Living creatures, such as plants and animals, have cell structures, and these cells have the ability to divide and proliferate. Every cell contains DNA or RNA, which is the genetic information of life. In contrast, viruses are more like free-form DNA or RNA. If it is separated from the host, the virus cannot reproduce itself. Dr Otto Yang, a professor of medicine, microbiology, immunology and molecular genetics at the University of California, Los Angeles, said that they had to invade living cells to replicate themselves. Otto said: "Viruses are packaged RNA and DNA, and they reproduce themselves by hijacking cells."
So is the virus alive or not?
Many philosophers and scientists have long argued about what life is. They roughly gave seven basic characteristics of life-life must be able to respond to external stimuli, grow and produce offspring, maintain an appropriate body temperature, have metabolism, be composed of one or more cells and be able to adapt to its environment.
But when you think about it, some life doesn't seem to meet all these conditions. For example, some hybrid animals, such as mules produced by horses and donkeys, cannot reproduce because mules are infertile. But if you look at other things, you will find that rocks can "grow", although in the opposite direction (rocks will be eroded and decomposed by weathering) But this is a very complicated method. If we adopt a much simpler definition, things will be completely different.
Amesh Adalja, an infectious disease physician at Johns Hopkins University Health Center, said: "You can put a cat, a plant and a stone in a room for several days. When you open the house again, you will find that the cats and plants have changed, but the stones have remained basically the same. "
Like this stone, if you put it in the room, most viruses will not change. In addition, Adaga noticed that creatures have survival instinct and spontaneous action, and they will take some measures to try to prolong their lives. For example, plants will stretch their roots to find water, and animals will run around looking for food.
But viruses, like stones, have no such active behavior to survive. Adaga said: "I don't think the virus is qualified as a kind of life. Essentially, they are inactive unless they can invade the interior of living cells. Of course, there are also some characteristics that make them close to the dividing line of life, that is, they have genetic material: DNA or RNA. This is of course fundamentally different from a stone, but it is also obvious that they are very different from bacteria because they have no survival instinct and spontaneous movement. "
In this regard, Otto Yang agreed. He said: "Without cells, the virus cannot replicate itself. At this level, viruses really can't be classified as life-if you define independent reproduction as a necessary condition of life. However, if this standard is relaxed and changed to' life can be defined as self-replication whether independently or with the help of the outside world', then viruses that can self-replicate with the help of cell bodies can of course be counted as a kind of life. "
Some people think that the earliest life form on earth is some kind of RNA molecule. Yang pointed out that under some suitable environmental conditions, RNA molecules can replicate themselves. Perhaps today's viruses were inherited from such an ancestor, but in the long process of evolution, they lost the ability to replicate themselves independently.