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Assalamualaikum everyone! Welcome back to another ATP video. In today's video, we are going to start a new chapter in microbiology: viruses. We will begin with an introduction to virology.
You must have caught the flu at least once, maybe twice. If not, well, you must be some sort of ultra-human specimen—congratulations! What causes these flu breakouts? Viruses. Here's what's cool about viruses: these little things manage to blur the line between what's living and non-living objects. We can't call them living since they can't replicate, metabolize, or perform most of the functions needed to classify something as living. In other words, they are not cells. We can't really call them non-living either because, once provided with the machinery needed—when they hijack living cells—boom! They can replicate and make their own proteins, just like living organisms. That is why they're called obligate intracellular parasites; they are obligated to be inside a cell in order to function.
Now, what does this all mean? Let's start. Viruses are basically genome packages. Now, for the composition: a virus consists of a nucleic acid genome (either DNA or RNA, but never both), a protein coat known as the capsid that encloses the genome and protects it from any chemicals or enzymes it may encounter. The capsid, along with the genetic material, are collectively called the nucleocapsid. In some cases, there is a lipid membrane known as the envelope, and glycoproteins on the surface of the envelope serve to identify and bind to receptor sites on the host's membrane.
Now, how can we classify viruses? The main characteristics we use to tell viruses apart include:
1. **Genome composition.** The virus can be further classified in three ways:
* The type of nucleic acid (if a virus contains RNA, then it's called an RNA virus; if it contains DNA, then it's referred to as a DNA virus).
* The number of strands (recall that viruses can only contain one type of nucleic acid at a time. You've heard of double-stranded DNA and single-stranded RNA, but what's special about viruses is that they can also contain single-stranded DNA or double-stranded RNA). You must keep in mind that if a virus contains single-stranded RNA, it cannot contain any other nucleic acid with it, like double-stranded RNA or single-stranded DNA, etc.
* Genome geometry (which can either be linear or circular).
2. **Capsid structure.** Depending on the structural organization of the capsomeres (which are the proteins making up the capsids), we can have three main types of viruses: helical, icosahedral, and complex. In helical viruses, the circular capsomeres are attached helically, creating a tube or a hollow space for the nucleic acid within—more like wrapping a long beaded bracelet around your wrist, the beads being the capsomeres. An example of that is tobacco mosaic virus. The second type is icosahedral, where the capsomeres arrange themselves in equilateral triangles that fuse together to form a 20-sided diamond. An example of that is poliovirus. Lastly, complex viruses are technically a combination of icosahedral and helical at the same time.
3. **Presence of an envelope.** Viruses that lack an envelope are called naked viruses, while viruses enclosed by the lipid bilayer envelope are called enveloped viruses. An example of that is influenza.
4. **Type of host.** Viruses can infect almost every type of living organism and are named accordingly. The most important example includes bacteriophages, which are viruses that infect bacteria.
5. **Method of action.** All of these properties (the presence of an envelope, type of capsid arrangement, type of host, etc.) can influence how a virus enters its host cell. If a virus has a complex structure (head and tail morphology), it combines to the host by the tail and injects its genetic material into the cell like a needle. This is most commonly seen in bacteriophages. If a virus is enveloped, the lipid bilayer can fuse with the cell membrane of the host and then infiltrate the cell that way. This is known as direct fusion. If a virus is naked or even enveloped, it can enter the cell through receptor-mediated endocytosis; receptors can't often tell that the particle is a viral particle and welcomes it in. The last two methods are most commonly seen in eukaryotic cells. Once the virus or its genetic material has infiltrated the cell, it uses the cell's machinery to make its own proteins and replicate—basically form more viral particles within the host cell. This can happen in one of two cycles: the lytic cycle and the lysogenic cycle. In the lytic cycle, the particle uses the host machinery to replicate and build more viruses to the point that the cell ruptures or lyses—hence the name lytic. In the lysogenic cycle (known as the sneaky cycle), the viral genetic material becomes part of the host cell's genetic material and stays hidden in the host's genetic material. Then, when the host makes new cells, it replicates its own genetic material, containing the viral genetic material within it. Now, this may seem harmless, but if the virus is triggered to go into the lytic cycle, then all those cells that replicated containing the viral genetic material with it would activate to form viruses.
It is important for examination purposes to understand that, just like bacteria, viruses like to exchange genetic material with each other. Let's now talk about the different ways in which they do this:
1. **Recombination:** Here, two viruses enter the cell and release their genetic material. The genetic material of both viruses is now exposed and can be exchanged between the viruses.
2. **Reassortment:** This occurs only in viruses with segmented genomes. The mnemonic for these viruses is called BOAR (B stands for bunyavirus, O stands for orthomyxovirus, A stands for arenavirus, and R stands for reovirus). Again, genetic material is exchanged between two virions. Note that in reassortment, the exchange is occurring with the same species of virus. When we talk about the influenza virus, we will go into more detail, but for now, understand that reassortment is implicated in pandemics and epidemics. A classic example is the H1N1 influenza virus pandemic in the early 2000s.
3. **Complementation:** Here, we also have two viruses, but one of them is functional and the other is defective. The functional virus provides a capsid to the defective one. Think of it as a rich friend giving his poor friend a jacket. A classic example is Hepatitis B giving Hepatitis B antigen to Hepatitis D. This is why you will see later that Hepatitis D needs Hepatitis B to survive.
4. **Phenotypic mixing:** Just like complementation, here we are exchanging the protein capsid, but in this case, both viruses are functional, so the exchange can happen in either direction. Here, the two friends are equal and both help each other.
Well, that was a doozy! So let's do a quick recap. Viruses are genome packages protected by capsids, which may or may not be surrounded by a lipid bilayer envelope. They can be classified according to genome composition, capsid structure, presence of envelope, and type of host it infects. Through the lytic cycle, the virus uses the cell's machinery to replicate and build proteins, rupturing the cell and traveling along the body to infect other cells. Or they can stay hidden within the host cell genome until triggered into the lytic cycle, and this is known as the lysogenic cycle.
That's it for the introduction to virology. We hope you found it beneficial. Don't forget to like, share, and subscribe to receive our latest updates, and as always, thanks for watching!