[Biology Class Notes] on Virus Pdf for Exam

A non-cellular, infectious agent which is microscopic in structure and can live by depending on the host cell is termed as a virus. Due to this parasitic nature and combined features of both living organisms and non-living substances, it cannot be classified under a living organism or non-living.

Due to the lack of required cellular material, the virus cannot replicate on its own outside the host cell. To survive, it enters the host body and attaches to a specific cell, then injects the genetic material, reproduces by using host genetic material, and finally the host cell splits and releases the new viruses.

In simpler terms, a virus is a non-cellular, infectious entity with genetic material and protein which invades and reproduces by using the bacteria inside the organism, plants, and animals.

History of Virus

The virus identification goes back to February 12, 1982 and it is discovered despite limited technological intervention. This was done in St. Petersburg, Russia by Dmitri Ivanovsky. This discovery was made even before the invention of Electron Microscope.

Six years later, this discovery by Dmitri Ivanovsky was confirmed by a Dutch microbiologist, Martinus Beijerinck. Finally, until the middle of the 20th century with the help of Electron Microscope, the virus was confirmed. This study was totally credited to Ivanovsky and named the study as Virology.

Habitats of Virus

A virus lacks cellular material or strength to sustain by itself. Hence it cannot replicate on its own outside the host cell it feeds on. It adopts a unique mechanism of survival such that it enters the host body, attaches to a specific host cell and injects its genetic material. It then reproduces by using this genetic material which eventually leads to the host cell splitting up and releasing new viruses. During this time, the host body, for instance the human body is on an extra high alert, working overtime to destroy these viral cells. 

The viruses are found in every organism including the prokaryotes. The virus which infects the prokaryotes is known as Phage. These are spread most widely with diverse entities in the biosphere which are even more numerous than prokaryotic organisms. Phages are found in soil, intestines of animals, or seawater. Majority of marine organisms are infected by phages. Foods such as fermented vegetables, meats and drinking water also contain phage.

Structure and Features of Virus

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The structure of the virus varies according to the organism it is living in. A virus particle is covered with a protective protein coat called capsid as the virus contains DNA or RNA. The shape of the capsid changes according to the type of virus. The proteins present in the viral genes within the genome constitute a viral capsid. Some viruses are coated with both phospholipids and proteins, this envelope is made by using a part of the host’s cell membrane. This envelope covers the capsid and protects the viral membrane from the host’s immune system and it also has receptor molecules which bind with the host cells. This structural formation makes it easy for the virus to depend on the host immune system for survival.

As an example, below is the figure of Cytomegalovirus:

The cytomegalovirus has a capsid which covers the genetic material of the virus. This has an envelope which surrounds the capsid that is made up of phospholipids and proteins. This envelope is the special feature of this virus structure as not all viruses have an envelope protecting the capsid.

Classification of Viruses

Viruses are classified based on the morphology, chemical composition, and mode of replication. The viruses modify according to the host it lives and the hosts range from vertebrates to protozoa, plants, fungi to bacteria, and humans.

1. Helical Viruses

The viruses with helical symmetry undergo replication in which the identical protein subunits self-assemble into a helical array around the nucleic acid, which forms a spiral path. The capsid in these viruses form rigid, highly elongated rods or flexible filaments. As an example, the helical virus which is extensively studied for its structure and function is Tobacco Mosaic Virus.

2. Icosahedral Viruses

These viruses resemble the shape of an Icosahedron which is a polyhedron with 20 equilateral triangular faces and 12 vertices. The structural fragments of a polyhedron repeat five times within each 360° of rotation within any of the fivefold axes. In these viruses, the structural polypeptide chains are arranged in oligomeric structures which are known as capsomeres. These capsomeres arrange into an icosahedral shell. An example of this is the Adenovirus, in which the isometric shell is made up from 20 equilateral triangles.

3. Complex Viruses

These viruses consist of a capsid which is a combination of helical and icosahedral pattern and they have extra structures like protein tails or a complex outer wall. DNA of these viruses codes for proteins and aid in building the viral capsid. Example of this virus is the ‘moon lander’ shaped complex virus which infects Escherichia Coli bacteria.

4. Enveloped Viruses

The viruses of this kind tend to envelop themselves in a portion of the cell membrane of their host. The outer membrane of the host cell or the internal membrane like a nuclear membrane or endoplasmic reticulum is used for the formation of a Viral envelope. Influenza Virus, HIV, Varicella Zoster Virus are examples of Enveloped Viruses.

Characteristics of Virus

Virus is a sub-microscopic particle that lives by infecting the cells of a living organism. When compared with prokaryotic these are smaller in size and range from 20-300 nm. Individual virus is known as a Virion. Viruses do not have cells, cell membranes, cytoplasm, ribosomes, and many other cell organelles required for survival. Viruses can replicate only by infecting a host cell and these are usually a normal strand of genetic material with a protective coating.

Despite not having features like a living organism, they have the genetic material to evolve. The viral infections in some cases cannot be treated with normal antibiotics, hence it is always a good idea to have vaccination for a strong immunity.

2020 will go down in human history. Let us not even ask why! The year that shook the entire world with a pandemic and has affected every dimension of human existence. Everyone was locked up in their houses, education shifted to online mode, economies and populations were devastated. There was so much unprecedented loss on a global scale! On the other hand, it made us take a relook at our lives, nurture our relationships and even adjust to new routines for our physical and mental well-being. After all, every coin has two sides!

But what was responsible for this shift in global consciousness? A tiny virus named SARS Covid-19. Even if we haven’t been affected by Covid, we have experienced viral infections at some point or the other in our lives right?