In our last few chapters, we spent some time learning some detailed information about our prokaryotic cells as well as our eukaryotic cells. As we move into chapter six, we want to consider another group of pathogens that we discuss in microbiology class. I'm careful to not say another group of organisms or cells because in chapter six, we're really looking at the disease-causing agents. that are not composed of any type of cell.
The main acellular pathogens we're going to discuss will be viruses, but there are some other acellular pathogens. We will discuss prions just a little, but there are some others that we really just won't have time to get into into this particular class. So moving into our discussion, let's start with viruses. On this slide, I have listed for you Five characteristics of a virus.
Number one characteristic, all viruses are acellular, meaning they are not composed of cells. Since they are not composed of cells, they lack all of the required materials to reproduce on their own. Viruses are considered not living because they cannot function without a living cell to attack. or live within or reproduce within.
Viruses are considered obligate intracellular parasites. Obligate because they absolutely must live intracellularly, meaning within inside of another cell. One interesting thing about viruses is not only do they infect a cell so that they can reproduce and quote live, but they actually have a very narrow range of hosts in which they can infect. So what this means is if we're looking at a virus that infects humans, that virus is typically not going to infect some other species. Even more specifically, if we have a virus that attaches specifically to human brain cells or human liver cells, then that virus can't infect a human kidney or some other organ of that nature.
Looking at the makeup of a virus, all viruses must contain either DNA or RNA. And the or is very important here. Humans contain both DNA and RNA, as well as bacteria and any other eukaryotic cell. Viruses are some of the... only pathogens on earth that contain either DNA or RNA.
The DNA or RNA is always surrounded by a protein coat called a capsid and sometimes surrounded by an envelope. So if we look at this image here in the bottom right of your screen, it shows you the three parts of a virus. So all viruses will have part one here, the RNA or DNA, the nucleic acid, and the protein capsid that surrounds. So all viruses have the genomic material, the DNA or RNA, and the capsid.
Some viruses have a phospholipid membrane called an envelope on the outside. And we're going to look at this in just a little bit more detail as we move into the next slide, the detailed structure of a virus. Since, as I said in the last slide, all viruses contain DNA or RNA surrounded by a capsid. If that's all they have, then that virus is considered naked. If the virus has, in addition to the protein coat, also that extra fatty envelope, we call it an envelope virus.
So looking here at this picture, this picture represents a naked virus. So we have the DNA, which is the core genomic material in the center, surrounded by the capsid. The capsid itself is made of individual proteins called capsomeres.
The picture in the bottom right hand shows what we would see if we had a virus that was not naked, it's enveloped. So in this particular example we have RNA in the center surrounded by the protein capsid and then that all is surrounded by the phospholipid membrane. making up the envelope. Notice in either the naked or the envelope virus, you can have some glycoproteins extending called spikes. The spikes actually are very important beyond the scope of this class.
The spikes are very often what allows a virus to attach to and infect a host. So for example, when we think about things like the flu virus, The particular flu virus is named by those different spikes. So H1N1.
H1 and N1 are different types of spikes found on an influenza virus. And so that's where that naming process comes from. When we're thinking about these viral structures, they come in one of three variety of shapes.
The viral capsid can be helical, as shown in this picture. It can be icosahedral, as shown here. Or it can be considered complex, meaning it doesn't fall into one of the other two categories.
Viruses are classified by the type of DNA or RNA found within the capsid of the virus. So I have on this slide some common family names of viruses. You do not need to memorize every single one of these. What I want you to see from this slide is the way we categorize our viruses. So what these represent, DS is double-stranded DNA.
So you can have a group of double-stranded DNA viruses that are envelope or you can have double-stranded DNA viruses that are naked. You can have single-stranded DNA viruses, you can have double-stranded RNA viruses, or two different types of single-stranded RNA viruses. So recall back to your knowledge that you obtained in your General Biology or Principles of AMP1 course. You learned about how DNA stores our genetic information in order to produce a protein. that DNA must be copied into mRNA.
The mRNA must move into the cytoplasm outside of the nucleus where that mRNA is converted into a protein on the ribosome. So I want to make sure you understand it's important that you remember that information you've learned before. If you're not familiar with that, I highly recommend you watch perhaps a Khan Academy video.
understand the process of how proteins are made. That's going to be crucial in your understanding of how viruses can be replicated. Since viruses are not cells and do not contain all of the cellular machinery that you have within your cells, they are simply going to hijack and use all of your cell machinery to replicate.
So what can a virus do to one of our cells when it infects a cell? The simplest way to approach this is to first look at how a bacteriophage, so that's this word here, a bacteriophage is a phage or virus that infects bacteria. So what we're looking at is how one of these bacteriophages infects a bacteria cell.
The most simplistic viral life cycle is considered a lytic cycle and it's represented by the bacteriophage. The lytic cycle is considered virulent because cell death is always the end result. With the lytic cycle, we have five steps.
