Biology
Viruses and Protozoa
Viruses
A virus is a microscopic infectious agent consisting of a nucleic acid molecule(DNA or RNA) in a protein coat and replicates only inside the living cells of an organism. The study of virus is called virology. The viruses become active once they enter the host cell
Based on the structure viruses can be classified as
• Icosahedral (e.g. Reo virus and Picorna virus)
• Helical (e.g. Paramyxovirus and orthomyxovirus)
• Radial symmetry (e.g. Bacteriophage)
• Complex virus (Pox virus)
Based on Genetic material present viruses can be classified as
• DNA virus – Single stranded DNA virus like Parvovirus and Double stranded DNA virus like Adenovirus
• RNA virus – Single stranded RNA virus like Poliovirus, Rabies virus and Double stranded RNA virus like Reo virus. Single stranded RNA virus can be positive or negative sense.
Based on the type of host viruses can be classified as
• Animal/human viruses – e.g. Influenza, Polio, rabies etc
• Plant viruses – e.g. potato virus, beet yellow virus etc.
• Bacteriophage
Based on mode of transportation viruses can be classified as
• Through respiratory route – Rhino virus, Swine flu etc.
• Through blood – Hepatitis B, HIV etc
• Through faeco-oral route – Hepatitis A, Polio etc
• Through sexual contacts – Retro virus
Based on replication viruses can be classified as
• Replication in cytoplasm of host – All RNA viruses except influenza
• Replication in nucleus of host – All DNA viruses except Pox virus
• Replication through single stranded RNA – All RNA viruses except Reo and tumor causing viruses
• Replication through double stranded DNA – All DNA viruses, Reo virus and some tumor causing RNA viruses
• Replication In nucleus and assembly in cytoplasm – Influenza, Pox
Baltimore classification of viruses
• Group I – Viruses contain double stranded DNA. e.g. Adenoviruses, Herpesviruses, Poxviruses)
• Group II – Viruses contain single stranded DNA. e.g. Parvoviruses
• Group III – Viruses contain double stranded RNA. e.g. Reoviruses
• Group IV – Viruses contain single stranded RNA with positive polarity. e.g. Coronaviruses, Picornaviruses
• Group V – Viruses contain single stranded RNA with negative polarity. e.g. Orthomyxoviruses
• Group VI – Viruses have two copies of single stranded RNA to be converted to dsDNA by reverse transcriptase e.g. Retroviruses
• Group VII – Viruses have partial double stranded DNA with RNA intermediate in life-cycle e.g. Hepadnaviruses
Protozoa
Protozoa are unicellular and eukaryotic organisms. Most are free living and some are parasites. Some of the human diseases caused by Protozoa are Malaria which is caused by Anopheles mosquito, Amoebiasis which is gastrointestinal infection caused by poor hygiene because of dirty water and food, African sleeping sickness caused by Tsetse fly, Toxoplasmosis which infects eye, brain and lymph nodes caused by undercooked meat.