VIRUSES - General Principles; Focus on Bacterial viruses Non-living obligate intracellular parasites OR the simplest living things there are (depends upon your definition of life )
Viruses They ALWAYS contain nucleic acid (DNA or RNA) and a protein coat (the capsid ), and SOMETIMES have an envelope made of lipids, carbohydrates, and proteins (basically similar in structure to an enhanced plasma membrane, but with a DIFFERENT FUNCTION!) Viruses are very specific regarding the types of cells they can infect; they generally infect certain types of cells in a particular species. This is their HOST RANGE. Viruses can be seen by electron microscopy ONLY and range from 20 to 1,000 nm in length.
Viral Structure A VIRION is a mature, infective particle consisting of nucleic acid and protein coat (and envelope depending upon the species). Viruses can have many arrangements of nucleic acids as their genomes. They may have either RNA or DNA (never both), and the nucleic acid may be single or doublestranded, and linear or circular. The protein coat is referred to as the CAPSID, which consists of subunits called capsomeres. If the virus has an envelope, spikes are usually present (carbohydrate-lipid complexes whose function we will discuss later.)
General Morphology HELICAL VIRUSES (capsomeres are assembled in a helical configuration) usually resemble long rods, and their capsids are hollow cylinders surrounding the nucleic acid. POLYHEDRAL viruses are many-sided; usually the capsid is an icosahedron (20 triangular faces). ENVELOPED viruses may be helical or polyhedral, and acquire their envelope by taking part of the plasma membrane of the host cell as they bud from the cell (leave by exocytosis). If a virus lacks an envelope, it is said to be naked. Enveloped viruses are usually animal viruses. COMPLEX viruses have a polyhedral head with a complex tail attached - bacteriophages are complex viruses.
Taxonomy of viruses Based on Type of nucleic acid, life-cycle strategy, and morphology. Viruses are not assigned a place in the normal overall classification system; however, they are generally grouped into families, genera, and species. (Exactly how depends upon who you ask!) A viral species is a group of viruses sharing the same genetic information and ecological niche. Viral species are not given binomial names, they are given normal names (like influenza virus, HIV) that do not have to be underlined or italicized.
Taxonomy Examples RNA VIRUSES Species: HIV (Human Immunodeficiency Virus) Family: Retroviridae; Genus: Lentivirus Species: Poliovirus Family: Picornaviridae; Genus: Enteroviridae Species: Rabies Virus Family: Rhabdoviridae; Genus: Lyssavirus DNA VIRUSES Species: HPV (Human papillomavirus) Family: Papovaviridae; Genus: Papillomavirus Species: HSV (Herpes simplex virus) Family: Herpesviridae; Genus: Simplexvirus
Viral cultures To grow a virus in the lab, its host cells must be present. It is easiest to grow bacteriophages; phages are incubated on a lawn of bacteria and plaques develop where they are multiplying. Animal viruses may require a living host in order to grow; otherwise, cell cultures of host cells can be used.
Viral replication strategies: life cycles Viruses do not have enzymes for key metabolic reactions (e.g. protein synthesis or energy production). Therefore, they must take over a host cell, and use its cellular machinery to produce viral enzymes and other components. Viral infection turns a cell into a factory for producing more viruses.
Focus on Bacteriophages: Viruses that infect bacteria Discovered in 1915 Bacteriophage means Bacteria-eater Term is often shortened to phage Almost all carry their genomes as double-stranded DNA. Almost always have a complex morphology. Most widely studied are the viruses of E. coli, including the T-even phages and phage lambda.
Bacteriophages: Lytic and Lysogenic Cycles LYTIC CYCLE: A phage causes the lysis and death of its host bacterium as it replicates. T-even phages, (T2 and T4 are widely studied), use the lytic cycle. In the LYSOGENIC CYCLE, a phage incorporates its nucleic acid into the host cell chromosome and remains dormant for a period of time. Phage lambda is a popular example of this type. These are called TEMPERATE phages.
Stages in the lytic cycle Attachment - Sites on the bacteriophage tail fibers attach to receptor sites on the bacterium. Penetration - The tail sheath contracts to force the tail core through the cell wall, and phage DNA enters the cell (the capsid stays outside!!) Biosynthesis - transcription, translation, and replication of viral DNA occurs. Maturation - New Phage DNA and capsids are assembled into virions. Release - Phage lysozyme destroys the cell wall, the cell bursts, and the new virions are released.
Stages in the lysogenic cycle Attachment/ Penetration - same as in lytic cycle Viral DNA recombines with the bacterial chromosome to form a PROPHAGE. The prophage replicates with the bacterial DNA, and will continue to do so until something (an adverse environment for example) triggers it to excise itself; it then continues with the steps of the lytic cycle. Thus, the steps specific to the lysogenic cycle can be viewed as additional steps added into the middle of the lytic cycle.