Animal Viruses: RNA Virus Synthesis & Replication: Videos & Practice Problems

RNA viruses replicate primarily in the cytoplasm using an enzyme called replicase, which is an RNA-dependent RNA polymerase. Plus single-stranded RNA (ssRNA) serves as mRNA, directly translated into viral proteins, including replicase. In contrast, minus ssRNA cannot be translated and requires the replicase to synthesize plus ssRNA. Double-stranded RNA viruses also need replicase for mRNA synthesis. Understanding these mechanisms is crucial for grasping viral replication and pathogenesis.

Animal Viruses: RNA Virus Synthesis & Replication

In this video, we're going to continue to talk about animal viruses and animal virus infections by focusing specifically on RNA virus synthesis and replication. And so the majority of RNA viruses that have an RNA genome are going to replicate in the cytoplasm of the host cell. RNA virus synthesis and replication is often referred to as a replicase enzyme. This replicase enzyme is an RNA-dependent RNA polymerase, which means that this is an enzyme that uses RNA as a template to synthesize new RNA molecules. Really, this replicase enzyme uses RNA to build more RNA. We'll be able to see how replicase is used by RNA viruses as we move forward in our course to talk about the synthesis and replication of plus single-stranded RNA, the synthesis and replication of minus single-stranded RNA, and then the synthesis and replication of double-stranded RNA. I'll see you all in our next video.

Replication of (+) Single Strand RNA (ssRNA) Viruses

In this video, we're going to talk more details about the synthesis and replication of plus single stranded RNA, or plus ssRNA viruses. And so first we need to recall that the plus ssRNA, or the plus single stranded RNA, is really just a normal messenger or mRNA molecule as we've discussed it in so many of our previous lesson videos. And so that means that this messenger RNA can be directly translated by a host cell's ribosome to form those viral proteins. And so what you can see is down below in our image on the left hand side over here, we're showing you a plus ssRNA molecule, which recall is the same exact thing as a messenger RNA, an mRNA. And so because the mRNA is the virus's genome, it's able to be translated to form viral proteins. And so these viral proteins that are translated include the enzyme replicase. And so this viral replicase enzyme is able to use the plus ssRNA, the messenger RNA, as a template to make multiple copies of a complementary minus ssRNA molecule. And so notice here we're showing you that the plus ssRNA is used to make a complementary strand of minus ssRNA. And then the replicase enzyme, once again, can function to use the minus ssRNA molecules as a template to replicate the plus ssRNA genomes. And so what you'll notice here is that the replicase enzyme is able to use the complementary minus ssRNA to build more plus ssRNA. And, again, this is what is needed to replicate the original genome, to make more copies of that original genome. And so, what's important to note is that the replicase enzyme actually does not enter during the initial viral infection. And so notice that the replicase enzyme is nowhere to be found over here on the left hand side. Instead, this replicase enzyme is only going to be translated after entry into the cell. And so the replicase enzyme is only going to be translated after the plus ssRNA has entered into the cell. And then this translated replicase enzyme can be used to make minus ssRNA, which can be used by the replicase to make more plus ssRNA. And so once again, recall that this replicase is an RNA dependent RNA polymerase, which means that it can use RNA to build more RNA. And once again, it can use the minus ssRNA to build more RNA. And so that is what is needed to replicate these RNA genomes. And so this here concludes our brief lesson on the synthesis and replication of plus single stranded RNA or plus ssRNA viruses. And we'll be able to get some practice applying these concepts and then learn about the synthesis and replication of minus single stranded RNA as we move forward in our course. So I'll see you all in our next video.

