All living things are composed of one or more cells. Unlike cells, viruses, viroids and prions are acellular, nonliving parasites that require a living host to reproduce.

Cellular Organisms: Prokaryotes & Eukaryotes

All living things are composed of cells that comprise the two main categories of living organisms, prokaryotes and eukaryotes.

Prokaryotic Cells

Prokaryotes include the biological domains of Eubacteria (bacteria) and Archaea (ancient bacteria-like organisms). These simple microscopic cells were the first living organisms to evolve.

Prokaryotes are primarily distinguished by the fact that they lack a membrane bound nucleus. Their genetic material is naked within the cytoplasm, ribosomes their only type of organelle.

Eukaryotic Cells

Eukaryotes all belong to the biological domain Eukarya. This domain contains the organisms that we are the most familiar with, such as animals (including humans), plants, fungi as well as some single celled microscopic organisms.

The most noticeable feature that differentiates these more complex cells from prokaryotes is the presence of a nucleus; a double membrane-bound control center separating the genetic material from the rest of the cell.

Acellular Particles: Viruses, Viroids & Prions

Although they may sometimes behave like living things, acellular particles, such as viruses, are not alive. Viruses, viroids and prions are not considered to be living organisms because they are incapable of carrying out all life processes.

Acellular particles…

• are not made of cells
• cannot reproduce on their own
• do not grow or undergo division
• do not transform energy
• lack machinery for protein synthesis
• are so small that they can only be seen with an electron microscope

What Is a Virus?

Viruses infect living cells and once inside, transform the cell essentially into a factory for making more viruses.

Viruses are composed of nucleic acid, proteins and, in some cases, lipids as well. Nucleic acid, which can be either DNA or RNA, encodes the genetic information that is necessary to make copies of the virus.

Viruses reproduce via four basic steps, as follows:

1. Delivery if the viral genomes into a host cell
2. Commandeering the host cell’s transcription and translation machinery
3. Utilization of host cell building blocks to copy viral genomes and synthesize viral proteins
4. Viral genomes and proteins self-assemble and exit host cells as new infectious particles.

What Is a Viroid?

Viroids are smallest known agents of infectious disease. Whereas viruses are made up of nucleic acid encapsulated in protein (capsid), viroids are uniquely characterized by the absence of a capsid.

In spite of their small size, viroid ribonucleic acids (RNAs) can replicate and produce characteristic disease syndromes when introduced into cells. Thus far, this type of acellular particle has only been identified in association with plants and is not currently a threat to humans.

What Is a Prion?

A prion is a disease-causing agent responsible for various fatal neurodegenerative diseases called Transmissible Spongiform Encephalopathies. Prions are an abnormal form of a normally harmless protein found in mammals and birds.

These abnormal proteins can enter the brain through infection or it can arise from a mutation in the gene that encodes the protein. Once present in the brain, prions cause normal proteins to refold into abnormal shapes. As these abnormal proteins multiply, they destroy neurons and eventually cause brain tissue to become riddled with holes.

More Information on Microbes

You can also find other informative microbe-related articles at Suite101, including Prokaryotic and Eukaryotic Cells, Viral Pneumonia, What is a Virus and the Bacterial Cell Wall or see the excellent websites of Science Prof Online or Cells Alive.

Sources

Bauman, R. (2005) Microbiology.

Park Talaro, K. (2008) Foundations in Microbiology.