Glossary P-S

Parasite: Any organism which in its natural habitat feeds and lives on or in a different organism.

Plasma: The clear fluid of blood and lymph in which cells are suspended; it also contains dissolved proteins.

Plasma Membrane: a delicate structure which encloses the cell, separating the contents of the cell from the surrounding environment.

Plasmids: Any extrachromosomal replicating unit or hereditary determinant, also called paragene.

Polio: A highly contagious infectious disease caused by a filterable virus and occurring most commonly in children ; in its acute form it involves the spinal cord causing paralysis.

Protease: Any enzyme which acts upon the peptide bonds of proteins and peptides; a protein-splitting or hydrolyzing enzyme.

Prevalence: The number of people with a specific condition in a given population.

Protein: Proteins are nitrogen-rich compounds that are formed from complex series of amino acids. They are fundamental to many of the processes of life, and are the fundamental building blocks for much of the substance of cellular and non-cellular life.

Provirus: A virus that has become and integral part of the host cell chromosome and is transmitted from one cell generation to another.

Rate of infection: The degree of speed or progression of an epidemic.

Receptor site: A structure of protein on the outside of the cell membrane - receptor sites allow cells to identify

each other and serve as attachment points for biological substances that produce a alterations in cell function. Viruses use receptor sites to identify a host cell and attach to it.

Retrovirus: A large group of RNA viruses.

Rhinovirus: The virus that causes the common cold.

Ribosome: One of the minute granules free in the cytoplasm or attached to the endoplasmic reticulum of a cell, containing a high concentration of RNA. Ribosomes are the cell's "factories" - they are the place where all protein synthesis occurs. Messenger RNA (mRNA) messages from the nucleus are translated by the ribosomes, which build the encoded proteins.

RNA: The abbreviation for ribonucleic acid, a form of genetic material. RNA is much less stable than DNA, and mutates frequently during its reproduction. For this reason, all life forms (with the exception some viruses) use it only as a temporary messenger molecule to carry information that is permanently stored in DNA.

RNA virus: A specialized set of viruses that use RNA, rather than DNA, for the storage of their genetic information.

"Sense" strand: Most genetic material, both DNA and RNA, appears as two chains or strands of nucleotides wound together into a double helix - the common picture of DNA. Each nucleotide - A, T, C and G - has an attractive opposite (A attracts T, C attracts G). As a result, one strand, the "sense" strand, contains the information (for

example, ATG-AAA) and the other strand, the "antisense" strand contains the opposite of this information (TAC-TTT - according to the pairing rules). Antisense RNA is the "antisense" half of a complete double RNA strand. RNA viruses consist of two types - "sense" RNA viruses, whose genetic material consists of the "sense" half of a complete strand, and "antisense" RNA viruses, which have the "antisense" half. Sense RNA viruses can have their genetic material read out directly by the ribosomes of their host cells - antisense RNA viruses must first copy themselves into a "sense" strand of RNA.

Serum hepatitis: A form of viral hepatitis transmitted by injection or exposure to human blood or blood products contaminated with hepatitis viruses.

Shingles: Infection of the ganglia of the posterior roots of the spinal nerves or the fifth cranial nerve by the varcella-zoster virus (shingles), which also causes chicken pox; it is marked by a painful eruption of vesicles usually on one side of the body along the course of one or more cutaneous nerves.

Subcelluar: Below the level of a complete cell. The various organelles and structures that make up a complete cell exist at the "subcellular" level.

Strain: A group or stock of microorganisms made up of descendents of a single isolation in a pure culture.

Synthesis: The formation of a new compound by putting together simpler compounds or elements.