Viruses infect and live inside the cells of living organisms. When viruses infect the cells of their host, they may cause disease. For example, viruses cause AIDS Acquired immune deficiency syndrome , influenza flu , chicken pox, and the common cold. Therapy is sometimes difficult for viral diseases.
Antibiotics have no effect on viruses and only a few antiviral drugs are available for some diseases. One of the best ways to prevent viral diseases is with a vaccine , which produces immunity. But vaccines are available for only a few diseases. Mimivirus, shown in the Figure below , is the largest known virus, with a diameter of nm. Protein filaments measuring nm stick out from the surface of the virus, which increases the diameter of the virus to about nm. This is bigger than a small bacterial cell.
The virus appears hexagonal under an electron microscope; the viral shape is icosahedral having 20 faces or sides. The largest known virus, called mimivirus, is so large that scientists first mistook it for a bacterium.
It was first discovered in amoeba, in , and was identified as a virus in Scientists believe that mimivirus may cause certain types of pneumonia in humans.
Viruses can replicate only by infecting a host cell. They cannot reproduce on their own. Viruses are not cells; they are a strand of genetic material within a protective protein coat called a capsid. They infect a wide variety of organisms, including both eukaryotes and prokaryotes. Viruses can be found almost anywhere there is life, including living within prokaryotes.
A phage is a virus that infects prokaryotes. Phages are estimated to be the most widely distributed and diverse entities in the biosphere, even more numerous than prokaryotic organisms. Viruses called bacteriophages are able to infect bacterial cells and use them as hosts to make more viruses. After multiplying, these viruses assemble and occasionally remove a portion of the host cell's bacterial DNA.
Later, when one of these bacteriophages infects a new host cell, this piece of bacterial DNA may be incorporated into the genome of the new host. There are two types of transduction: generalized and specialized. In generalized transduction, the bacteriophages can pick up any portion of the host's genome.
In the example shown in the accompanying diagram a bacteriophage attacks a bacterial cell by attaching itself to the outer wall. It then uses a specialized protein to digest a small hole in the wall of the bacterial cell and inject its nucleic acid molecule into the bacterial cell's cytoplasm. Once there, the bacteriophage's nucleic acid takes over the synthetic machinery of the bacteria and directs the full scale production of more phage nucleic acid and more phage proteins.
When these components are ready, they are assembled into new bacteriophages and then released as the host cell bursts open, or lyses. In this way a single infecting bacteriophage can give rise to over new offspring. The Lysogenic Cycle Other bacteriophages and many of the viruses do not automatically take over the host cell and begin making new "offspring". Instead, the invading nucleic acid somehow joints up with the host cell's DNA. A break occurs in the host chromosome and a piece of phage or viral DNA is inserted.
Enzymes splice the two molecules together, like splicing recording tape, and the recombinant molecule is a hybrid of host genetic information and virus genetic information. From this moment on, every time the host cell makes a new copy of its own DNA it also inadvertently makes a copy of the viral or phage DNA.
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