Viruses, Viroids, and Prions
- obligate intracellular parasite – require a host to cause damage
- filterable – small enough to be filtrated
- contains an outer protein coat and inner genome
- has only 1 kind of nucleic acid (RNA or DNA, but never both)
- lacks metabolic abilities
Viruses can be found either inside a cell (intracellular) or outside of a cell (extracellular). If it is found extracellular, the virus is called a virion. A virion contains a protein coating called a capsid, which surrounds the core of the virus containing the nucleic acid (either DNA or RNA). Together with the capsid and the DNA or RNA core is called a nucleocapsid. Some virions also contain an envelope which is made up of a phospholipid membrane. Both the capsid and the envelope are important in protection and providing shape to the virus.
How are they Classified?
- RNA/DNA viruses
- bacterial viruses (also known as bacteriophage) because the bacterial host cell is eaten up by the virus cell
nature of the capsid coat
whether it has an envelope or not
Genome of the Virus
Are either single or double stranded DNA or RNA. The overall size of it is much much smaller than that of bacteria.
Host of Viruses
Specialists. Some viruses only affect a certain kind of host, animal, plant etc, and kind of cells within that host. So viruses infecting HIV patients only the helper T lymphocytes (helper T cells, responsible for the communication among other immunity cells) specifically because of the glycoprotein 120 (gp120). Glycoproteins are seen at the surface of the T helper cells. Human immunodeficiency virus (HIV) are able to recognize the GP120 and binds to the GP120.
Generalists. Some viruses infect a variety of cells in different hosts, like rabies.
Size of a Virus
They range from the smallest being 10nm to the largest of 400 nm
Protein coat containing capsomeres.
Polyhedral – most common is the icosahedron (20 Sides)
Complex – These are neither helical or polyhedral, example is the bacteriophage
Virus gains the envelope from their host. Envelopes are made up of proteins, phospholipid bilayer, and some glycoproteins protruding out (looking like spikes) at the surface of the virus.
These are very small, circular RNA (may appear linear), and infectious in plants. They do not contain a capsid.
Prions are proteinaceous infective particles. Prions do not contain nucleic acid. Prions contain a single protein called PrP. The sequence of amino acid that makes up PrP can fold into at least an alpha-helices, cellular PrP and a disease causing form called beta-pleated sheets called prion PrP. All mammals have PrP. Development of prion PrP only occurs in excess production of PrP or due to the formation of prion PrP.
Prion PrP induces the normal cellular PrP into a prion PrP by changing the shape of the cellular PrP.
Prion PrP leads to prion disease such as the mad cow disease or bovine spongiform encephalitis, scrapie, kuru, Creutzfeldt-Jakob disease, among others. These diseases are only transferrable by ingestion of infected tissues and transplanting such organs.