The Status and Structure of Viruses

• A virus is a very small infective agent that consists of a core of nucleic acid and is dependent on a living host.

  • Viruses contain the nucleic acids necessary to make copies of themselves, but to reproduce they must invade living cells and commandeer their metabolic machinery.

• A virus is a subcellular particle consisting of a DNA or RNA genome surrounded by a protein coat, called a capsid.

  • In some viruses the capsid is surrounded by an outer envelope.

Classification of Viruses

• Viruses are classified based on host range, the types of host species they can infect; the type of nucleic acid they contain; and whether the nucleic acid is single-stranded or double- stranded.

  • Other factors used to classify viruses include size and shape, presence of an envelope, and method of transmission from host to host.

Viral Replication

• Bacteriophages, or phages, are complex viruses that infect bacteria.

  • They typically consist of a long molecule of dsDNA coiled within a polyhedral head.

• In a lytic cycle, the virus destroys the host cell.

  • The five steps in a lytic cycle are attachment to the host cell, penetration of viral nucleic acid into the host cell, replication of the viral nucleic acid, assembly of newly synthesized components into new viruses, and release from the host cell.

• The nucleic acid of some phages becomes integrated into the DNA of its bacterial host.

  • The phage is then called a prophage.

  • A temperate virus alternates between a lytic and a lysogenic cycle.

  • In a lysogenic cycle, the genome of a temperate virus is replicated along with replication of the host’s DNA without causing death of the host cell.

• Bacterial cells that carry prophages are lysogenic cells.

  • In lysogenic conversion bacterial cells containing certain temperate viruses exhibit new properties.

Viral Diseases

• Most plant viruses are ssRNA viruses that do not have envelopes.

  • Plant viruses penetrate plants through damaged cells and are also transmitted by infected seeds.

  • Viruses can be spread among plants by insect vectors.

  • Viruses spread through the plant via plasmodesmata.

• Viruses enter animal cells by membrane fusion or by endocytosis.

  • Diseases caused by DNA viruses include herpes, respiratory infections, and gastrointestinal disorders.

  • RNA viruses cause influenza, upper respiratory infections, AIDS, and some types of cancer.

• Retroviruses, such as HIV, use reverse transcriptase to transcribe their RNA genome into a DNA intermediate that becomes integrated into the host DNA.

  • Copies of the viral RNA are then synthesized.

Evolution of Viruses

• According to the progressive, or escape, hypothesis, viruses may have originated as mobile genetic elements that could have escaped and moved from one cell and entered another through damaged cell membranes.

  • Viruses may trace their origin to animal cells, plant cells, bacterial cells, or archaeal cells.

• The regressive, or reduction, hypothesis asserts that viruses are remnants of cellular organisms; they evolved from free-living, complex cellular ancestors.

  • As they became parasitic, the viruses gradually lost some of the genes necessary for protein synthesis.

  • Genes they no longer needed, like those for protein synthesis, were gradually lost through evolution.

• According to the virus-first hypothesis, viruses predate or coevolved with their current cellular hosts.

  • Viruses may have initially existed as self-replicating units.

  • As they evolved, viruses produced enzymes for the synthesis of membranes and cell walls, leading to the formation of the first cells.

• Evidence suggests that polydnaviruses evolved from viruses that infected wasps.

  • They have become mutualistic partners with their wasp hosts.

Subviral Agents

• Satellites, viroids, and prions are smaller than viruses.

  • Satellites reproduce only with the help of a helper virus.

• A viroid consists of a short strand of RNA with no protein coat.

  • Many viroids cause plant diseases.

  • A prion consists only of protein.

  • Prions cause fatal degenerative brain diseases, such as transmissible spongiform encephalopathies (TSEs).