INTRODUCTION

• Viruses are defined as non-cellular infectious entities which contain either RNA or DNA, normally encased in proteinaceous coat, and reproduce only in living cells.
• They replicate in living organisms and are responsible for many diseases such as influenza, hepatitis, smallpox and AIDS.

CHARACTERISTICS OF VIRUS

Viruses are infectious particles with both living and non-living characteristics.

LIVING CHARACTERISTICS

• Viruses possess genetic material in the form of DNA or RNA.
• Viruses can replicate inside a host cell and show gene mutations similar to living organisms.
• Viruses are capable of causing various diseases in plants and animals.

NON-LIVING CHARACTERISTICS

• Viruses are a-cellular and do not possess any cytoplasm or cellular organelles.
• They do not carry out metabolism on their own and are completely dependent on the host cell’s metabolic machinery.

STRUCTURE OF VIRUS

• Viruses are a-cellular in nature and ultra-microscopic in size.
• Structurally, they consist of two components;

1. Nucleoid: is the inner core of nucleic acid and may contain either DNA or RNA but not both.
2. Capsid: is the outer protein coat encasing the nucleoid.

CLASSIFICATION OF VIRUS

Viruses are classified differently according to various aspects such as;

• On the basis of type of nucleic acid
• On the basis of type of host
• On the basis of mode on nutrition

ON THE BASIS OF TYPE OF NUCLEIC ACID

On the basis of nucleic acid viruses are divided into two major types:

1. DNA viruses
2. RNA viruses

DNA VIRUSES

• These are viruses containing DNA as its genetic material.
• DNA virus are further classified as;

1. Single-stranded (ss) DNA virus
2. Double-stranded (ds) DNA virus

RNA VIRUSES

• These are viruses containing RNA as its genetic material.

ON THE BASIS OF TYPE OF HOST

According to the type of host being infected, viruses are categorized into four major types

1. Animal viruses
2. Plant viruses
3. Bacteriophage
4. Insect viruses

ANIMAL VIRUSES: The viruses which infect, live and replicate inside the animal cell including man are known as animal viruses. Their genetic material is either RNA or DNA.

FOR EXAMPLE: Influenza virus, Rabies virus, Mumps virus, Poliovirus etc.

PLANT VIRUSES: The viruses that invade plant cells and cause infection in plants are called plant viruses. Their genetic material is RNA which remains enclosed in the proteinaceous coat.

FOR EXAMPLE: Tobacco mosaic virus, Potato virus, Beet yellow virus, Turnip yellow virus etc.

BACTERIOPHAGE: Viruses which infect bacterial cells are called bacteriophage or bacteria-eaters. They contain DNA as genetic material.

FOR EXAMPLE: Escherichia virus T4, M13 bacteriophage, Bacteriophage MS2 etc.

ON THE BASIS OF MODE OF TRANSMISSION

AIR-BORNE VIRUS: Virus transmission occurs through air via the respiratory route. FOR EXAMPLE: Swine flu, Rhinovirus etc.      

SEXUALLY TRANSMITTED: Virus transmission can occur through sexual contact with the infected individual. FOR EXAMPLE: HIV, Retrovirus etc.

TRANSFUSION-TRANSMISSION: Virus transmission occurs through blood transfusion. FOR EXAMPLE: Hepatitis B virus, HIV etc.

REPLICATION CYCLE OF VIRUS

• Viruses cannot replicate itself. They depend on the host’s biosynthetic machinery to undergo replication.
• Replication of bacteriophage in host involve following steps:

1. Attachment
2. Penetration and uncoating
3. Replication and assembly
4. Release

VIROIDS

T.O. Diener in 1971 discovered a new infectious agent that was smaller than the virus and mainly caused potato spindle tuber disease. The infectious agent did not contain any RNA, also lacked the protein coat, and hence was named viroid.