Viruses are not living beings, even though they act like it. Viruses cause many serious diseases by injecting new genetic instructions straight into our own cells.

Why this is important…Viruses cause plenty of impactful diseases – polio, influenza, and COVID to name a few. Anything that can shut down the world for 2 years, deserves our attention to understand it.

Foundations

Viruses get lumped into a large group of things we call “germs.” Unlike bacteria, protists, or fungi, viruses are not alive. They are pathogens, completely dependent on host cells for everything from energy processing to reproduction. We still consider them “biological” however, since they contain their own genetic material.

Viruses are incredibly small. One tool molecular biologists use to learn about things at this scale is radioactive material. This allows molecules to be tracked around cells. Using radioactive viruses even helped establish that DNA was the molecule of inheritance.

Key terms

Isotope – a chemical element with a different number of neutrons, which increases the atomic weight; some isotopes are unstable and are therefore radioactive

Details

Viruses are smaller than microscopic. Hundreds can fit on the surface of a bacterial cell. The first virus was described in 1892. It attacked tobacco plants. Since then, over 9000 other viruses have been described.

Virus structure is simple. A short strand of genetic material (DNA or RNA) is surrounded by a protein shell (capsid). Some viruses also have an outer lipid layer (envelope) stolen from their host’s cell membrane.

When a virus attacks a cell, it takes over the cell, causing it to make more viruses. The virus has to bind to surface proteins of the cell. This triggers the release of the virus’ genetic material into the cell. Once inside, the viruses’ DNA or RNA instructs the cell’s protein-making machinery to construct more protein coats. It also forces the cell to copy the genetic material of the virus. Once these 2 parts are assembled into new virus particles, the cell often bursts, releasing the viruses to attack new cells. No wonder they make us feel miserable!

Stop & Think #5: Do you think viruses could be used to treat bacterial infections? Explain why or why not.

Diagrams of virus structure. Both show nucleic acid core, protein coat and outer envelope with embedded proteins.

FIGURE 12-4. Virus structure. Left: general diagram of a virus that has a fatty envelope around it. Right: diagram of the coronavirus, which also has a fatty envelope. The spike proteins help the virus stick to specific cells. ("Virus structure" by Graham Beards is licensed under CC BY-SA 3.0 || "Coronavirus" by Scientific Animations is licensed under CC BY-SA 4.0)

To combat viruses, our bodies have a couple defenses.

Cells infected with viruses release a signal molecule called interferon. This tags the infected cell for destruction by our own immune cells. It also signals neighboring cells to make anti-viral proteins that chop up viral DNA. This selfless act by the infected cell prevents the spread of the virus.

Immune cells can also produce proteins to attack virus particles. If our immune system recognizes the protein coat of a virus, then it can make proteins (antibodies) that stick to it and drag it down. Antibodies will be discussed further in a later section.

Viruses in research

Viruses helped solve the debate over which was the molecule of inheritance. Both proteins and DNA had been known to science since the late 1800s, but most scientists assumed proteins were the molecule of inheritance. The reason was simple – proteins were more variable. The complexity of inheritance seemed like it needed a complex molecule. In the 1940s, experiments using bacteria suggested that DNA was the molecule of inheritance, but many scientists were still not convinced.

The simple structure of viruses – made of just proteins and DNA – helped solved the debate. In 1952, Alfred Hershey and Martha Chase performed an elegant experiment that convincingly showed that DNA was the molecule of inheritance.

They understood that viruses hijacked other cells, forcing them to make more viruses. This is how viruses work – they take over a cell and insert their own instructions. They reasoned whichever molecule was injected into the cell must contain instructions. Since viruses were only made of DNA and protein, they just needed a way to track which one was inserted.

But how could they track individual molecules? The answer was to label viruses with isotopes. One batch of viruses was tagged with radioactive sulfur, which labeled proteins. The other batch of viruses was tagged with radioactive phosphorous, which labeled DNA.

Radioactive isotopes allow molecules to be tracked. What makes one phosphorous atom different from another? Nothing, unless it has a different number of neutrons in its nucleus, making it radioactive. If it gives off radioactive particles, these can be detected as different from the phosphorous that does not. For the rest of the story, watch Video 12a.

To further explore isotopes, visit this PhET Interactive Simulation site. If you select the “Isotopes” simulation you can drag extra neutrons into an atom to see how it changes its stability and atomic mass.

VIDEO 12a: Hershey and Chase Experiment

After watching the video, you should be able to:

describe isotopes and how they can be used to track molecules
explain how their experiment demonstrated that DNA was the molecule of inheritance

The next year, 1953, James Watson and Francis Crick famously announced their discovery of the structure of DNA. This started the field of molecular genetics. Since then, much more has been discovered, such as the processes of replication, transcription and translation.

Oddly, viruses can also be used to heal people. Since viruses are excellent at getting into small spaces and delivering genetic instructions into specific cells, they are potential medical tools. Although the process is still being perfected, viruses can be used to genetically alter cells. This has been used successfully to genetically engineer plants and animals, inserting genes with specific traits into cells. It has also been used, with much more limited success, to introduce genes to human cells that lack the correct genetic instructions. This practice is known as gene therapy. The goal is to correct genetic diseases by inserting the proper version of genes into cells that lack any working copies.

Stop & Think #6: What potential problems might occur during gene therapy?

Learning objectives

Describe how viruses reproduce
Describe how isotopes differ from one another
Explain the role of isotopes and viruses in discovering the role of DNA.

Stop & Think Questions Explained

5. At this point, you probably don’t have enough information to be certain, so you have to speculate. There are viruses (phages) that attack bacteria specifically. We still are not sure how big of a role they play in our own health. It is possible that we could use phages instead of antibiotics someday.

6. Inserting DNA is risky. It could interrupt another gene and cause damage. There are other potential risks, but that is one that should make sense to you based on genetics you know.