And cannot be seen

à Viruses have different shapes: they may be simple (rod-shaped, spiral, spherical, polyhedron) or very complex

à Viruses are obligatory parasites: as they don't multiply or reproduce unless they are inside a host cell

à Viruses are highly-specific organisms: Every virus has certain living organisms and cells. Viruses which attack plants cannot attack animals or humans, and those which attack the brain cannot attack the liver

à They are non-living organisms (pathogens – disease-causing) which are more simple than viruses

à They consist of small ring-shaped RNA strips in the form of double helix

à They do not have protein coat (capsid)

à Viroids enter the nucleus of the infected cell, and directs metabolism to produce more Viroids. They cause many diseases to plants such as:-

- Potato spindle tuber disease

- Cucumber pale fruit disease

à Viroids may spread quickly, but they cannot be determined because they do not destroy host cells like viruses

à Prions are non living organisms which are more simple than Viroids, as they are composed of only protein and do not have any nucleic acids.

à They spread inside the infected tissues of organisms causing disease which destroy their central nervous system, as they turn brain into a sponge-like mass full of pores like sieves. This may cause death

à Prions infection is common in sheep and livestock, they cause mad cow disease. Prions causes the infection of human with two diseases whose symptoms are like those of mad cow.

à Mad cow Prions can transfer to human from meat products of cows suffering from the disease (Hamburgers, sausages, cosmetics and medicine containing substances extracted from cows…etc)

à The transfer of Prions to human wasn't proved till now due to the extent of incubation period of mad cow disease in human

Дата добавления: 2015-10-23