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Viruses can reproduce only within a host cell: an overview
Viruses are obligate intracellular parasites; that is, they can reproduce only within a host cell. An isolated virus is unable to reproduce--or do anything else, for that matter, except infect an appropriate host cell. Viruses lack the enzymes for metabolism and have no ribosomes or other equipment for making their own proteins. Thus, isolated viruses are merely packaged sets of genes in transit from one host cell to another.
Each type of virus can infect and parasitize only a limited range of host cells, called its host range. This host specificity depends on the evolution of recognition systems by the virus. Viruses identify their host cells by a "lock-and-key" fit between proteins on the outside of the virus and specific receptor molecules on the surface of the cell. (Presumably, the receptors first evolved because they carried out functions of benefit to the organism.) Some viruses have host ranges broad enough to include several species. Swine flu virus, for example, can infect both hogs and humans, and the rabies virus can infect a number of mammalian species, including raccoons, skunks, dogs, and humans. In other cases, viruses have host ranges so narrow that they infect only a single species. For instance, there are several phages that can parasitize only E. coli.
Viruses of eukaryotes are usually tissue specific. Human cold viruses infect only the cells lining the upper respiratory tract, ignoring other tissues. And the AIDS virus binds to a specific receptor on certain types of white blood cells.
A viral infection begins when the genome of a virus makes its way into a host cell (FIGURE 18.3). The mechanism by which this nucleic acid enters the cell varies, depending on the type of virus. For example, the T-even phages use their elaborate tail apparatus to inject DNA into a bacterium (see the chapter-opening drawing on p. 328). Once inside, the viral genome can commandeer its host, reprogramming the cell to copy the viral nucleic acid and manufacture viral proteins. Most DNA viruses use the DNA polymerases of the host cell to synthesize new genomes along the templates provided by the viral DNA. In contrast, to replicate their genomes, RNA viruses must use special virus-encoded polymerases, ones that can use RNA as a template. (Cells generally have no native enzymes for carrying out such a process.) We will describe the replication of DNA and RNA viruses in more detail later in the chapter.
Fig 18-3. A simplified viral reproductive cycle. A virus is an obligate intracellular parasite that uses the equipment of its host cell to reproduce. In this simplest of viral cycles, the parasite is a DNA virus with a capsid consisting of a single type of protein. After entering the cell, the viral DNA uses host nucleotides and enzymes to replicate itself. The viral DNA uses other host resources to produce its capsid proteins by transcription and translation. The new viral DNA and capsid proteins assemble into new virus particles, which leave the cell.
Regardless of the type of viral genome, the parasite diverts its host’s resources for viral production. The host provides the nucleotides for nucleic acid synthesis. It also provides enzymes, ribosomes, tRNAs, amino acids, ATP, and other components needed for making the viral proteins dictated by viral genes.
After the viral nucleic acid molecules and capsomeres are produced, their assembly into new viruses is often a spontaneous process, a process of self-assembly. In fact, the RNA and capsomeres of TMV can be separated in the laboratory and then reassembled to form complete viruses simply by mixing the components together again. The simplest type of viral reproductive cycle is completed when hundreds or thousands of viruses emerge from the infected host cell. The cell is often destroyed in the process. In fact, some of the symptoms of human viral infections, such as colds and influenza, result from cellular damage and death and from the body’s responses to this destruction. The viral progeny that exit a cell have the potential to infect additional cells, spreading the viral infection.
There are many variations on the simplified viral reproductive cycle we have traced in this overview. We will see several examples as we take a closer look at some bacterial viruses (phages), animal viruses, and plant viruses.
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Phages reproduce using lytic or lysogenic cycles
The phages are the best understood of all viruses, although some of them are also among the most complex. Research on phages led to the discovery that some double-stranded DNA viruses can reproduce by two alternative mechanisms: the lytic cycle and the lysogenic cycle.
The Lytic Cycle
A phage reproductive cycle that culminates in death of the host cell is known as a lytic cycle. The term refers to the last stage of infection, during which the bacterium lyses (breaks open) and releases the phages that were produced within the cell. Each of these phages can then infect a healthy cell, and a few successive lytic cycles can destroy an entire bacterial colony in just hours. A phage that reproduces only by a lytic cycle is a virulent phage. FIGURE 18.4 (p. 332) uses the virulent phage T4 to illustrate the steps of a lytic cycle. The figure and legend describe the process, which you should study before proceeding.
