Animal viruses behave differently than those bacteriophages we just took a look at. In fact, just down to their structure, animal viruses look different. So animal viruses tend to have viral envelopes. Right? Those membranous, accessory structures that are often derived from the host cell's own membrane. They also tend to have RNA genomes unlike those bacteriophages that had DNA genomes. Now, animal virus replication involves entry into the cell just like we saw with bacteriophages but this time, the whole virus is going to enter the cell, and it's going to do that through cell surface protein receptor recognition. Right? So, there are a variety of specific ways that this can happen but think of endocytosis. Think about those endocytotic processes we took a look at, way back in cell signaling. And what happens there is you have little molecules, ligands, bind to these cell surface receptors and that causes the membrane to fold inward and pinch in. Well, similar to that, these viral cell surface proteins are going to bind to receptors on the host cell and it's going to cause the host cell to take the virus inside. Right? These viruses are very tricky. They trick the host cells. The viral RNA, once inside the cell, serves as a template for replication by the viral RNA polymerase. So that RNA will serve as a template from which mRNAs will be synthesized by a special viral RNA polymerase. Retroviruses are a special type of animal viruses. And these are viruses with RNA genomes that actually instead of just putting that RNA out there to make copies of mRNA from it, instead of doing that, these retroviruses actually do something called reverse transcription, right? So transcription is when you take DNA and you copy over the message into an RNA code. But what these retroviruses do is they take their RNA code, that's their genome, and they turn it back into DNA which they insert into the host cell's genome. So let's just cover that one more time. These viruses have an RNA genome. They reverse transcribe that RNA into DNA and then insert that DNA into the host cell's DNA genome. Now they use a special enzyme called reverse transcriptase to do this. It's a special enzyme that can catalyze that RNA to DNA transcription. In fact, because of the discovery of reverse transcriptases, we've actually been able to advance a lot of fields of biotechnology. These are very helpful enzymes. And much like we were talking about the prophages, which are those viral genomes inserted into the bacterial genome. Right? Those precursor phages more or less. Well, when a retrovirus has inserted its viral genome into the host cell's genome through that reverse transcription process, we call that a provirus. So provirus, very similar to prophage except to become a provirus, you have to have this special reverse transcriptase enzyme. So in a way, proviruses are kind of a little fancier than prophages. Now, we can see examples of this right here with a very famous virus, human immunodeficiency virus better known as HIV. HIV is an animal virus. It's also a retrovirus. It has a lipid bilayer viral envelope accessory structure, and it has that reverse transcriptase enzyme. You can see it right here represented by this little blob right here. And it has an RNA viral genome. It interacts with this particular type of cell surface receptor allowing its viral genome to enter the cell. And here we can see the steps of reverse transcription playing out whereby the viral genome is converted to DNA. That DNA is inserted into the host cell's genome and from there, viral products can be produced. And ultimately, the ultimate goal is to produce the proper viral products to then create a new virus which will leave the host cell taking some of its membrane with it to act as a viral envelope and it will go infect a new cell.
Alright. Let's turn the page.
