Viruses are noncellular, intracellular parasites. A virus isn’t a cell; it’s a piece of nucleic acid surrounded by a protein coat. Viruses are parasites; they depend on the resources of host organisms to reproduce. In order to reproduce, a virus must wait for the opportunity to enter a cell. It can then make that cell its host.

Once inside, a virus reprograms its host cell to replicate the viral genome and to synthesize viral proteins for making more viruses.

The range of potential hosts for a virus varies. Hosts can span several species, or the host might be restricted to just one single type of tissue cell of a single species.

Just as the range of potential hosts varies for a virus, the structure of a virus varies. However, one feature is the same in all viruses: a protein coat that surrounds the viral genome. The viral genome is a single nucleic acid molecule containing anywhere from a few genes to a few hundred genes. Depending on the virus, the genome is DNA or RNA, single-stranded or double-stranded.

The protein coat is called a capsid, and it’s made of smaller protein subunits. Some viruses have an additional covering surrounding the capsid. A viral envelope is a protein-studded lipid membrane derived from the membrane of the host cell.

The influenza virus is a single-stranded RNA molecule with an envelope surrounding its capsid. The proteins of the envelope help attach the virus to the host cell membrane. The viral envelope then fuses with the host’s cell membrane and the capsid and viral genome enter the cell. The capsid is removed and the viral genome reprograms the cell to stop replicating its own genome.

Instead, the viral genome is replicated and viral proteins are synthesized. The proteins are used to make the virus’s structural components, such as the capsid and the proteins of the envelope. The proteins are transported to the cell membrane. The capsid proteins gather around the viral genome. Then they both merge toward the proteins and the viruses “bud” from the cell.

Another well-known animal virus is HIV, the human immunodeficiency virus. HIV is the virus that causes AIDS. HIV is a retrovirus. A retrovirus has an RNA genome and contains reverse transcriptase, an enzyme that transcribes DNA from RNA.

The DNA corresponding to the virus’s RNA can be incorporated into the cell’s DNA. This viral DNA is considered a provirus. The provirus is replicated and transcribed to RNA. The viral RNA is used as the genome for a new virus, or as a template for the production of viral proteins.

Bacteria can also get viral infections. Bacteriophages, or phages, are viruses that infect bacteria. For example, the phage shown here infects E. coli a species of bacteria found in the human intestine. The capsid of the phage consists of a polyhedral head and a helical tail, composed of protein.

Some phages have two life cycles. One cycle, the lytic cycle, destroys the host cell. The other cycle, the lysogenic cycle, does not harm the host cell.

The lytic cycle is called “lytic” because it results in the lysis of the host cell. Lysis is the bursting of the cell caused by destruction of the cell membrane. Let’s look at the lytic cycle of a double-stranded DNA virus.

The cycle begins with the attachment of the virus to the host cell. The virus then injects its DNA into the host. The host’s DNA is cut into pieces and the host’s resources are used to replicate viral DNA and synthesize viral components. The viral components and the replicated viral DNA then come together and assemble new viruses. The new viruses leave the host, bursting the host’s cell membrane. The new viruses repeat the cycle with new host cells.

The lysogenic cycle doesn’t destroy the host cell. In this cycle, when the virus injects its DNA into the host cell, it doesn’t take over the host’s resources. Instead, the virus incorporates its DNA into the host cell’s DNA, making it a prophage. When the host cell replicates its DNA, the viral DNA is replicated along with it. The host cell divides, and both daughter cells carry the viral DNA. The prophage can either continue to be incorporated in the host’s DNA or it can separate from the host’s DNA and enter the lytic cycle.

Viruses are so small that only the largest viruses can be seen with a light microscope. Now imagine infectious molecules that make viruses look like giants. Viroids are RNA molecules several hundred nucleotides long that attack plant cells. They lack a protein coat and replicate using their host’s resources. Viroids affect plant metabolism, causing stunted or abnormal growth. Viroids caused diseases that killed over 30 million coconut trees in the Philippines from the early 1900’s to the 1980’s.

The origin of viroids is uncertain. Similarities have been found between viroids and certain RNA genes. Nucleotide sequences of viroids are similar to introns in those RNA genes. Some scientists believe that viroids have evolved from introns that escaped from the cell.

One of the most intriguing infectious agents is the prion. Prions are protein molecules that cause transmissible disease. Biologists speculate that a prion is a misfolded protein that can affect the structure of similar normal-folded proteins by just coming into contact with them. Scientists think that a specific prion causes mad cow disease. Mad cow disease, also known as bovine spongiform encephalopathy, results in the degeneration of the brain of cows. Research is still being done on how exactly prion diseases spread.

Prions are fascinating because they don’t follow the central dogma of biology. They don’t carry their reproduction information in DNA or RNA. They are merely proteins.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education