Virology: Introduction, Diagram, Stages, Types, Life Cycle, FAQs

Virology is the study of viruses and how they interact with living organisms, such as humans, animals, plants, and microbes. It is the study of how viruses damage cells, impair our immune systems, resulting in disease. Virologists also study how viruses evolve and spread in order to develop vaccines and treatments.

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1. What is Virology?
2. Classification of Viruses
3. Types of Viruses
4. Diagram of the Viral Replication Cycle
5. Lytic Cycle (fast and destructive)
6. Lysogenic Cycle (slow and hidden)
7. Host-Virus Interactions

This topic is commonly asked in entrance and competitive exams and is important to biology. This chapter will cover the basic concepts of virology, including the types and structures of viruses, their transmission, and their effects on living organisms. Virology is an important topic in the 11th class of biology.

What is Virology?

The study of viruses and their effects on humans, animals, and plants is known as virology. This field of study aids in our understanding of how viruses divide, induce illnesses, and are repelled by our bodies. In order to prevent or treat viral infections, scientists are always working to develop new vaccines and medications. Virology is important because it protects people against viral diseases like COVID-19 and the flu.

Also Read:

• Biological Classification

• Composition of Bacterial Cell Wall

• Plant Kingdom

Classification of Viruses

They are classified based on many characteristics, including their viral genetic material, the presence or absence of the envelope, and many other structural and functional properties.

DNA Viruses vs. RNA Viruses

The following points explain the key differences between DNA and RNA viruses, based on their genetic material, replication methods, and examples.

• DNA Viruses: These are viruses whose genetic material is represented by DNA. Examples are the ones of the Herpesviridae family, which causes herpes simplex and varicella-zoster and the Adenoviridae family, responsible for respiratory infections.

• RNA Viruses: These include viruses with RNA as their genetic material. This grouping includes viruses of the Retroviridae family, which includes HIV, and the Orthomyxoviridae family, the causative agent of influenza.

Enveloped vs. Non-enveloped Viruses

The pointers below describe how viruses differ in structure based on the presence or absence of an outer envelope and how it affects their infection process.

• Enveloped viruses have capsids enveloped by a lipid envelope originating from the host cell membrane and containing viral-derived glycoproteins. Examples of these include the families Herpesviridae and Retroviridae.

• Non-enveloped Viruses have no lipid envelope and thus are generally more resistant to environmental conditions. Examples include the family Picornaviridae, which includes poliovirus and rhinovirus and is the causative agent of the common cold, and the family Adenoviridae.

Types of Viruses

The types of viruses, classified based on their genetic material, host range and structure, are given below-

• Animal viruses are obligate intracellular parasites that infect animal cells and tissues. They consist of genetic material (DNA or RNA) encapsulated by a protein coat (capsid). An example is the influenza virus, the rabies virus, and the coronavirus responsible for COVID-19.

• Plant Viruses are submicroscopic infectious particles that replicate only inside the living cells of plants. They have genetic material (DNA or RNA) encased in a protein coat called a capsid. Some well-known plant viruses include Tobacco Mosaic Virus (TMV), Cucumber Mosaic Virus (CMV), and Potato Virus Y (PVY)

• Bacteriophages are viruses that exclusively infect bacteria. The term phage is derived from Greek and means "bacteria-eater". Phages generally possess a narrow host range so that they can often only infect and kill a few strains of a species. Examples include T4 phages that infect bacteria such as E. coli.

• Retroviruses are a type of RNA virus that contains the enzyme reverse transcriptase, which allows their genetic information to be integrated into the host DNA. Retroviruses have a genome consisting of two RNA molecules, which may or may not be identical, from which they code for DNA. One such famous retrovirus is HIV, which causes AIDS.

• Arboviruses are arthropod-borne viruses and are transmitted by biting arthropod vectors. The viruses are genetically diverse and are from several different virus families, including Bunyaviridae, Togaviridae and Flavivirus.

Diagram of the Viral Replication Cycle

The diagram and explanation show how viruses infect host cells and replicate inside them, following a step-by-step infection process.

Lytic Cycle (fast and destructive)

The viruses in this case multiply quickly and kill the host cell in order to release new virus particles. The steps that follow explain it.

1. Attachment and Injection: This virus, also known as a bacteriophage, attaches to the host bacterium and injects its DNA into it.

2. Decision Point: The virus can go into the lytic or lysogenic phase after it has regained its circular shape.

3A. Virus Production: The viral DNA makes copies of its own genome by using the host's cells to divide.

4A. Cells burst: When cells burst, they release new viral particles.

Lysogenic Cycle (slow and hidden)

These show the lysogenic cycle, in which the viral DNA hides inside the host genome and slowly copies itself without any effort.

2. Decision Point: The phage DNA can also enter the lysogenic cycle.

3B. Integration: The DNA of the virus is integrated into the host genome.

4B. Normal Replication: The host bacterium divides its own cell and copies the viral DNA along with it.

5. Reactivation: The prophage leaves the host chromosome due to induction and enters the lytic cycle,

Host-Virus Interactions

Various mechanisms, such as receptor-mediated endocytosis, direct fusion with the cell membrane, or directly injecting their genetic material into the host cell, mediate the entry of viruses into cells. Major events are mentioned below in the table:

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