Such a virus is known as previous are called Prophage.  Bacteria and virus. whose chromosomes' become prophages are called lysogenic viruses. The lysogenic viruses under certain circumstances may bring about lysis of host.