Properties of viruses
Viruses are obligate intracellular parasites. Probably there are no cells in nature that escape infection by one or more kinds of viruses. Viruses that infect bacteria are called bacteriophages.
Outside the cell, they consist of particles called virions. Virions range in size from as small as the poliovirus, which is 30 nm in diameter (about the size of a ribosome) to as large as the vaccinia virus which, at 230 nm, is larger than some bacteria. The virion consists of an outer shell, the capsid, made of protein. The capsid is responsible for protecting the contents of the core establishing what kind of cell the virion can attach to infecting that cell. Some viruses contain other ingredients (e.g., lipids, carbohydrates), but these are derived from their host cells. an interior core containing the genome; either DNA or RNA The genes are few in number (3 - 100 depending on the species). They encode those proteins needed for viral reproduction that the host cell will not supply. Often, one or more proteins (enzymes) needed to start the process of reproduction within the host cell.
The virion attaches to the surface of the host cell (usually binding to a specific cell surface molecule that accounts for the specificity of the infection). Example: HIV-1, the cause of AIDS, binds to the chemokine receptor CCR5 found on human lymphocytes and macrophages. Once inside the cell, the virions are uncoated. Viral genes begin to expressed leading to the synthesis of proteins needed for replication of the genome synthesis of new proteins to make new capsids and cores.
Either DNA or RNA, never both.
- DNA viruses can be further divided into those that have their genes on a double-stranded DNA molecule (dsDNA). Example:
- smallpox - those that have their genes on a molecule of single-stranded DNA (ssDNA)
- Adeno-Associated Virus
- RNA viruses occur in four distinct groups:
- Those with a genome that consists of single-stranded antisense RNA; that is, RNA that is the complement of the message sense. This is also called negative-stranded RNA. Examples: measles, Ebola
- Those with a genome that consists of single-stranded sense RNA; that is, the RNA has message sense (can act as a messenger RNA - mRNA). This is also called positive-stranded RNA. Examples: poliovirus
- Those with a genome made of several pieces of double-stranded RNA. Example: reovirus.
- Retroviruses - their RNA (also single-stranded) is copied by reverse transcriptase into a DNA genome within the host cell. Example: FIV