WORD OF THE MONTH – VIRUS
By Bruce Palmer, from quarantine in Cutten, CA
Here we are, stuck at home, needing to isolate ourselves from our friends, relatives, the Eureka Chapter and the rest of society. What better word than VIRUS? The word virus comes directly from the Latin, virus, a gooey substance or a poison. Recently, the term has been applied to problems in our computers, but technically, it is a biological term applied to very small particles containing only DNA or RNA and no cell organelles. It is still debated as to whether they are living, and in fact the existence of viruses is one of the obstacles to defining just what life is.
The discovery and early study of viruses is related to plants, not animals. The leaves of tobacco plants have a disease now called the tobacco mosaic virus. In the late nineteenth century, researchers tried to isolate the agent, assuming it was a bacterium. It was not. In fact, they discovered that whatever the agent was could not be seen with the best light microscopes and that it passed through thin porcelain filters (thus the term used when I was a child, filterable virus). It was not until within my lifetime, in 1935, that the tobacco mosaic virus was crystalized to show that it was a biological substance; it could be crystallized but still be viable. It took the perfection of the electron microscope since World War II to obtain images of viruses. You may have seen the scenes on TV of somebody looking through a light microscope, followed by a photo of the COVID-19 virus. That is extremely misleading. The practical limit of light microscopes is about 1,000X. Viruses require a magnification of at least 100,000X and detailed images like those we see on TV are closer to 500,000X Only electron microscopes can resolve objects that small (newer electron microscopes can resolve objects at 1,000,000X).
Plants have a number of viruses, including witches’ brooms (on our rhodies), burls such as those on our redwoods and lack of coloring on parts of leaves and tulip flowers. The early Dutch tulip traders valued mottled flowers most highly. Viruses in plant tissue are seldom fatal, unlike some in animals.
Viruses are tricky. They have no way to reproduce themselves, so they hijack a regular cell’s reproductive mechanisms in order to multiply. Most viruses have a DNA helix surrounded by protein. They invade the DNA of a cell and cause it to reproduce myriad copies of the virus while the cell DNA is replicating itself. Covid-19, though, is a retrovirus. Discovered only recently (HIV is a retrovirus), retroviruses contain RNA, not DNA. The usual order of copying and assembly in cells is that DNA makes various RNAs which make proteins. Retroviruses contain reverse transcriptase that can make DNA from an RNA template. That harmful DNA then produces new virus particles directly or invades the cell’s DNA and acts like a regular virus. Regardless of how COVID-19 works, here we are, confronting a virus humans have never encountered before and to which we have no immunity. We must cancel our meetings and the flower show. Hopefully, we will be together again in September.