Are viruses considered to be cells

Scientists have argued for hundreds of years over how to classify viruses, says Luis Villarreal , professor emeritus at the University of California, Irvine, where he founded the Center for Virus Research. In the s, viruses were believed to be poisons.

In the s, they were called biological particles. Throughout, viruses have rarely been considered alive. More than definitions of life exist today, and most require metabolism, a set of chemical reactions that produce energy. Viruses do not metabolize. They do not have cells. They cannot reproduce independently.

A coronavirus, for example, is a nanoscale sphere made up of genes wrapped in a fatty coat and bedecked in spike proteins. Still, viruses have many traits of living things. They are made of the same building blocks. They replicate and evolve. Once inside a cell, viruses engineer their environment to suit their needs — constructing organelles and dictating which genes and proteins the cell makes.

Recently discovered giant viruses — which rival the size of some bacteria — have been found to contain genes for proteins used in metabolism, raising the possibility that some viruses might metabolize. Plus, almost every rule that excludes viruses from the land of the living has its own exceptions. For example, Rickettsia bacteria are classified as living but, like viruses, can multiply only within other cells.

All living things, in fact, rely on other living things. A single rabbit cannot replicate on its own, but a rabbit is definitely alive, right? For these reasons and others, the debate over whether viruses are alive or not continues today. In , virologists Marc H. Or maybe a virus can be both nonliving and alive. Despite their potential to kill, these potent pathogens are in fact considered to be non-living, as alive as the screen that you are reading this article on.

How is this possible? How can something as nasty as a virus spread so fast, reproduce, and infect other living things, but not be considered a living creature? The answer has been a subject of debate since the moment viruses were first named in There is no single undisputed definition of life. Does it multiply through cellular division? Does it have a metabolism? Read more: What came first, cells or viruses? They fail the second question for the same reason.

Unlike living organisms that meet their energy needs by metabolic processes that supply energy-rich units of adenosine triphosphate ATP , the energy currency of life, viruses can survive on nothing. In theory, a virus can drift around indefinitely until it contacts the right kind of cell for it to bind to and infect, thus creating more copies itself. Most viruses are only 20— nanometers in diameter, whereas human egg cells, for example, are about micrometers in diameter, and the E.

Viruses are so small that they are best viewed using an electron microscope , which is how they were first visualized in the s. Viruses generally come in two forms: rods or spheres. However, bacteriophages viruses that infect bacteria have a unique shape, with a geometric head and filamentous tail fibers. No matter the shape, all viruses consist of genetic material DNA or RNA and have an outer protein shell, known as a capsid. There are two processes used by viruses to replicate: the lytic cycle and lysogenic cycle.

Some viruses reproduce using both methods, while others only use the lytic cycle. In the lytic cycle, the virus attaches to the host cell and injects its DNA. Then fully formed viruses assemble. These viruses break, or lyse, the cell and spread to other cells to continue the cycle. Like the lytic cycle, in the lysogenic cycle the virus attaches to the host cell and injects its DNA. In humans, viruses can cause many diseases.

For example, the flu is caused by the influenza virus. Typically, viruses cause an immune response in the host, and this kills the virus. However, some viruses are not successfully treated by the immune system, such as human immunodeficiency virus, or HIV.

This leads to a more chronic infection that is difficult or impossible to cure; often only the symptoms can be treated. Unlike bacterial infections, antibiotics are ineffective at treating viral infections. Viral infections are best prevented by vaccines, though antiviral drugs can treat some viral infections. Most antiviral drugs work by interfering with viral replication. Some of these drugs stop DNA synthesis, preventing the virus from replicating. Although viruses can have devastating health consequences, they also have important technological applications.

Viruses are particularly vital to gene therapy. Because some viruses incorporate their DNA into host DNA, they can be genetically modified to carry genes that would benefit the host. Some viruses can even be engineered to reproduce in cancer cells and trigger the immune system to kill those harmful cells.