Replication of (-) Single Strand RNA Viruses

In this video, we're going to talk in more detail about the synthesis and replication of minus single-stranded RNA viruses, or minus ssRNA viruses. Now, first, we need to recall from our previous lesson videos that minus ssRNA, unlike plus ssRNA, cannot be directly translated. Recall that it's only the plus ssRNA or the mRNA that can be directly translated. And so, the minus ssRNA, once again, cannot be directly translated. However, it can be used as a template to synthesize a plus ssRNA molecule or an mRNA molecule. And that is going to require the function of the replicase enzyme to use the minus ssRNA as a template to build a plus ssRNA. However, since the minus ssRNA cannot be translated to make the replicase enzyme, this means that the replicase enzyme must actually enter into the host cell during the initial viral infection. And so this is very different than what we saw with the plus ssRNA viruses. Because recall from our last lesson video, with plus ssRNA viruses, the replicase enzyme does not enter during the initial viral infection. However, with minus ssRNA viruses, the replicase enzyme must enter during the initial viral infection. And so if we take a look at our image down below, what you'll notice on the far left-hand side, we're showing you the minus ssRNA viral genome. And what you'll notice about the minus ssRNA viral genome is that it cannot be directly translated. And so that means that the replicase enzyme must enter during the initial viral infection. And this replicase enzyme that enters during the initial viral infection can use the minus ssRNA genome as a template to produce a plus ssRNA molecule, which recall is really just the messenger RNA that can be translated. And so this newly synthesized plus ssRNA can either be translated to make viral proteins or it can be used as a template by the replicase enzyme to replicate the minus ssRNA genome. And so what you'll notice is if you take a look at our image down below is that the replicase enzyme that enters during the initial viral infection can use the minus ssRNA template to build a complementary plus ssRNA, which once again is really just the same thing as the messenger RNA or the mRNA. And the mRNA, we know, can be translated directly. And so you can see translation can lead to the synthesis of viral proteins, including the viral replicase enzyme. And the plus ssRNA, once again, can be used as a template by the replicase enzyme to replicate the original genome, the minus ssRNA genome. And so what's really important to note is that after the synthesis of viral proteins and the replication of the original viral genome, these components need to assemble with one another. And what's really important is that when the new viral particles assemble, it's important that the replicase enzyme is packaged with the minus ssRNA genome inside of the virus. And this will allow the replicase enzyme to be present during the initial viral infection, which is critical for minus ssRNA viruses. And so this here concludes our brief lesson on the synthesis and replication of minus single-stranded RNA viruses or minus ssRNA viruses. And we'll be able to get some practice applying these concepts as we move forward in our course. So I'll see you all in our next video.

A positive ssRNA virus:

• A. Must first be converted to a mRNA before it can be translated.
• B. Can be used to translate viral proteins.
• C. Is used to synthesize dsRNA before replication.
• D. Is not recognized by host ribosomes.

Answer: (B)

Replication of Double-Stranded RNA Viruses

This video, we're going to talk more details about the synthesis and replication of double-stranded RNA viruses or dsRNA viruses. Now, we don't typically think of RNA as being a double-stranded molecule. However, in some cases, some viruses can have a double-stranded RNA genome. And so, first, we need to recall from our previous lesson videos that the double-stranded RNA genome is going to have a plus or coding strand, as well as a minus or non-coding template strand. And so, this plus-minus double-stranded RNA genome is a double-stranded RNA molecule. And double-stranded RNA molecules cannot be directly translated. However, the double-stranded RNA molecule can act as a template to make plus ssRNA. And recall that this plus ssRNA is the same exact thing as the messenger RNA. And the messenger RNA, we know, can be directly translated. Now, using the double-stranded RNA to make plus ssRNA is going to require the function of the replicase enzyme, the RNA-dependent RNA polymerase.

And so, because the double-stranded RNA molecule cannot be directly translated, it's somewhat similar to the minus ssRNA. And what this means is that the replicase enzyme must enter with the plus-minus double-stranded RNA genome as part of the initial viral infection. And so, if we take a look at our image down below over here on the left-hand side, notice we're showing you the double-stranded, the plus-minus double-stranded RNA genome. And this plus-minus double-stranded RNA genome cannot be directly translated. And so what that means is that the replicase enzyme, which in this image is shown here as these purple structures, the replicase enzymes must enter during the initial viral infection, and so they must be present initially.

And so, the replicase enzymes that are present initially, they can use the plus-minus double-stranded RNA genome as a template to make plus ssRNA. And once again, the plus ssRNA is the same exact thing as the messenger RNA. And so, the messenger RNA or this plus ssRNA we know can be translated directly to make viral proteins, or the plus ssRNA can be used to replicate the original double-stranded RNA genome, and that is going to require the function of the replicase enzyme.

