Viruses

Based on genetic material

Viruses are classified by the type of nucleic acid they carry:

1. DNA Viruses Contain deoxyribonucleic acid (DNA) as their genetic material.
   • Examples:
     • Adenoviruses (cause respiratory infections).
     • Herpesviruses (cause herpes and chickenpox).
2. RNA Viruses Contain ribonucleic acid (RNA) as their genetic material.
   • Examples:
     • Influenza virus (causes flu).
     • HIV (causes AIDS).
3. Retroviruses Contain RNA but use reverse transcriptase to convert RNA into DNA during infection.
   • Example: HIV.

Based on host type

Viruses are classified by the organisms they infect:

1. Animal Viruses Infect animals and humans.
   • Examples: Rabies virus, Zika virus, and Ebola virus.
2. Plant Viruses Infect plants, causing mosaic diseases and other symptoms.
   • Examples: Tobacco mosaic virus (TMV), Cauliflower mosaic virus.
3. Bacteriophages Infect bacteria.
   • Examples: T4 bacteriophage, Lambda phage.

Based on structure

Viruses differ in their shape and structure:

1. Helical Viruses Rod-shaped, with genetic material inside a helical capsid.
   • Example: Tobacco mosaic virus (TMV).
2. Icosahedral Viruses Spherical in appearance but composed of 20-sided faces.
   • Example: Adenovirus.
3. Complex Viruses Have a combination of helical and icosahedral structures, sometimes with additional components like tails.
   • Example: Bacteriophages.
4. Enveloped Viruses Possess an outer lipid envelope derived from the host cell membrane.
   • Examples: Influenza virus, HIV.

Based on mode of transmission

1. Airborne Viruses Spread through respiratory droplets.
   • Examples: Influenza, COVID-19.
2. Vector-borne Viruses Transmitted by insects or animals.
   • Examples: Dengue virus (mosquitoes), Rabies virus (animal bites).
3. Waterborne Viruses Transmitted through contaminated water.
   • Examples: Hepatitis A, Norovirus.
4. Sexually Transmitted Viruses Spread through sexual contact.
   • Examples: HIV, Human Papillomavirus (HPV).

Based on diseases caused

1. Respiratory Viruses: Affect the respiratory tract.
   • Examples: Rhinovirus (common cold), SARS-CoV-2 (COVID-19).
2. Hepatotropic Viruses: Affect the liver.
   • Examples: Hepatitis A, B, and C viruses.
3. Neurotropic Viruses: Affect the nervous system.
   • Examples: Rabies virus, Poliovirus.

1. Tobacco mosaic virus
2. Bacteriophage virus
3. Influenza virus
4. HIV virus
5. COVID-19 virus

1. Viruses do not have cellular structures, which mean that they lack certain important organelles like nucleus, cytoplasm, golgi bodies, etc.

   1. They can only reproduce inside the living cells hence they are parasitic. Look at the example below (Bacteriophage reproduction)

      

      (a) Virus attach itself to a host cell and inject DNA into the cell (b) The virus DNA undergoes replication inside the host cell and forms viral components (c) The viral components are assembled to form new viruses which are complete with a protein coat (d) The host cell bursts to release new viruses

2. They have a simple structure consisting of either DNA or RNA but not both, surrounded by a protein or lipoprotein coat.
3. They can be described as living or non-living.
4. They are highly specific to their hosts i.e. each virus recognizes only certain types of cells.
5. Viruses are capable of replicating themselves only when they are inside the host cell.

1. They possess genetic material (RNA or DNA).
2. They reproduce when they are in the host cell (replication).
3. They are capable of identifying their hosts and attack them.
4. They can undergo mutation (they mutate) i.e. they undergo different changes in shape.
5. Like other parasites, they are specific to host.

1. They can crystallize outside the host.
2. They are metabolically inert in isolation.
3. They are non-cellular i.e. they lack cell organelles.
4. They do not perform necessary life processes such as respiration, excretion nutrition etc.

Generally viruses have a very simple structure consisting of the following:

1. RNA or DNA which may be single stranded or double stranded. They form a structure called core.
2. A protective coat of protein surrounding the core called capsid.
3. A nucleocapsid which is a combined structure of core and capsid.
4. Envelope – an additional layer of lipoprotein layer around the capsid.
5. Capsids are made up of identical repeating units known as capsomeres.

Bacteriophage is a virus that attacks and kills bacteria. Some of them have head with a tail sheath.

Bacteriophage virus

1. Viruses are used in developing vaccines, for example, vaccines for measles, polio and rubella are made from viruses that have been attenuated (weakened).
2. Viruses are used as biological weapons to kill organisms.
3. They are used as vectors in genetic engineering to transfer genes from one organism to another for improving or treating the defective genes.
4. Bacteriophages are viruses that attack bacteria and hence they help in controlling infections and diseases.

1. Viruses are pathogens, they cause disease and infections such as rabies, muscles, chickenpox and polio also they destroy living
2. They can reproduce very fast, leading to large scale
3. Viruses can attach themselves often and this become difficult to destroy for example there are many different types of viruses that cause the common cold and