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Prokaryotes and Viruses

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Biology

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 fulfill 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 cells at all. They do not meet the criteria of a living organism. The criteria of a living organism are:

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).

Differences between prokaryotes and eukaryotes

Eukaryotic and prokaryotic cell structures differ. They have some organelles in common, such as the plasma membrane and the cytoplasm. However, a number of organelles with a specialised structure to produce ATP (mitochondria) or carry out photosynthesis (in the chloroplast) are only present in the eukaryotic cells.

Prokaryotes and Viruses flagellum plasmid StudySmarterFigure 1. An example of a bacterium - StudySmarter Originals.

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

Prokaryotes and Viruses mitochondria eukaryote StudySmarterFigure 2. An example of a eukaryotic cell - StudySmarter Originals.

Prokaryotes: bacteria

Here we will briefly cover the classification and reproduction of bacteria.

Classification

Bacteria can be classified through the gram stain or by their shape. Let’s see how these classifications work.

Gram stain

Bacteria can be sub-divided into two main groups: gram-negative and gram-positive. Bacteria are classified in this way by using a gram stain. The Gram stain (which is purple) colours the bacteria’s cell wall, and this determines the overall outcome of the stain.

When we apply the purple Gram stain, it will colour the Gram-positive bacterium in a distinct purple, and the Gram-negative one in a pale red colour. Why do Gram-positive bacteria retain the purple colour? This is because Gram-positive bacteria have a thick peptidoglycan cell wall.

Where does the red colour come from in the Gram-negative bacteria? From the counterstain, safranin.

Safranin is used as a coutnerstain in the Gram test to help distinguish between the two types of bacteria. Scientists can use other counterstains depending on the nature of the experiment/the stain.

Examples of Gram-positive bacteria include Streptococcus. Examples of Gram-negative ones include chlamydia and Helicobacter pilorii.

By shape

Bacteria can also be classified by their shape. Round bacteria are known as cocci, cylindrical as bacilli, spiral-shaped ones as spirilla, and comma-shaped bacteria as vibrio. There are also other less common types of bacteria such as star or rectangular shaped ones.

Reproduction

Bacteria mostly reproduce asexually. The most common form of reproduction in bacteria is called binary fission.

Binary fission is a process in which a bacterial cell copies its genetic material, grows, and then splits into two cells, making an exact replica of the mother cell.

Bacterial conjugation involves two bacteria, but it isn't a form of reproduction. During bacterial conjugation, genetic information in the form of plasmids is transferred from one cell to another via pili. This often gives the receiving bacteria an advantage, such as antibiotic resistance. This process doesn’t produce a new bacteria. It’s more like a ‘buff’ version of the previous one.

Prokaryotes: archaea

While you won’t need to know too much about Archaea, let’s highlight a few things. Next to bacteria, archaea are the other pillar of prokaryotes. They can be found in extreme environments like geysers and vulcanoes. They evolved to function best in those environments. Archaea are mostly unicellular.

Some research suggests that archaea could be the origin of eukaryotes, as they share traits with both prokaryotes and eukaryotes.

Viral structures

Viruses are non-living microbes. 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. They are made of genetic material in the form of DNA or RNA, a protein capsule around the genetic material called a capsid, and, sometimes, a lipid membrane. They also have enzymes to break down the cell wall or the membrane, and can also have an enzyme that transcribes RNA to DNA. This enzyme is called reverse transcriptase. 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.

Prokaryotes and Viruses Virus structure StudySmarterFigure 3. A virus structure. Source: DEXI, Wikimedia Commons.

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.

For more information on the replication please visit our explanation on Viral Replication.

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

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.

Prokaryotes and Viruses - Key takeaways

  • Prokaryotes are almost exclusively unicellular organisms, they don’t have a nucleus.
  • Prokaryotes (like bacteria) are living cells. Viruses are not defined as living.
  • Both viruses and bacteria can cause infections, but in different ways.
  • Viruses need a host to reproduce.
  • Bacteria are much bigger than viruses.

Prokaryotes and Viruses

Viruses can infect both prokaryotes and eukaryotes, causing disease or cell death.

Viruses are not considered alive as they are not capable of replicating without a host cell.

They can both cause diseases in eukaryotes.

These are called bacteriophages. 

Final Prokaryotes and Viruses Quiz

Question

What do eukaryotes and prokaryotes have in common in comparison to viruses? 


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Answer

They are living cells.

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Are there unicellular eukaryotes?


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Answer

Yes, they are called protists.

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Besides bacteria, what is the second type of prokaryotes?



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Answer

Archaea

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What cells can viruses infect?


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Answer

Eukaryotes and prokaryotes

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How big are viruses? 


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Answer

20-400nm 

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Why are viruses not alive?


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Answer

They can’t reproduce without a host cell as they do not have ribosomes. 

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How do bacteria multiply mathematically?


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Answer

Exponentially 

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How are bacteria studied?


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Answer

They are put on to a medium with nutrients to multiply and are then often viewed under a microscope. 

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What is the difference between gram-positive and gram-negative bacteria?


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Answer

Gram-positive bacteria have a thicker cell wall.

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What is the most common type of bacteria reproduction?


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Answer

Binary fission. 

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How many orders of magnitude are bacteria bigger than viruses?


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Answer

Bacteria are measured in micrometers whereas viruses are measured in nanometers. These differ by a factor of 1000. 1 micrometers is a thousand nanometers. 

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Which cells can be viewed through a light microscope?


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Answer

Eukaryotic and prokaryotic cells but not viruses. 

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Which types of genetic material can viruses have?


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Answer

DNA or RNA 

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What device is needed to view viruses?


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Answer

Electron microscope 

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What gives bacteria genetic advantages?


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Answer

Plasmids, which are transferred from one cell to another via pili.

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