Fig 18-4. The lytic cycle of phage T4. Phage T4 has about 100 genes, which are transcribed and translated using the host cell’s machinery. One of the first phage genes translated after infection codes for an enzyme that chops up the host cell’s DNA (step 3); the phage DNA is protected from breakdown because it contains a modified form of cytosine that is not recognized by the enzyme. The entire lytic cycle, from the phage’s first contact with the cell surface to cell lysis, takes only 20-30 minutes at 37°C.
After reading about the lytic cycle, you may wonder why phages haven’t exterminated all bacteria. Actually, bacteria are not defenseless. Natural selection favors bacterial mutants with receptor sites that are no longer recognized by a particular type of phage. And when phage DNA successfully enters a bacterium, various cellular enzymes may break it down. Enzymes called restriction nucleases, for example, recognize and cut up DNA that is foreign to the cell, including certain phage DNA. The bacterial cell’s own DNA is chemically modified in a way that prevents attack by restriction enzymes. But just as natural selection favors bacteria with effective restriction enzymes, natural selection favors phage mutants that are resistant to these enzymes. Thus, the parasite-host relationship is in constant evolutionary flux.
There is still another important reason bacteria have been spared from extinction as a result of phage activity. Many phages can check their own destructive tendencies and, instead of lysing their host cells, coexist with them in what is called the lysogenic cycle.
The Lysogenic Cycle
In contrast to the lytic cycle, which kills the host cell, the lysogenic cycle replicates the phage genome without destroying the host. Phages that are capable of using both modes of reproducing within a bacterium are called temperate phages. To compare the lytic and lysogenic cycles, we will examine a temperate phage called lambda, written with the Greek letter l. Phage l resembles T4, but its tail has only one short tail fiber.
Infection of an E. coli cell by phage l begins when the phage binds to the surface of the cell and injects its DNA (FIGURE 18.5). Within the host, the l DNA molecule forms a circle. What happens next depends on the reproductive mode: lytic cycle or lysogenic cycle. During a lytic cycle, the viral genes immediately turn the host cell into a l-producing factory, and the cell soon lyses and releases its viral products. The viral genome behaves differently during a lysogenic cycle. The l DNA molecule is incorporated by genetic recombination (crossing over) into a specific site on the host cell’s chromosome. It is then known as a prophage. One prophage gene codes for a protein that represses most of the other prophage genes. (This is the repressor protein Nancy Hopkins studied as a graduate student; see p. 232.) Thus, the phage genome is mostly silent within the bacterium. How, then, does the phage reproduce? Every time the E. coli cell prepares to divide, it replicates the phage DNA along with its own and passes the copies on to daughter cells. A single infected cell can quickly give rise to a large population of bacteria carrying the virus in prophage form. This mechanism enables viruses to propagate without killing the host cells on which they depend.
Fig 18-5. The lysogenic and lytic reproductive cycles of phage l, a temper-ate phage. After entering the bacterial cell and circularizing, the l DNA can either integrate into the bacterial chromosome (lysogenic cycle) or immediately initiate the production of a large number of progeny phages (lytic cycle). In most cases, the lytic pathway is followed, but once a lysogenic cycle begins, the prophage may be carried in the host cell’s chromosome for many generations. Phage l has a single, short tail fiber, not shown in this diagram.
The term lysogenic implies that prophages are capable of giving rise to active phages that lyse their host cells. This occurs when, occasionally, the l genome exits the bacterial chromosome. Once free in the cell, the l genome initiates a lytic cycle. It is usually an environmental trigger, such as radiation or the presence of certain chemicals, that switches the virus from the lysogenic to the lytic mode.
In addition to the gene for the repressor protein, a few other prophage genes may also be expressed during lysogenic cycles, and the expression of these genes may alter the phenotype of the host bacteria. This phenomenon can have important medical significance. For example, the bacteria that cause the human diseases diphtheria, botulism, and scarlet fever would be harmless to humans if it were not for certain prophage genes that induce the host bacteria to make toxins.
This post was edited by Abstraction on Dec 11 2008 12:45am