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Influenza virus particles.

Eye of Science/Science Source


From Grolier's New Book of Knowledge

In a simple animal virus structure (above), the nucleic acid (DNA or RNA) is enclosed by a protein shell called the capsid. Many viruses develop spikes to help them attach to specific cell surfaces.

Eye of Science /
Science Source

The invisible, infectious "somethings" called viruses—first named in the late 1800s—proved to be the subjects of the great biological detective stories of the 20th century. Yet, despite the many breathtaking discoveries of the past 100 years, mysteries remain for virus hunters of the 21st century. For that matter, biologists have yet to resolve the long-standing debate as to whether viruses are "alive" in the first place. Certainly, viruses fail to fit into any of the five recognized kingdoms of life, being neither bacteria, fungi, protist, plant, nor animal. If they are alive, viruses may well be the most numerous of life-forms on our planet.

What Is a Virus?

The name virus means "poison," and that is all that early biologists knew about the mysterious agent that sickened those who came in contact with it. Then, in 1892, the Russian botanist Dmitry Ivanovsky isolated the first known virus, in the sap of tobacco plants with mosaic disease. First, Ivanovsky filtered the plant juices with an ultrafine sieve to remove any bacteria. He expected to find the culprit behind tobacco-mosaic disease inside his filter. Instead, he found that the supposedly purified plant juice remained infectious. When he applied it to new plants, they, too, developed mosaic disease. Ivanovsky realized he had discovered an infectious agent smaller than any microorganism known to science at that time.

Before the end of the 19th century, biologists found a similar "contagious fluid" in cows with foot-and-mouth disease. Importantly, the scientists discovered that a tiny amount of the substance, injected into an animal, multiplied and spread throughout the animal's body. In other words, viruses appeared to replicate—a sign that they were some sort of organism.

At first, biologists thought that viruses were simply bacteria too small to see with even the most powerful light microscope. In the early years of the 20th century, researchers discovered many more diseases caused by these ultratiny microbes. Yet, unlike ordinary microorganisms, viruses refused to grow in laboratory cultures. They thrived only in the living cells of their hosts—plants, animals, or other microbes such as bacteria. As a result, scientists concluded that viruses were some sort of cell parasite.

Biologists of the 1930s found ways to study viruses, though they still could not see them. The researchers grew large amounts of virus material inside chicken and mouse embryos, and performed chemical tests to identify the substances of which the material was made. To their amazement, the scientists found that viruses consisted of only nucleic acids and proteins. Unlike all known organisms, viruses lacked the other building blocks of life: carbohydrates and lipids (fats). Finally, in the 1940s, the development of powerful electron microscopes enabled microbiologists to see the size and shape of different viruses and study their details. What they saw was a bizarre zoo of particles shaped like rods, cubes, spheres, threads, and polyhedrons—some with strange geometric "heads" and tadpole "tails."

Chemistry of a Virus

Life as we know it reproduces, or produces offspring, according to the instructions written in long molecules of nucleic acid called DNA, or deoxyribonucleic acid. DNA is the genetic material that cells copy before dividing into two, and that parents pass to their offspring. Cells translate the instructions carried in DNA using a second type of nucleic acid called RNA, or ribonucleic acid. All cells, from that of the simplest bacterium to those of the most complex plants and animals, contain DNA and RNA.

Viruses, by contrast, contain just one type of nucleic acid—some RNA, others DNA, but never both. Some viruses contain only a single strand of nucleic acid. Some have two strands, and still others have loops or scattered bits and pieces. Be it RNA or DNA, a virus' snippet of genetic material contains just enough instructions, or genes, to direct its host cell to make more viruses.

A virus' nucleic acid lies in a hollow core surrounded by a capsule, or capsid, made of protein. In some viruses, this capsid consists of a single type of protein. Others have several types. In either of these cases, the individual protein molecules curl into subunits, or building blocks, called capsomeres, which chemically link to assemble the capsid and give a virus its distinctive shape.

Some viruses have an additional outer membrane, or envelope, made of fatty acids and additional proteins. However, the virus does not make this outer coat itself. Rather, it steals it from the outer membrane of whatever cell it infects. Viruses that lack such envelopes are referred to as naked.

Some enveloped viruses display special molecules called spikes on their surface. When such a virus bumps into the right cell, the spikes fasten onto the cell's surface like a cocklebur sticking to a hiker's sock. The HIV virus that causes AIDS has such a spiked envelope. The spikes attach only to special molecules found on the surface of white blood cells known as T lymphocytes.