Attachment, penetration, biosynthesis, maturation, and lysis. In step one, attachment, the tail fibers of the bacteriophage specifically bind to the bacterial cell wall and membrane. The bacteriophage then inserts its DNA inside of the cell. That's when we move here into penetration. Once inside of the host cell, the phage or viral DNA is then going to use the cell machinery to make many copies of all of its components.
So recall a virus has at least two components. the DNA or RNA and the proteins, correct, forming the capsid. So in this biosynthesis step, this virus must make sure that its DNA is shown in blue is copied, but also all of the protein components to make the capsid must be produced as well.
Once everything is synthesized and maturation occurs, where everything is assembled, once all new virions are assembled, they can then lead to lysis of the host cell where the new bacteriophages will leave. So end result, one bacteriophage infects one cell, that one host cell dies releasing many newly formed viruses. So the lytic cycle, as I said, is kind of a good starting point.
It's the simplest way we can see reproduction with viruses. The other option is considered the lysogenic cycle. In the lysogenic cycle, the virus is considered temperate.
because it can become part of the host cell chromosome for a at least part of the life cycle. So we'll still have attachment and penetration where the bacteriophage infects the host cell with DNA. The temperate part of this life cycle is now the phage DNA is incorporated into the host cell DNA.
So we now have a host cell with a prophage. meaning the virus is part of the cell. Now when this cell replicates, all offspring will have host cell and viral DNA in the new cell. This is, you can think of, a way for the virus to hide inside of the host cell. Whenever conditions are optimal for the virus, the virus that will then come back out of the host cell DNA.
And then enter into the rest of the lytic cycle where we'll see biosynthesis, maturation, and then release. Animal cells pose a little bit different process or few different mechanisms that must happen in order for a virus to infect a host animal cell. And really where that comes into play is sometimes we'll see a step called uncoating. So we still see attachment shown here. With attachment and then penetration, sometimes the entire virus will be pulled into the host cell by the method of endocytosis.
So we'll end up with the virus inside of one of our cells, and then we got to get the virus out of the membranous coat. Once the uncoating occurs, the viral genome has a few options. If the viral DNA is released, that DNA must move into the host cell nucleus, where it can be copied inside of the host cell nucleus.
The DNA will then transcribe to mRNA, move out into the cytoplasm to produce proteins. If the viral... Genome is RNA to begin with.
It may never need to go into the nucleus. It can make proteins and be copied directly out inside of the host cell. There are a few different viral enzymes that can be used. If a virus has DNA on the inside, then the viral DNA can use all of our own enzymes, things like polymerase.
If the viral genome is RNA... We can't copy RNA directly. As humans, we only make RNA from DNA. So a virus that contains RNA will bring with it its own enzymes, RNA-dependent RNA polymerase.
The last type of enzyme that some RNA viruses will bring with it is called reverse transcriptase. Reverse transcriptase will convert RNA into DNA. That DNA can then move and hide within your host cell chromosome, forming a temperate prophage, which we saw in the lysogenic cycle. With release in animal host, very often a portion of the host membrane will come off with the virus that has formed.
So the envelope actually comes from your own cell membrane. What I really want you to think about as you're trying to understand how a virus can replicate is how amazing it is that that virus can take over your cell, use your enzymes, your nucleotides, even steal some of your membrane to make copies of itself. I have in your PowerPoint slideshow a video.
I highly recommend that you watch this video. to give you a little bit more of a visual understanding of what's going on with the replication. On my YouTube page, there's also an older video of mine explaining how viruses reproduce and it would serve as another reference point for you. The last couple things to go over in this slideshow are the different types of viral infections. Some viral infections are considered persistent.
This occurs when a virus can stay within a tissue of an infected person. Examples of this would be herpes, hepatitis C, and HIV. Once someone has herpes, they may not always have a cold sore, but that virus is always present within their body.
A latent infection example would be chickenpox and shingles. Latent infections can stay hidden or dormant inside of your body until a stressor poses. something that causes that virus to come back and cause a new effect. A chronic infection is a viral disease that has symptoms that seem to constantly occur over time and often get worse over time. HIV is an example of a chronic infection.
The last thing for us to discuss are the two other acellular pathogens just to mention. Viroids and prions the prions are the ones that are really interesting prions are considered infectious protein particles they do not have any rna or dna they are a single piece of protein that once ingested or blood transfused into the human body will cause mutations and spontaneous formation of other prion proteins on neurological cells within the body. What will happen is once the cell starts having these different messed up proteins on the outside, cell death will occur.
So in people who are infected with prions, they will begin to see loss of brain cells. The disease that occurs due to prion infections is called a transmissible spongiform encephalopathy. Common names for these diseases include Kuru, Creutzfeldt-Jakob disease, and mad cow disease. There are some very interesting YouTube videos that talk about how these diseases are, how they were beginning to be understood, and so I highly recommend that you watch some of those videos.
The takeaway from this chapter is number one, make sure you understand the composition of a virus and the basic characteristics of a virus. Then make sure you understand the way that viruses can replicate and how that replication will differ depending on whether or not the virus contains DNA or RNA and whether or not the virus is naked or envelope.