When a particular protein is to be made, the DNA sequence of the relevant gene acts as a template for the creation of a complementary molecule made from another nucleic acid, RNA. In eukaryotes-creatures, like humans, mushrooms and kelp, with complex cells-as in their simpler relatives, the bacteria and archaea, the genes that describe proteins are written in double-stranded DNA. Others have roots that comfortably predate LUCA itself.ĭisparate origins are matched by disparate architectures for information storage and retrieval. Some viral lineages seem to have begun quite recently. It is more like a lifestyle-a way of being which different genes have discovered independently at different times. Being a virus is not something which provides you with a place in a vast, coherent family tree. T HE IDEA of a last universal common ancestor provides a plausible and helpful, if incomplete, answer to where humans, oak trees and their ilk come from. For scientists wanting to reprogram cells and mend metabolisms they offer inspiration-and powerful tools. For scientists seeking to understand life’s origin, they offer a route into the past separate from the one mapped by humans, oak trees and their kin. The legacy they have left in the human genome helps produce placentas and may shape the development of the brain. They may have been responsible for some of the most important events in the history of life, from the appearance of complex multicellular organisms to the emergence of DNA as a preferred genetic material. They spur evolution, driving natural selection and allowing the swapping of genes. They shape the balance of species in ecosystems ranging from those of the open ocean to that of the human bowel. The virions in the surface waters of any smallish sea handily outnumber all the stars in all the skies that science could ever speak of.īack on Earth, viruses kill more living things than any other type of predator. There are hundreds of billions of stars in the Milky Way galaxy and a couple of trillion galaxies in the observable universe. The number of copies of their genes to be found on Earth is beyond astronomical. No other biological entities are as ubiquitous, and few as consequential. The fact that viruses have only a tenuous claim to being alive, though, hardly reduces their impact on things which are indubitably so. In 2002 he became the first person in the world to take an array of nonliving chemicals and build a virion from scratch-a virion which was then able to get itself reproduced by infecting cells. Viruses, he says, “alternate between nonliving and living phases”.
That is why, asked whether viruses are alive, Eckard Wimmer, a chemist and biologist who works at the State University of New York, Stony Brook, offers a yes-and-no. It copies itself in whatever way it does simply because it has copied itself that way before, in other cells, in other hosts. The simplest purposes of the simplest life-to maintain the difference between what is inside the cell and what is outside, to move towards one chemical or away from another-are entirely beyond it.
Others immediately set about producing enough virions to split their hosts from stem to stern.
Some bide their time, letting the cell they share the life of live on.
It is truly alive only in the cells of others, a virtual organism running on borrowed hardware to produce more copies of its genome. A virion is just an arrangement of matter. An individual animal, or plant, embodies and maintains the restless metabolism that made it. The tiny particles, “virions”, in which those genes come packaged-the dot-studded disks of coronaviruses, the sinister, sinuous windings of Ebola, the bacteriophages with their science-fiction landing-legs that prey on microbes-are entirely inanimate. Viral genes have no cells, no bodies, no metabolism of their own. In viruses the link between metabolism and genes that binds together all life to which you are related, from bacteria to blue whales, is broken.
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