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understood only within the last half century, but the first step on this path of discovery was taken by the Russian botanist Dmitri Ivanovsky in 1892. He was trying to find the cause of tobacco mosaic disease, which results in the destruction of photosynthetic tissues in plants and can devastate a tobacco crop. Ivanovsky passed an extract of diseased tobacco leaves through a fine porcelain filter, a technique that had been used previously by physicians and veterinarians to isolate disease-causing bacteria.

To Ivanovsky's surprise, the disease agent in this case was not retained on the filter. It passed through, and the liquid filtrate still caused tobacco mosaic disease. But instead of concluding that the agent was smaller than a bacterium, he assumed that his filter was faulty. Pasteur's recent demonstration that bacteria could cause disease was the dominant idea at the time, and Ivanovsky chose not to challenge it. But, as often happens in science, someone soon came along who did. In 1898, the Dutch microbiologist Martinus Beijerinck repeated Ivanovsky's experiment and also showed that the tobacco mosaic disease agent could diffuse through an agar gel. He called the tiny agent contagium vivum fluidum, which later became shortened to virus.

Almost 40 years later, the disease agent was crystallized by Wendell Stanley (who won the Nobel prize for his efforts). The crystalline viral preparation became infectious again when it was dissolved. It was soon shown that crystallized viral preparations consist of proteins and nucleic acids. Finally, direct observation of viruses with electron microscopes in the 1950s showed clearly how much they differ from bacteria and other organisms. The simplest infective agents of all are viroids, which are made up only of genetic material.

Viruses reproduce only with the help of living cells

Whole viruses never arise directly from preexisting viruses. Viruses are obligate intracellular parasites; that is, they develop and reproduce only within the cells of specific hosts. The cells of animals, plants, fungi, protists, and prokaryotes (both bacteria and archaea) can serve as hosts to viruses. Viruses use the host's synthetic machinery to reproduce themselves, usually destroying the host cell in the process. The host cell releases progeny viruses, which then infect new hosts.

Viruses outside of host cells exist as individual particles called virions. The virion, the basic unit of a virus, consists of a central core of either DNA or RNA (but not both) surrounded by a capsid, or coat, composed of one or more proteins. Because they lack the distinctive cell wall and riboso-mal biochemistry of bacteria, viruses are not affected by antibiotics.

There are many kinds of viruses

There are four ways to describe viruses:

► Whether the genome is DNA or RNA

► Whether the nucleic acid is single-stranded or double-stranded

► Whether the shape of the virion is a simple or complex crystal

► Whether the virion is surrounded by a membrane

Some of these variations are shown in Figure 13.1.

Another important descriptor of a virus is the type of organisms it infects. Most viruses have relatively simple means of infecting their host cells. Some can infect a cell but postpone reproduction, remaining inactive in the host cell until conditions are favorable.

Bacteriophage reproduce by a lytic cycle or a lysogenic cycle

Viruses that infect bacteria are known as bacteriophage or phage (Greek phagos, "one that eats"). They recognize their hosts by means of proteins in the capsid, which bind to specific receptor proteins or carbohydrates in the host's cell wall. The virions, which must penetrate the cell wall, are often equipped with tail assemblies that inject the phage's nucleic acid through the cell wall into the host bacterium. After the nucleic acid has entered the host, one of two things happens, depending on the kind of phage:

► The virus may reproduce immediately and kill the host cell.

► The virus may postpone reproduction by integrating its nucleic acid into the host cell's genome.

We saw one type of viral reproductive cycle when we studied the Hershey-Chase experiment (see Figure 11.3). That was the lytic cycle, so named because the infected bacterium lyses (bursts), releasing progeny phage. The alternative fate is the lysogenic cycle, in which the infected bac-

Lysogenic Cycle Steps

13.1 Virions Come in Various Shapes (a) The tobacco mosaic virus (a plant virus) consists of an inner helix of RNA covered with a helical array of protein molecules. (b) Many animal viruses, such as this adenovirus, have an icosahedral (20-sided) capsid as an outer shell. Inside the shell is a spherical mass of proteins and DNA. (c) Not all virions are regularly shaped.These wormlike virions of the influenza A virus infect humans, causing chills, fever, and sometimes, death.

13.1 Virions Come in Various Shapes (a) The tobacco mosaic virus (a plant virus) consists of an inner helix of RNA covered with a helical array of protein molecules. (b) Many animal viruses, such as this adenovirus, have an icosahedral (20-sided) capsid as an outer shell. Inside the shell is a spherical mass of proteins and DNA. (c) Not all virions are regularly shaped.These wormlike virions of the influenza A virus infect humans, causing chills, fever, and sometimes, death.

terium does not lyse, but instead harbors the viral nucleic acid for many generations. Some viruses reproduce only by the lytic cycle; others undergo both types of reproductive cycles (Figure 13.2).

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