Acellular organisms include prions, viruses and viroid's
Prions
1. Prions are proteinaceous infective particles.
2. The term prion was coined by Stanley B. Prusiner.
3. It is a stable structure of abnormally folded proteins.
4. It affects the central nervous system.
5. It is responsible for causing Bovine spongiform encephalopathy(mad cow disease) in cattle's and Creutzfeldt-Jacob disease in humans.
Viruses
1. Meaning of virus-venom or poisonous fluid.
2. The term virus coined by Pasteur.
3. D.J. Ivanowsky(1892) led the foundation of discovery of virus. He identified causative agent of Mosaic disease, which are smaller than bacteria as they can pass through bacterial filter paper.
4. M. W. Beijrinek(1898) found that the extract of infected plant causes diseases in healthy plants. He called the fluid as Contagium vivum fluidum(Infectious living fluid).
5. W. M Stanley(1935) crystalized viruses and demonstrated that crystals contain largely proteins, inert outside the host cell.
6. Being acellular, they are not placed any where in Whittaker's five kingdom of classification.
7. Viruses are not truly living as they are living only inside the host body, else non cellular organisms inert crystalline structure outside the cells.
8. They need a compulsory host to complete their life cycle(obligate parasite).
9. Once inside a living cell, they take over the host cell machinery to replicate themselves, killing the host.
10. In addition to proteins, viruses also contain genetic material, either DNA or RNA, never both.
11. It is a nucleoprotein and the genetic material is infectious.
12. The protein coat is called capsid. It protects the nucleic acid and is made up of small subunits called capsomeres, which are arranged in helical or polyhedral geometric forms.
Based on host cell viruses are classified into fallowing
a) Plant viruses: single stranded RNA
b) Animal viruses: Double or single stranded RNA and double stranded DNA.
c) Bacterial virus(Bacteriophages): Double stranded DNA.
d) Human virus cause diseases such as mumps, small pox, Herpes, Influenza and AIDS.
e) Plant virus cause mosaic, leaf rolling and curling, yellowing, vein clearing, dwarfing and stunted growth.
Viroid's
1. These are infectious RNA molecules.
2. Discovered by T.O. Diener in 1971.
3. Smaller than viruses and responsible for spindle fiber disease in potato.
4. Unlike viruses, they lack protein coat and exists as free RNA, hence named viroid.
5. Viroid RNA is of low molecular weight.