Many human diseases, including the cold and the flu, are caused by viruses, made up mostly of a coat made of protein that surrounds either DNA or RNA.
How do they multiply?
Cells multiply using either mitosis or meiosis, processes which require much more complex machinery than viruses contain. So, how do they make more viruses? A virus reaches a cell and injects its own DNA or RNA into the cell, where it can then use the components of that host cell to make more viral genetic material. In the lytic cycle of the virus, the viral genetic material is then translated and used to create lots of new viruses, which all eventually break out of the host cell, killing it. Some viruses avoid killing the host cell by extending a part of the cell’s membrane and pinching off. Then, those viruses can go off and infect more host cells, creating even more viruses.
Some viruses choose not to immediately break out of the host cell. Instead, they insert their genetic material into the material of the host cell, which then gets passed onto all the offspring (or daughters) of that cell. This makes more and more viral genetic material, which can then all at once make lots of viruses and break out of cells in large numbers. This is called the lysogenic cycle.
Viruses are so different from all other “life” (and I put life in quotes because it is up for debate whether viruses are actually living) that many scientists have found the following question very interesting: how did viruses evolve? Did they evolve from bacteria and animals, or did they evolve completely separately? We’ll look at the hypotheses to this question in the next post!