Prions, a class of infectious agents, do not possess ribosomes. They are not cells or viruses, but singular, misfolded proteins. Their simple structure explains why they lack the cellular machinery, like ribosomes, found in other microbes.
The Structure of a Prion
A prion is an abnormally shaped version of a protein found in the body, mainly in the nervous system. The normal protein, PrP^C (cellular prion protein), has a structure of flexible coils called alpha-helices. The infectious form, PrP^Sc (scrapie prion protein), has the same amino acids but is folded into a rigid shape with many beta-sheets, making it stable and resistant to being broken down by enzymes.
As a single protein molecule, a prion lacks the components of a biological cell, such as a nucleus or organelles like ribosomes. It is a standalone particle of protein, with no genetic material like DNA or RNA. This simplicity distinguishes it from other infectious agents.
How Prions Propagate Without Cellular Machinery
Prion propagation does not involve manufacturing new material. When an infectious PrP^Sc particle encounters a normal PrP^C protein, it acts as a template. This interaction forces the normal protein to refold into the infectious PrP^Sc conformation.
This conversion sets off a chain reaction, where new PrP^Sc proteins convert other normal PrP^C proteins. This process is an exponential cascade of misfolding that uses the host’s existing supply of normal proteins. The resulting buildup of abnormal proteins accumulates in brain tissue, causing the cell damage seen in prion diseases.
Comparing Prions to Other Infectious Agents
Bacteria are complete, single-celled organisms. They possess their own DNA and ribosomes, allowing them to metabolize nutrients and reproduce independently through cell division.
Viruses are packets of genetic material, either DNA or RNA, inside a protein coat. Lacking ribosomes, they cannot reproduce on their own. A virus must invade a host cell and use its ribosomes to read the viral genetic code and manufacture new virus particles.
Prions are different from both. They have neither genetic material to provide instructions nor ribosomes to build new components. Their infectious cycle depends entirely on forcing existing host proteins to change shape. While bacteria build their own proteins and viruses borrow the machinery, prions bypass protein synthesis.