Microbiology, part 47: Acellular Infectious Agents - Viral Classification

Hi, I'm Cathy with Level Up RN. In this video, we're going to talk about the different classes of viruses and how they replicate. And at the end of the video, I'm going to give you guys a little quiz to test your understanding of some of the key concepts I'll be covering, so be sure to stay for that. And if you have our Level Up RN microbiology flashcards, go ahead and pull them out so you can follow along with me, and pay close attention to the bold red text on the back of the cards because those are the things that you are likely to get tested on.

The first type of virus that we're going to talk about is a positive-sense single-strand RNA virus, which is often referred to as a sense strand or positive strand. So this viral RNA acts as mRNA and can be translated directly to make viral proteins. And then in order to replicate the genome, the virus uses its own enzyme, which is called RNA-dependent RNA polymerase, or RDRP. So RDRP synthesizes a negative strand, which is complementary to the positive strand. And then that negative strand serves as a template to make more positive strands.

Next, we have a negative-sense single-strand RNA virus, which is referred to as an anti-sense strand or negative strand. So unlike the positive strand that we just talked about, this negative strand cannot be used to directly make viral proteins. Instead, the virus needs to use its enzyme, RDRP, to synthesize a complementary positive strand. And then this positive strand can be translated to make viral proteins. And that positive strand is also used as a template to synthesize more negative strands.

Next, we have a double-stranded RNA virus, which contains a positive strand and negative strand. In order to make viral proteins, RDRP uses the negative strand as a template to synthesize a positive strand. And then this positive strand is translated to make viral proteins. And then RDRP copies both strands in order to replicate the genome.

Next, we have a double-stranded DNA virus. So DNA viruses are able to use enzymes in the host cell to replicate the genome and produce viral proteins. So RNA-dependent RNA polymerase is not required. So basically, a double-stranded DNA virus is able to be replicated and transcribed, just like a normal chromosome, using host enzymes. However, for a single-stranded DNA virus, the host enzymes need to synthesize a complementary strand in order to produce a double-stranded DNA molecule. Then this double-stranded DNA can be replicated and transcribed just like normal.

The last type of virus we're going to talk about is a retrovirus. The most well-known example of a retrovirus is HIV. The retrovirus's genome consists of two copies of positive-sense single-stranded RNA. In addition to the viral genome, retroviruses carry three important enzymes, which include reverse transcriptase, integrase, and protease. Let's take a look at how a retrovirus uses these enzymes to replicate. The virus first attaches to the cell and enters the cell via membrane fusion. After the virus enters the cell, reverse transcriptase synthesizes a double-stranded DNA molecule from the single-stranded RNA. Next, that double-stranded DNA molecule is integrated into the host chromosome by the enzyme integrase, forming a provirus. When this provirus is transcribed, this produces viral RNA, and translation of the viral RNA produces viral proteins. The viral RNA and proteins then assemble to form a new immature virus. Then this newly formed immature virus is released, and the enzyme protease breaks up the proteins in the immature virus, which creates a mature virus.

All right. It's quiz time, and I have three questions for you.

Question number one. RNA viruses must carry which enzyme to allow for the replication of the viral genome?

The answer is...RNA-dependent RNA polymerase, or RDRP.

Question number two. Blank is an enzyme used by a retrovirus to synthesize a double-stranded DNA molecule from a single-stranded RNA molecule.

The answer is...reverse transcriptase.

And number three. During replication of a retrovirus, the double-stranded DNA molecule produced by reverse transcriptase is integrated into the host chromosome forming a blank.

The answer is...provirus.

All right. That's it for this video. I hope it was helpful. Take care and good luck with studying.

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