A virus is a nucleic acid encased in a protein shell and is inactive except within a host cell. Viruses penetrate host cells and inject their genetic information in the form of DNA or RNA. In the litic cycle, the virus uses the cell's functions to replicate its genetic information and create more viruses. In the lysogenic cycle, the virus leaves its genetic material dormant inside the cell as the cell reproduces until environmental conditions revive it and initiate the lytic cycle.
If you want to make a biologist twitch, ask them where in the classification system should they put a virus or is a virus alive? Because I can't answer that question. Viruses are weird things that are kind of in and out of a biologist's definition of what's alive because viruses don't do anything except when they're inside of a regular host cell. They're kind of like, this is why they named computer programs that and infect other computer systems viruses, it's because they're kind of like an individual program whose only purpose is to make more of that individual program. But when it's on the floppy disk it doesn't do anything or when it's on the CD-ROM it doesn't do anything or in the Zip driver it doesn't do anything, of regular cells like an entire computer with all its operating system and everything else. Now the virus structure is typically some kind of protein coat wrapped around either DNA or RNA inside of that coat. Many viruses will have a capsule or a stolen portion of its host membrane. Let's take a look at this slide over here and we can see this is a particular kind of virus called bacteriophage because it infects and kills bacteria. And here we can see the protein coat with the DNA inside and what happens is that the external proteins, their job is to land on the bacteria that it's going to infect and insert its DNA inside of the bacteria. Let's look at this typical route of infection and here we see a eukaryotic cell. We can talk as it's got a nucleus and we see the virus comes in here. It binds to the plasma membrane, it does this because it has proteins on the exterior surface that will stick to particular proteins on the membrane of the target cell. When it gets inside it releases it's RNA and it winds up copying that RNA and then that RNA in this case it's an RNA virus. The RNA is then used to guide the building of proteins and more RNA, that ultimately lead to the creation of more viruses that get released. If it uses DNA, then that in stead of having RNA in here we'll have DNA, the DNA will get opened up and transcribed and again the newly transcribed RNA will then be used to guide the construction of proteins and then the release of the virus. There's two basic life cycles followed by viruses and some viruses do both. One is the lytic life cycle and here we see a bacteriophage and what it's doing is injecting its DNA into a bacterial cell that's the green little box. Here's the DNA of the bacteria, sometimes they'll incorporate their DNA and then they'll use the instructions of the viral DNA to build a bunch more of the DNA and proteins that make up the bacteria and here you seeing it dissembling it until it's got enough and it just burst open the cell as it just fills it up with more and more viruses, it's basically taking over the cell and turned it into a virus factory and this process of filling it up until it pops or lysis is why it's called the lytic life cycle. Now some viruses will enter into the lysogenic cycle or lysogenic phase and here it begins the same, the virus infects and injects its DNA or RNA but instead of immediately attacking and destroying the cell, instead it gets kind of sneaky and it just sits there doing nothing until the cell gets ready to make a copy of itself, well because it's added its DNA into the cell's DNA, when the cell does it's normal DNA replication you wind up with two copies of not just the bacterial DNA but the viral DNA and each daughter cell will be essentially infected and it may sit there doing nothing, and just allowing the bacteria to make more and more, more, more copies of it until environmental triggers cause one of these daughter cells to enter the lytic cycle. As a little side note HIV the virus that's associated with the disease called AIDS, HIV is what is called a retrovirus, it uses RNA and when it infects one of our helper T cells, it uses that RNA and an enzyme called reverse transcriptase which guides the reverse version of transcription and it turns the RNA makes a DNA copy of it. Inserts that into our DNA and that's why it can sit there and the helper T cells doing nothing which is why some people get infected and show no symptoms really for months or even years until eventually some of the cells start becoming lytic and start popping open and start eventually killing the host cells, so that's viruses.