Alpha-helix -- A short, helical arrangement of a polypeptide chain that is a common secondary structure in proteins. In the alpha-helix, the polypeptide folds by twisting into a right handed screw so that all the amino acids can form hydrogen bonds with each other.

Base pair -- Either of the two pairs--guanine and cytosine, adenine and thymine--of purine-pyrimidine bases joined by hydrogen bonds that make up DNA. In RNA, uracil replaces thymine.

Crystallography, x-ray -- An analytical technique in which x-ray diffraction is used to obtain information about the identity or structure of a crystalline substance.

Culture [bacterial] -- To grow bacteria in a special medium, such as agar, which allows for their rapid reproduction. The term can also refer to the colony of bacteria resulting from this process, or to the laboratory cultivation of living tissue cells.

DNA, or deoxyribonucleic acid -- The primary genetic material of all cellular organisms and the DNA viruses. Located predominantly in cell nuclei, it is composed of two chains of nucleotides--deoxyribose and phosphate backbones with side chains of purine (adenine or guanine) or pyrimidine (cytosine and thymine) bases projecting inward. Hydrogen bonds link adenine to guanine, and cytosine to thymine. The two linked strands are twisted in a double helix.

Nucleic acids -- A large molecule composed of nucleotide subunits.

Potexvirus -- A genus of plant viruses that cause mosaic and ringspot symptoms, with transmission occurring mechanically.

RNA, or ribonucleic acid -- A single-stranded nucleic acid found in the cell nucleus and cytoplasm, which plays a key role in protein synthesis. (It also constitutes the genetic material of the RNA viruses.) It is similar to DNA but has ribose sugar, rather than deoxyribose sugar, and uracil, rather than thymine, as one of the pyrimidine bases. There are several classes of RNA molecules, including messenger RNA, transfer RNA, and ribosomal RNA, each serving a different purpose in the cell.

Spectroscopy -- The production and investigation of light in different parts of a spectrum; also the process or technique of using a spectroscope in such studies.

Tobacco mosaic virus, or TMV -- Member of a group of plant viruses with rigid, rod-shaped particles containing one molecule of linear RNA, easily sap-transmitted and transmitted in nature by contact and in seed. Frequently used in molecular biology and virology studies for its nonpathogenicity to animals and ease of growth.

Unit cell -- The simplest polyhedron that embodies all the structural characteristics of a particular crystal, and by indefinite repetition makes up its lattice.

Virus -- Any one of a group of sub-microscopic infectious agents characterized by a lack of independent metabolism and by the ability to replicate only within living host cells. Whether regarded as extremely simple microorganisms or extremely complex molecules, like living organisms they have the ability to pass genes through multiple generations and to mutate. The individual virus particle consists of a strand or strands of nucleic acid (which may be either DNA or RNA) and a protein shell. Viruses are classified into three main subgroups based on their host, bacterial viruses, plant viruses, and animal viruses. Viruses are then further classified by their origin, means of transmission, or illnesses they produce.

X-ray diffraction -- A scattering of x-rays by the atoms of a crystal that produces an interference effect so that the diffraction pattern gives information on the structure of the crystal or the identity of the crystalline substance.