Facing the New World: A COVID-19 Reality Part 2
Written by: Akshaya, Kavya, Haritha
Viruses are the most abundant living organisms on the planet. Viruses are extremely unique in that they are neither living nor dead. How is this possible you may ask? Well, the state of their living depends on whether they are present in the host organism or not. Viruses are also known to be intelligent organisms as they are able to mimic host cells in multiple ways (2). More specifically, viruses possess structural resemblance to host cells by containing similar markers that trick the body to not fight against them (2). We talk more about these mimicry organisms further below when we discuss how viruses enter the cell. Viruses use the host cells in order to replicate and produce progeny, or multiplied viruses (1). In terms of the structure of a virus, they do not contain organelles, nor do they have a cytoplasm or membrane. Depending on the virus, they contain either DNA or RNA, and they are either single or double-stranded (1).
Delving into the parts of a virus, we will be discussing common structural features most viruses are composed of. Firstly, all viruses contain genomes, and a capsid, or protein shell which is the outer, surrounding layer of the virus. The capsid’s function is in protecting the viral genome as well as in aiding the virus to enter the host cell and to let the progeny out (2). But viruses such as Influenza, HIV, and coronaviruses, contain another layer known as an envelope. This coating is actually part of the membrane from the previous cell of the host that was infected by the virus (1). This allows for the virus to escape detection from our immune system, as the outer layer has human cell markers. Additionally, the reason soap can help against these viruses is that the envelope of the viruses become greasy when interacting with soap. To explain this in more detail: Soap contains a lipid-soluble part (referred to as hydrophobic - meaning ‘water-fearing’) and a water-soluble hydrophilic - ‘water-loving’) part. Typically, molecules that are hydrophobic interact with each other and repel water. Molecules that are hydrophilic like to interact with water. The hydrophobic part of soap binds to the hydrophobic part of the virus envelope. The virus, now bound tightly to the soap, gets washed away when the water-soluble (hydrophilic) part of the soap interacts with and dissolves in the water and gets washed away. The virus interacts with our cells in a similar way. Our cells also contain a hydrophobic layer that interacts with the hydrophobic envelope of the virus and allows it to pass though. (Now you know why you should wash your hands after touching something to prevent yourself from catching COVID-19)
Now that we know about the general structure and parts of a virus, we can discuss how viruses spread. For this to happen, they first have to enter the cells by binding to the proteins on the surface of the cells. Let’s talk further about molecular mimicry of viruses which we briefly touched on earlier. Molecular mimicry is found in molecules that show resemblance to the host and microbe through sequence and structure (3). This becomes a defense mechanism for pathogenic organisms as it aids in the survival and spread of the viruses. Additionally, in order to enter the cell, viruses mimic the cytokines of the host by utilizing decoy receptors (3). This acts as an advantage to the virus by changing the immune system’s response to the host.
Once the replicated viruses or progeny of the viruses are ready to exit the host cell, they leave the cell by making holes in the membrane (1). This process essentially leaves the infected cells destroyed and unable to function anymore. For viruses that are enveloped, they can use the process of budding in order to leave the host cell. This involves covering themselves with the membrane of the cell and simply diffusing through the outer membrane. Although this process does not cause structural damage, the cells are weakened (1).
Here is an image that gives a general overview of how retroviruses replicate in the host cell:
Where do viruses come from?
There are three main theories about how viruses originated:
1. Progressive Hypothesis:
The first is called the ‘Progressive Hypothesis’. According to this hypothesis, viruses originated from certain parts of the genetic sequence that gained the ability to move from organism to organism (4). These translocated sequences are known as mobile genetic elements. When retroviruses, which have single-stranded RNA (ssRNA), enter a host cell, an enzyme called reverse transcriptase converts this strand of RNA to single-stranded DNA (ssDNA) (4).
You may be wondering, what is the difference between RNA and DNA? Why are these viruses called retroviruses? DNA is the main genetic component of our cells that is responsible for storing information in the form of genetic sequences (A, T, C, G). These genetic sequences contain information to carry out cellular functions. The central dogma (the principal rule) of biology is that DNA goes to RNA, which then can be used to make proteins and enzymes responsible for cell structure and function. In other words, RNA serves as the intermediate between DNA and protein. DNA usually exists in a double-stranded form and then goes through a process known as transcription to create single-stranded RNA, specifically called messenger-RNA (mRNA). Once the mRNA is created, it then undergoes another process called translation that creates proteins, essential for carrying out many reactions in the cells. In comparison, RNA usually exists in a single-stranded form. Therefore, when the RNA from the retroviruses enters our cells, their RNA genetic material is first transcribed to DNA and then back to RNA (thus the name retrovirus). For more information on the central dogma, please refer to Intro to gene expression (central dogma) (article)
For more information on DNA v. RNA, take a look at this video DNA vs RNA (Updated)
2. Regressive Hypothesis:
This hypothesis states that the remnants of cell organelles are viruses (organelles are comparable to organs of the body that carry out specific functions for the cell). The evidence for this is found in certain bacteria such as Chlamydia and Rickettsia, which share a common “free-living” ancestor with viruses. A “free-living” organism, in this case, refers to its ability to survive without depending on the host. As time progressed (evolution), the free-living ancestor lost certain genes for self-replication and became dependent on the host (4). In other words, the organism slowly lost its ability to replicate on its own as a result of losing organelles and relies on another organism to be able to multiply.
3. Virus-First Hypothesis:
This is the third hypothesis. According to this hypothesis, viruses and their hosts co-evolved together. Viruses consisted of self-replicating units and existed before cells did. Over time, the units became more complex, and eventually enzymes for replication and cell membrane production evolved (4).
Types of viruses
Viruses can be classified using various characteristics (such as RNA or DNA, positive or negative-stranded, double-stranded or single-stranded). Coronaviruses are positive-stranded viruses (5). Specifically, COVID-19 contains a single-stranded RNA genome. Coronaviruses are divided into four sub-categories: alpha, beta, delta, and gamma. The alpha and beta coronaviruses arise from bats and rodents whereas the delta and gamma coronaviruses originate from birds (5). The COVID-19 strain is considered to be a beta coronavirus, as it is suspected to have originated from bats (as previously discussed in part 1).
How viruses evolve to different strains:
The virus genome must be replicated in order for it to spread. This replication process is error-prone and can alter the sequences of the virus. In reality, these errors (usually referred to as mutations) occur frequently. Usually these errors do not result in significant changes. However, if the mutation allows for the virus to become more virulent (its ability to infect the host), that particular strain will begin to circulate and infect more people (6). In other words, if the mutation gives the virus an advantage in infecting the host, the new strain will spread faster than the original strain. This is the case with COVID-19. This particular strain contains mutations that differentiate it from other existing coronaviruses and seems to be more virulent. Interestingly, as the virus spreads, there is an increasing possibility that different strains of the COVID-19 can come into existence as mutations accumulate (6). So far, scientists have found multiple strains of COVID-19 around the world (there is some discrepancy on how many strains exactly). Therefore, it is important to curtail the spread of the virus as the more widespread it becomes, the more mutations it can accumulate and evolve.
Now that we have talked about viruses in general, we will talk about how viruses spread and measures to control the spread in the next post.
Stay Informed, Stay Tuned, and Stay SAFE!
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