Virology 100x: Let’s start from the very beginning

Tobacco Mosaic Virus

The tobacco mosaic virus; the first virus discovered and studied.

Let me get one thing straight first – viruses are NOT the same thing as bacteria. They’re just not. Okay? Great.

What are viruses, then? Well, I could tell you that they’re nucleoprotein complexes that make use of cells to replicate; I could tell you that they are obligate intracellular parasites – that is to say, they’re obliged to be parasites inside our cells; I could even tell you that they do not undergo cell division. But I won’t.

Oh. I already did.

Never mind then.

The point is, viruses are all that and more. One of the most interesting things in the world, we still can’t decide if viruses are living organisms or not. They carry genetic material, and are able to replicate themselves (but only after they’ve infected a host cell). However, outside of cells, they become metabolically inert – they can’t make their own proteins or copy their own DNA (or in the cases of some cells, their own RNA). Yet they can ‘die’ – if they’re outside of their host cell for long enough, or are otherwise exposed to adverse conditions, they can no longer successfully infect cells. This straddling of the line between life and non-life is very likely to be a clue to how life originated; most viral genes are so unlike the genes of the rest of the planet that they may well have come from a common ancestor before cell-based life existed.

The core of any virus is its genetic material – either DNA or RNA; this may be wrapped up with viral proteins to form a nucleoprotein complex (from combining “nucle”ic acid + “protein”). Viral proteins are also incorporated into the protective coat surrounding the nucleoprotein complex to give rise to the basic viral particle (or the virion). Some viruses, particularly those that infect mammals, may also have an additional lipid envelope over its protective coat.

You may think that all viruses are harmful to us, but this is not true. Because of the proteins embedded in their protein coat (or lipid envelope), viruses are very specific about which species they will infect, much like how only one specific key can open one specific lock. In fact, there are viruses, known as bacteriophages, which are beneficial to us because they are able to infect, and thus cause the death of bacteria that are harmful to us. It has been suspected that some “healing waters” may have been effective due to the presence of viruses that infect pathogenic bacteria. Viruses also infect amoeba, fungi, plants, and of course, other animals. In fact, every species on Earth has a whole host of viruses that is specifically only able to infect that species.

lock and key

In this case, the pink object refers to a viral surface protein, while the orange object refers to a host cell surface protein/glycoprotein. Only when viral protein keys are able to fit into host protein/glycoprotein locks can the infection of a cell start to happen, so viruses are VERY specific about who they are able to infect.

The gold standard for determining a new virus is by cultivating and isolating the virus, and viewing it under an electron microscope. Unfortunately, this species specificity means that it is very difficult to cultivate human viruses in order to study them, because they won’t infect non-human cells. The best way to go about studying human viruses would of course be by infecting humans, but this is ethically problematic, to say the least. One partial solution would be to culture human cells in a dish, and throw some human viruses on top to infect the human cells. This helps with studying how viruses infect cells in the first place, and what effects they have on cells in isolation. However, viruses are very finicky, and it first takes a lot of trial and error, and later, precision and time, to properly grow them. This still doesn’t answer how viruses act in the whole body, though – especially in the presence of our immune system, which has been implicated in a large amount of the cellular damage it causes as it tries to rip the virus from our bodies. Scientists thus have to use genetically near-identical species such as monkeys, or modify mice and other model organisms such that they carry human proteins, and are then able to be infected by our human viruses.

Because viruses are basically protein coats around genetic material, they are therefore also ideal as a mechanism for transferring genes to our own genomes. These engineered viruses are known as viral vectors, and have been modified such that they no longer cause disease. Viral vectors have a wide range of applications, including gene therapy, cancer treatmentvaccine development, and even in basic neuroscience research. Virus-like particles, which are viruses emptied of their genetic material, are also being investigated for use in nanotechnology applications like drug delivery and diagnosis.