Acellular Infectious Agents: Viruses, Viroids & Prions - Online Tutor, Practice Problems & Exam Prep

Acellular infectious agents, including viruses, viroids, and prions, are non-cellular entities that cannot be classified as living organisms. Viruses are obligate intracellular parasites composed of DNA or RNA encased in a protein coat, capable of infecting all life forms. Viroids consist solely of a circular strand of RNA and primarily infect plants, while prions are misfolded proteins that induce normal proteins to misfold, leading to diseases like mad cow disease. Understanding these agents is crucial for grasping their impact on health and disease.

Acellular Infectious Agents: Viruses, Viroids & Prions

In this video, we're going to begin our lesson on acellular infectious agents, including viruses, viroids, and prions. Now recall from our previous lesson videos that the term acellular or non cellular is just a term that means not consisting of or containing cells or cellular structures. And so anything that is acellular is not going to be made of cells, and that includes these acellular infectious agents, viruses, viroids, and prions. Now by definition, anything that is acellular and is not made of cells is also not going to be considered living. Because recall that the smallest and most fundamental unit of life is the cell. And so if it's not consisting of cells, it's not technically by definition considered alive. And so all of these acellular infectious agents are not technically considered alive because they lack many of the features of life, including not being cellular. And so these acellular infectious agents, once again, are noncellular or acellular, not composed of cells. And so they're noncellular objects that will contaminate and affect living cells. And once again, this includes viruses, viroids, and prions are the three groups of acellular infectious agents. And so if we take a look at our image down below at our map of the microbial world, recall that in our previous lesson videos, we already introduced the cellular organisms, including the prokaryotes such as bacteria and archaea, as well as the microscopic eukaryotes, including fungi, the protists, algae, and protozoa, as well as helminths. And so this green region that I've highlighted here on the map represents the living aspect of the microbial world, the living cellular components, the living cellular organisms. But then notice over here on the right-hand side is where the acellular infectious agents fall that are not composed of cells and are not technically considered alive. And this once again includes the viruses, viroids, and prions. And as we move forward in our course, we're going to talk more about each of these groups. But for now, this here concludes our brief introduction to acellular infectious agents, and we'll be able to talk more about them as we move forward. So I'll see you all in our next video.

Viruses

In this video, we're going to begin our introduction to viruses. But later in our course, in a different video, we are going to talk a lot more about the details regarding the structure and function of viruses. This video here is just an introduction. A virus can be defined as an obligate intracellular parasite that is made of DNA or RNA, but not both. This DNA or RNA will be packed into a protein coat, and sometimes the DNA and RNA can be packed into a lipid envelope. Now, we have said that a virus is an obligate intracellular parasite, and to define an obligate intracellular parasite, we can define it as substances that can only replicate inside a host that they infect or harm. Thus, this virus can only replicate and reproduce when it is inside the host that it infects and harms. All forms of life can actually be affected or infected by different types of viruses, including bacteria, archaea, and eukarya. All forms of life can be infected by different types of viruses. Frequently, viruses will kill the cells that they infect. However, they can also silently remain inside of the host without killing them. This is an idea that we'll get to talk more about later in our course. But if we take a look at our image below at our introduction of viruses, notice on the left-hand side, we are showing you one type of virus known as a bacteriophage, which is a type of virus that infects bacteria. Notice that it has a protein coat on the perimeter in pink color, and then on the inside, there is genetic material either DNA or RNA. This is a micrograph of an Escharichia virus T4 bacteriophage. Then, on the right-hand side, what we are showing you is a human virus, which takes on a different structure. However, it still has a protein coat and still is going to have genetic material. And of course, we're all familiar with the SARS CoV-2 virus that caused COVID-19, the COVID-19 pandemic. Here is a visual depiction of the SARS CoV-2 virus that caused COVID-19 disease in humans. This here concludes our brief introduction to viruses, these obligate intracellular parasites. But once again, later in our course, we're going to talk a lot more about the details concerning the structure and function of viruses. For now, I'll see you all in our next video.

Viruses:

• A. Contain both protein and nucleic acids.
• B. Infect only eukaryotic cells.
• C. Can grow in the absence of living cells.
• D. Are generally the same size as prokaryotes.
• E. Always kill the cells they infect.

Viroids

This video, we're going to begin our introduction to viroids. Now, viroids, like viruses, are also obligate intracellular parasites that must infect a living cell and be inside of the cell to replicate. Unlike viruses that are made up of DNA or RNA as well as protein and sometimes lipids, viroids are only made of a single short strand of RNA that forms a closed ring. Viroids are only known to infect plants and cause plant diseases. They are not known to infect other groups of life other than plants, and there's not much known about how viroids originated or how they can cause disease in plants. Therefore, they are under investigation, and there is a lot of research to find out more about these viroids.

Now, if we take a look at our image down below of the viroids, what you'll notice is we're showing you this red structure here which is representing the circular single-stranded RNA, which we call the viroid. Notice that it is able to base pair with itself, but it is going to complete a circle here, a closed ring. It can infect plants and cause plants to wilt and ultimately die.

Down below what you see right here is another example of a viroid, specifically potato spindle tuber viroid or PSTV, which is a viroid that infects potatoes. It can infect normal potatoes and cause the potatoes to become PSTV potatoes or potatoes that have been infected by the potato spindle tuber viroid. This here concludes our brief introduction to viroids, these single short strands of RNA that form a closed ring and are obligate intracellular parasites. Once again, we'll be able to talk more about viroids later in our course. This is just the introduction. So, I'll see you all in our next video.

