Prokaryotes and Viruses

If you have read our explanation on Cell Structure, you probably know that prokaryotes do not have a nucleus or any other membrane-bound organelles. Prokaryotes are almost exclusively unicellular organisms: they are made up of a single cell. Prokaryotes can, however, form something called colonies. These colonies are interlinked but don’t fulfil all criteria of a multicellular organism.

Eukaryotes, on the other hand, are cells with a nucleus. Most often eukaryotes are multicellular. The main types of eukaryotes are animals, plants, fungi, and protists. Protists are special eukaryotic cells that are unicellular organisms. Go to our explanation on the subject if you want to learn more about Eukaryotes.

Viruses are not considered living beings at all because they do not meet the criteria of a living organism. The criteria of a living organism are:

• Sensitivity and response to the environment.
• Autonomous reproduction - viruses cannot reproduce on their own, but rather need to invade another organism to reproduce.
• Growth and development.
• Homeostasis.
• Energy processing - viruses do not process energy themselves: they use the hosts cellular machinery to obtain the components they need to reproduce.

What types of prokaryotes are there?

There are two main types of prokaryotes: bacteria and archaea. The main differences are the cell membranes and the conditions in which these prokaryotes are found.

Bacteria have a phospholipid bilayer, whereas archaea have a monolayer. Archaea are only found in extreme conditions such as hot geysers. Bacteria, on the other hand, can be found absolutely everywhere on earth, even in the human body (good bacteria).

Viral structures

Viruses are non-living microbes, they are not cells and therefore they are neither prokaryotes nor eukaryotes. This means that they need some kind of host to reproduce as they can’t do it on their own. They do, however, have genetic material, either DNA or RNA. They introduce the DNA or RNA into the host cell. The cell is then manipulated into producing the virus parts, after which it usually dies.

Viruses have fewer components than cells. The basic components are:

• Genetic material (DNA or RNA)
• Initial proteins to help with host invasion. Retroviruses also carry the reverse transcriptase.
• Capsid (protein capsule that surrounds the genetic material)
• Lipid membrane surrounding the capsid (not always present)

Viruses do not have any organelles, which is the reason they cannot make their own proteins; they do not have any ribosomes. Viruses are much smaller than cells and you can almost never see them in a light microscope.

Differences between prokaryotes and eukaryotes

Eukaryotic and prokaryotic cell structures differ. They have some organelles in common, such as the plasma membrane, ribosomes and the cytoplasm. However, membrane-bound organelles are only present in eukaryotes.

Fig. 1. Schematic prokaryotic cell structure.

The eukaryotic cell structure is much more complex than the prokaryotic one. Prokaryotes are also usually single-celled, so they can’t ‘create’ specialised structures, whilst eukaryotic cells usually function together and create specialised structures. For example, in the human body, eukaryotic cells form tissues, organs, and organ systems (e.g. the cardiovascular system).

Fig. 2. Animal cells are an example of eukaryotic cells.

Prokaryotes, eukaryotes and viruses Venn diagram

Here's a Venn diagram aid to help you understand what prokaryotes, eukaryotes and viruses have in common and where they differ.

Fig. 3. Venn diagram comparing eukaryotic and prokaryotic cells and viruses.

Impact of viruses on prokaryotic and eukaryotic cells

Viruses can infect plants, animals, humans, and prokaryotes.

A virus often causes an illness in the host by inducing cell death. Most often, viruses only ever infect one species, like humans. A virus that infects prokaryotes will never infect a human, for example. However, there are instances where a virus can infect different animals.

A common example of the effect of viruses in prokaryotic cells are the bacteriophages. These are a group of viruses that only infect bacteria.

Viruses infect host cells by:

• Attaching to the host cell.
• Injecting their DNA or RNA into the host cell.
• The DNA or RNA is translated and transcribed into proteins which make up viral components called virions. The virions are released and usually, the host cell dies.
• The process is repeated with more and more virions.

Below you will find a diagram showing the infection through bacteriophages.

Fig. 4. Lytic cycle of a bacteriophage.

Studying viruses and prokaryotes

Bacteria are usually grown in cultures using a medium with nutrients in which they can quickly multiply. The multiplication of bacteria is exponential, because the number of bacteria always doubles: from one to four, to eight, etc. This means that bacteria replicate very quickly and can often be viewed under a light microscope.

Viruses, however, are much smaller and can’t simply grow on their own. They need a cell to grow in and can most commonly only be seen under an electron microscope. For comparison, the average size of bacteria is approximately 2 micrometers whereas the average size of a virus is between 20 and 400 nanometers.