Size and Shape

An individual virus, or virion, measures only a few nanometers, or billionths of a meter, across. Very few can be seen without the aid of a powerful electron microscope. The poliovirus, one of the smallest, measures about 30 nanometers (1.2 millionths of an inch). The smallpox virus, one of the largest, measures nearly 300 nanometers (12 millionths of an inch)—about the same size as the smallest known bacteria, and just visible under the most powerful lens of a light microscope. To envision the size of viruses compared to a human cell, think of the cell as an ocean liner, and viruses ranging in size from a rowboat to a tugboat.

So small are most viruses that biologists can count how many molecules they contain. The tobacco-mosaic virus, for example, consists of a single molecule of the nucleic acid RNA wrapped in about 2,200 molecules of one kind of protein.

Using an electron microscope, microbiologists have identified three basic virus shapes: helical, polyhedral, and complex. From the outside, the capsid of a helical virus looks like a thread or a stick, depending on its diameter and flexibility. If you could cut a cross section of the helical capsid, you would see that it has a hollow core that contains the virus' genetic material. The rodlike tobacco-mosaic virus is such a helical virus, as is the bullet-shaped rabies virus.

Polyhedral viruses can have a faceted, almost gemlike appearance, with sharp, distinct faces; or they can be spherical, like a soccer ball. A very close look shows that most polyhedral viruses are icosahedrons—polygons with 20 triangular faces. Like the helical rod, the polyhedron has a hollow core containing genetic material. The polio virus is a polyhedron, as are the viruses that cause herpes and colds.

True to their categorical name, complex viruses can have an extremely complicated structure made up of many kinds of protein. This is especially true of many bacterial viruses, or bacteriophages (meaning "bacteria eaters"). Their genetic material can be found inside a hollow polyhedral head attached to a rodlike tail. The tail, in turn, ends in a circular plate with spiderlike feet that attach to the host bacterium. Animal viruses likewise come in complex forms. The virus that causes smallpox, for example, has several thick layers of protein surrounding its strand of nucleic acid.

Classification. Virologists, scientists who study viruses, do not speak of virus species. But they do sort different kinds of viruses into groups according to their structure, chemistry, and other physical properties. On the simplest level, all viruses can be divided into two groups based on their nucleic acid: DNA viruses and RNA viruses. The experts also classify viruses by what they infect—so we have animal viruses, plant viruses, and bacterial viruses, or bacteriophages. Within each of these groups, certain viruses can infect only certain species. The HIV virus that causes AIDS, for example, infects a variety of primates, including humans.

On yet another level, viruses can be categorized according to structure, or morphology, as naked or enveloped, and as helical, polyhedral, or complex. The papilloma, or wart, viruses, for example, are said to be naked polyhedrons. The influenza viruses and HIV viruses are enveloped helical; and pox viruses are enveloped complex. Even more commonly, virologists classify viruses by the types of diseases they cause or the kinds of organs or tissues they infect. So they speak of rhinoviruses when talking about the 100-odd types of virus that can cause the common cold.

Importantly, although different types of viruses vary dramatically in size, shape, and complexity, the individual members, or virions, of any one kind of virus are alike, just as members of a living species are alike. At the same time, viruses can evolve surprisingly quickly—sometimes by picking up genetic material from the cells they infect. As a result, new strains of certain viruses such as influenza (the "flu") appear every year.

Viral Origins Any description of what viruses are leads to the question of how they first evolved. In other words, if viruses cannot replicate themselves, what produced the first virus? One of two hypotheses appears possible. Many biologists believe that viruses are no more than lost bits of genetic material that mistakenly get copied by living cells. The Nobel Prize–winning microbiologist Salvador Luria called them "bits of heredity looking for a chromosome."Alternately, viruses may be descended from more-complex, living parasites that became more and more dependent on their cell hosts for all the energy-fueled activities we associate with the various functions of "life."

Whatever the case, viruses have become a virtually inescapable part of all other living systems on this planet—most noticeably as the carriers of disease. Many biologists also believe that viruses played a vital role in the evolutionary process.

How Do Viruses Operate?

Viruses represent perhaps the purest form of parasitism. Outside of the living cell of its host, an individual virus is nothing more than a lifeless entity. It cannot produce so much as a molecule of ATP—the chemical fuel of life as we know it. So a virus cannot actively move or "seek out" the cells it infects. Instead, virus particles passively travel through the environment, typically attached to water droplets, specks of dust, or the bodies of carrier organisms, such as flies or mosquitoes. Sooner or later, a virus particle collides with the appropriate cell. What follows is a step-by-step chemical process that ends in the virus being multiplied hundreds, even thousands, of times.