Prions

In this video, we're going to introduce prions. And so the term prions is actually derived from proteinaceous infectious agents. And you can see that with the bolded terms here to help create that word, prions. And so by proteinaceous, what this is referring to is that these infectious agents are made of proteins and only proteins. And these prions, like viruses and viroids, are obligate intracellular parasites, which once again just means that they must infect a living cell and get inside of the cell in order to replicate themselves. And so, unlike viruses and viroids, these prions, once again, are made of only misfolded proteins. So they only consist of proteins and that's why they are proteinaceous. And, you can see that with the prions here. Now prions, again, they are misfolded proteins themselves. However, these misfolded prion proteins can cause normal proteins to misfold. And ultimately, a misfolded protein is going to lose its function, and that can cause disease. And so prions are linked to several slow degenerative diseases such as scrapie and mad cow disease.

And so if we take a look at our image down below over here on the left-hand side, notice that we're showing you the structure of a normal protein. And over here on the right, what we're showing you is the structure of the prion, which is going to be a misfolded protein itself. And so notice that the shape of this prion is different than the shape of the normal protein. And so over here on the right-hand side, we're showing you the brain of a healthy individual that's going to have normal proteins. However, if this individual is infected with a prion, then the prion can cause normal proteins to transform into prions. And so you can see that those normal proteins have transformed into prions, misfolded proteins that have lost their function, and that can lead to diseases. And so this represents the diseased brain.

And so this here concludes our brief introduction to prions, and once again, we'll be able to talk more about prions later in our course. But for now, I'll see you all in our next video.

This type of acellular infectious agent is only known to infect plants and is a small, circular, single-stranded nucleic acid molecule.

• A. Archaea.
• B. Prion.
• C. Virus.
• D. Viroid.

An infectious protein is known to cause neurodegenerative diseases in humans such as Creutzfeldt-Jakob disease ('mad cow disease'). This type of acellular infectious agent is known as a _________.

• A. Viroid.
• B. Prion.
• C. Virus.
• D. Protozoan.

Which of the following statements about acellular infectious agents is FALSE?

• A. Acellular infectious agents are non-cellular objects that contaminate and negatively affect living things.
• B. Acellular infectious agents are obligate intracellular parasites and they cannot reproduce on their own.
• C. Acellular infectious agents are considered the smallest forms of parasitic life.
• D. Acellular infectious agents are known to infect all forms of life.

Here’s what students ask on this topic:

What are the main differences between viruses, viroids, and prions?

Viruses, viroids, and prions are all acellular infectious agents, but they differ significantly in structure and function. Viruses are composed of DNA or RNA encased in a protein coat and sometimes a lipid envelope. They infect all forms of life and are obligate intracellular parasites, meaning they can only replicate inside a host cell. Viroids, on the other hand, consist solely of a single short strand of circular RNA and primarily infect plants, causing plant diseases. Prions are misfolded proteins that can induce normal proteins to misfold, leading to diseases like mad cow disease. Unlike viruses and viroids, prions do not contain nucleic acids and are made entirely of protein.

How do prions cause diseases in humans and animals?

Prions cause diseases by inducing normal proteins in the brain to misfold. These misfolded proteins accumulate and form aggregates, disrupting normal cellular functions. The accumulation of prions leads to brain damage and the characteristic symptoms of prion diseases, which include neurodegenerative conditions such as Creutzfeldt-Jakob disease in humans and mad cow disease in cattle. The misfolded prion proteins are resistant to proteases, enzymes that normally break down proteins, making them particularly difficult to eliminate from the body. This resistance contributes to the progressive nature of prion diseases.

What is the structure of a virus and how does it replicate?

A virus consists of genetic material, either DNA or RNA, enclosed in a protein coat called a capsid. Some viruses also have a lipid envelope surrounding the capsid. Viruses are obligate intracellular parasites, meaning they can only replicate inside a host cell. The replication process involves the virus attaching to a host cell, injecting its genetic material, and hijacking the host's cellular machinery to produce viral components. These components are then assembled into new viral particles, which are released from the host cell to infect new cells.

What are viroids and how do they differ from viruses?

Viroids are the smallest known infectious agents, consisting solely of a short strand of circular RNA without a protein coat. Unlike viruses, which can infect all forms of life, viroids primarily infect plants and cause plant diseases. Viroids replicate within the host cell by hijacking the host's RNA polymerase. The lack of a protein coat and the exclusive presence of RNA distinguish viroids from viruses, which have both nucleic acids and proteins. Viroids are also unique in their ability to form secondary structures through base pairing within their RNA strand.

Can prion diseases be transmitted between individuals?

Yes, prion diseases can be transmitted between individuals, although the modes of transmission vary. In humans, prion diseases can be transmitted through contaminated medical equipment, consumption of infected tissue, or inherited genetic mutations. For example, Creutzfeldt-Jakob disease can be transmitted through contaminated surgical instruments or corneal transplants. In animals, prion diseases like mad cow disease can spread through the consumption of infected feed. The infectious prion proteins are highly resistant to standard sterilization procedures, making transmission a significant concern in both medical and agricultural settings.