And so, if we take a look at our image down below, notice that the double-stranded RNA genome here can be used as a template by the replicase enzyme to make plus ssRNA. And again, this plus ssRNA is going to be the same thing as the messenger RNA. And so, it can be used directly in the process of translation to synthesize viral proteins, including the viral replicase enzyme. And also, the plus ssRNA or the messenger RNA here can also be used as a template by the replicase to replicate the original genome, which is the plus-minus double-stranded RNA genome. And so, once again, upon assembly of these proteins, viral proteins and viral genome, the replicase enzyme must once again be packaged into the virus in order for the replicase enzyme to be present during the initial viral infection.

And so this here concludes our brief introduction to the synthesis and replication of double-stranded RNA or dsRNA viruses, and we'll be able to get some practice applying these concepts as we move forward in our course. So I'll see you all in our next video.

Virus X, a (-) ssRNA virus, cannot replicate its genome without bringing what into the host cell?

• A. Replicase enzyme.
• B. Duplicase enzyme.
• C. RNA polymerase.
• D. Protease.

Answer: (A)

(+/-) dsRNA viruses are most similar to which other type of virus?

• A. dsDNA viruses.
• B. (+) ssRNA viruses.
• C. (-) ssRNA viruses.
• D. (+) ssDNA viruses.

Answer: (C)

Here’s what students ask on this topic:

What is the role of the replicase enzyme in RNA virus replication?

The replicase enzyme, also known as RNA-dependent RNA polymerase, plays a crucial role in RNA virus replication. It uses RNA as a template to synthesize new RNA molecules. For plus single-stranded RNA (ssRNA) viruses, replicase is translated from the viral mRNA and then used to create complementary minus ssRNA, which serves as a template to produce more plus ssRNA genomes. In minus ssRNA viruses, the replicase enzyme must enter the host cell during the initial infection to synthesize plus ssRNA from the minus ssRNA template. Double-stranded RNA (dsRNA) viruses also require replicase to generate plus ssRNA, which can be translated into viral proteins or used to replicate the original dsRNA genome.

How do plus single-stranded RNA viruses replicate their genomes?

Plus single-stranded RNA (ssRNA) viruses replicate their genomes by first using their RNA as mRNA, which can be directly translated by the host cell's ribosomes to produce viral proteins, including the replicase enzyme. This replicase enzyme then uses the plus ssRNA as a template to synthesize complementary minus ssRNA. The minus ssRNA serves as a template for the replicase to produce more plus ssRNA genomes. This process ensures the replication of the viral genome and the production of viral proteins necessary for assembling new viral particles.

Why must the replicase enzyme enter the host cell during the initial infection for minus single-stranded RNA viruses?

For minus single-stranded RNA (ssRNA) viruses, the replicase enzyme must enter the host cell during the initial infection because minus ssRNA cannot be directly translated into proteins. The replicase enzyme is required to synthesize plus ssRNA from the minus ssRNA template. The plus ssRNA can then be translated into viral proteins, including more replicase enzymes, and used as a template to replicate the minus ssRNA genome. Without the replicase enzyme present at the start, the virus would be unable to produce the necessary plus ssRNA for translation and replication.

How do double-stranded RNA viruses synthesize mRNA?

Double-stranded RNA (dsRNA) viruses synthesize mRNA using the replicase enzyme, which is an RNA-dependent RNA polymerase. The dsRNA genome consists of a plus (coding) strand and a minus (non-coding) strand. The replicase enzyme uses the dsRNA as a template to produce plus single-stranded RNA (ssRNA), which is equivalent to mRNA. This plus ssRNA can be directly translated by the host cell's ribosomes to produce viral proteins or used as a template to replicate the original dsRNA genome. The replicase enzyme must be present during the initial infection to facilitate this process.

What is the difference between plus and minus single-stranded RNA viruses in terms of replication?

The primary difference between plus and minus single-stranded RNA (ssRNA) viruses in terms of replication lies in their initial translation and the role of the replicase enzyme. Plus ssRNA viruses have genomes that function as mRNA, which can be directly translated by the host cell's ribosomes to produce viral proteins, including the replicase enzyme. This enzyme then synthesizes complementary minus ssRNA, which serves as a template to produce more plus ssRNA genomes. In contrast, minus ssRNA viruses cannot be directly translated. They require the replicase enzyme to enter the host cell during the initial infection to synthesize plus ssRNA from the minus ssRNA template. The plus ssRNA can then be translated into viral proteins and used to replicate the minus ssRNA genome.