Attachment Viruses bump into cells all the time. But nothing happens unless there exists an exact chemical match between the virus and the membrane of the cell. When the virus touches the right cell, it attaches. As mentioned, some viruses have molecular spikes that cling to the membranes of certain cells. Many bacteriophages have spiderlike tail fibers that chemically bind to the cell membranes of certain bacteria. In either case, a weak chemical bond forms between the virus and the target cell.

Entry Whether they consider viruses to be living or nonliving, all biologists agree that viruses seem downright clever when it comes to getting inside a cell. Among the most fascinating to observe are the complex bacteriophages that use spiderlike feet to land on a cell. Once attached, the tail of such a virus contracts, driving its core through the cell wall and injecting its nucleic acid into the bacterium. The now-empty capsid remains on the outside of the bacterium.

The viruses that enter animal cells do so without appendages such as "feet" and "tails." Many of these viruses have envelopes, stolen from a previous cell, that match and fuse with the membrane of the new host cell. Others "trick" the host cell into engulfing them as it would some bit of food. Still others simply slip in undetected.

Plant viruses usually need some sort of outside help to gain entry into a healthy cell. This requirement comes about because the living membrane of a plant cell lies protected inside a strong cell wall. Some plant viruses gain entry by hitching a ride in the body fluids of insect pests that bore through a plant's cell walls when they feed. A broken stem or leaf can likewise provide access to viruses. And a few plant viruses can directly enter seeds and other especially tender plant structures.

Multiplication and Release Once through the cell membrane, a virus unpacks its nucleic acid, which then chemically instructs the cell's machinery to begin manufacturing new viruses. It may do so in one or more of several ways. The uncoated DNA or RNA may become a direct template. In other words, the cell begins using the virus' nucleic acid as it would its own—as a blueprint for making both more DNA or RNA as well as the proteins needed to assemble intact virus particles.

Other viruses merge their genetic material with that of the cell they infect. If the virus' genetic material consists of DNA, it may simply patch itself into the cell's DNA. The process is a little more complicated for an RNA virus. Its RNA becomes a template for the building of a matching strand of DNA, which then splices into the cell's DNA.

A cell whose genome contains inserted viral genes may remain healthy and copy the virus' genetic material along with its own whenever it multiplies. Such viral genes remain latent, or quiet, until certain conditions arise that trigger them to pop out of the cell's genetic material and become active—that is, direct the cell to begin copying the genes thousands to millions of times over.

Whether a cell begins making virus particles immediately or after a period of latency, two things can happen. The copying may continue until the cell swells, and then, full of viral particles, bursts, releasing a new generation of viruses to infect nearby cells. This is the case with many naked viruses, such as the adenoviruses that cause respiratory infections in humans.

Other viruses, once assembled inside the host cell, push through the cell's membrane in a process called budding. In the process, a bit of the cell membrane sticks to the virus capsid to form an envelope. Budding may weaken a host cell, but does not immediately kill it. As a result, the cell may continue to shed virus particles indefinitely. The virus that causes infectious mononucleosis operates in this way.

The herpes virus that causes cold sores in people (herpes simplex I) is a good example of a virus that remains latent in certain cells while killing others. After an initial infection, the virus retreats to the long nerve cells that extend from a region near the temple to the lips. Instead of killing those nerve cells, the herpes simplex I virus inserts its genetic material into that of the nerve cells. At some later time, an outside stress, perhaps a cold, a sunburn, or even anxiety, triggers the viral DNA to start replicating. New cold-sore viruses then bud from the nerve cells to enter cells on the lips. Those cells simply multiply the virus until they burst, producing painful, fluid-filled blisters that are teeming with infectious virus particles.

Cells and Viruses Interact

Just as viruses have evolved ways to enter and commandeer most every kind of cell, organisms have evolved ways to rebuff their attacks, or at least bring them under control. Once exposed to a virus, the immune systems of animals produce antibodies custom-designed to destroy it. For example, should chicken-pox viruses enter the body of a person who has already had the disease, that individual's immune system will quickly mop up the viruses before they can make him or her sick a second time.

Similarly, exposure to one virus may trigger immunity to similar viruses. Such was the case with the relatively harmless cowpox virus that gave infected people natural immunity to the far deadlier smallpox. In 1796, the English physician Edward Jenner exploited this generalized immunity to create the world's first effective vaccine, injecting patients with cowpox virus to protect them from smallpox. Today, many vaccines contain killed or weakened viruses that signal the immune system to be "on the lookout" for others of their kind.