The weird thing that we have to understand here is when we look at the classification of all living organisms. There is bacteria, there's archa and there's ukaria. And even under ukaria there are the four kingdoms. Nowhere in this classification did I mention anything about viruses. Now the main reason why viruses are not put here is because in most cases viruses are considered non-living structures. On one hand if you check online there are some scientists who say that yes viruses should be put under living. Some scientists say no it should not. Some scientists are kind of on the fence about things. But for the purpose of Cambridge uh A levels, we would classify viruses as non-living structures. And in chapter 1 of AS uh it was mentioned that viruses are ascellular. Asellular meaning to say they lack important cell structures. For example, they don't have cytoplasm, no ribosomes, and they don't have their own cell surface membrane. They may have a lipid membrane which they obtain from the host cells. Viruses themselves cannot carry out living processes. They cannot respire and they cannot reproduce by themselves. They have to enter the host cell as you can see here and they will hijack the host cell and the whole cell will make more copies of the virus before the virus you know goes out of the cell. And the basic structure of the virus that was mentioned as well is all viruses will have the two basic structures where they have a protein code known as a capsit and they also have nucleic acid. The important thing you have to know is viruses will have either one of the nucleic acids DNA or RNA but never at the same time. So that's an important thing. One important question that they can ask in the exam is how do we classify viruses? So we have four types of virus over here. Some students will say oh we can classify the virus based on their shape, their size, what type of diseases they can cause or what type of organisms they can infect. That is true. But the main two ways that we classify viruses will be based on the type of nucleic acids where we classify them according to whether they have DNA or RNA. But in viruses, DNA and RNA can behave differently because as you can see here, those two at the top are DNA viruses. One DNA virus has a double helix, two strands. But that's the weird thing. another virus that's a DNA by the way but it's a single strand for the RNA viruses at the bottom here yes one of the RNA virus has a single strand as what we studied in chapter 6 but one of the RNA viruses there it has a double strand so in viruses DNA and RNA can behave a bit differently the common assumption is always oh DNA is double stranded and RNA is single stranded right but in viruses that entire rule is tossed out the window. The first way we classify virus is what type of nucleic acid it has. The second way we classify the virus after that is whether the nucleic acid is single or double stranded. So please be aware of how we do classification of viruses as well.