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Absorption spectrum

The relative strength of light absorption by a substance as a function of the wavelength of the radiation.

Amino acids

The molecular subunits of proteins.

Amino : imino

A tautomeric equilibrium between amino ( - NH2 ) and imino ( = NH) forms.


A substance that reacts with antibodies or cells of the immune system.

Atomic force microscope

A microscope that scans the topography of surfaces on a near-atomic scale with an extremely sensitive probe.


Viruses that attack bacterial cells. T2, T4, and f X are specific varieties of bacteriophage.


The science involving the application of physical principles and methods to the study of the structures and processes of living organisms.

Cellular ultrastructure

Assemblies of varied submicroscopic filaments forming scaffolds and shaping substructure in cells of higher organisms, called ultrastructure.


A small intracellular body containing chlorophyll that plays an essential role in photosynthesis.


Threadlike structures, each of which carries a linear array of genes.


A set of genetically identical individual organisms. In man, identical twins are a clone of two.



A unit of mass appropriate for individual atoms or molecules, defined as one twelfth of the mass of an atom of the carbon isotope 12; equivalent to 1.66 × 10-24 grams.

Density gradient centrifugation

A technique of centrifugation in which the centrifugal field is employed to establish a gradient of density in a solvent; particles of different density then separate in the centrifuge, each settling in the gradient at a point equal to its own density.

Density label

A means of labeling one group of molecules by altering its density relative to another similar group; often used in conjunction with density gradient centrifugation.

Deoxyribonucleic acids

See Nucleic acids.

Developmental biology

That part of biology which concerns the progression of an organism from the fertilized egg to the adult form.

DNA ligase

An enzyme that can link together DNA chains.


See Nucleic acids.

DNA polymerase

An enzyme that catalyzes the linking of nucleotides into a DNA chain.

DNA renaturation

The process by which two strands of a DNA double helix, having been separated in solution, selectively rejoin each other and pair up over time.

DNA repair

Mechanisms to effect the repair of chemical or mechanical damage to a cell's DNA; all cells have a variety of such processes.


A specific family of fruit flies used extensively in genetic research.

Electromagnetic radiation

Radiation composed of electromagnetic waves or, alternatively, photons; the electromagnetic spectrum comprises the whole range of electromagnetic wavelengths or photon energies.


A protein that serves as a catalyst for a particular biochemical reaction.


A cell with a definitive nucleus and mitochondria.

Exon : intron

Alternating tracts comprising the messenger RNA molecules in higher organisms as initially transcribed from DNA. The introns are subsequently deleted and the exons spliced together to form the functional messenger RNA molecules.


The functional unit of inheritance.

Genetic code

The code linking nucleotide sequences in nucleic acids with amino acid sequences in proteins.

Genetic map

A graphic presentation of the linear order and spacing of genes in a chromosome.



The complete genetic endowment of a member of a species.

Heavy water

Water in which all of the normal hydrogen atoms (atoms of the hydrogen isotope 1) have been replaced by deuterium atoms (atoms of the hydrogen isotope 2).

Infrared spectroscopy

See Spectroscopy.

Ionizing radiation

Radiation composed of particles or photons of sufficient energy to remove electrons from the atoms or molecules of the material through which they pass, leaving a trail of charged atoms or molecules, that is, a trail of ions.


One of two or more chemical substances having the same atomic composition but differing in structure.


One of two or more atoms of an element, differing in mass. Nonradioactive isotopes are stable. Radioactive isotopes are unstable and decay by radioactivity.

Keto : enol

A tautomeric equilibrium between keto ( = O) and enol ( - OH) forms.

Light scattering

The absorption and instantaneous reemission in varied directions of light without change of wavelength.

Messenger RNA

An RNA molecule the nucleotide sequence of which parallels that of one strand of a portion of a double-helical DNA molecule and that programs protein synthesis on a ribosome.


An instrument to measure and compare the transmission of light by microscopic areas of a specimen.


A unit of length convenient for cellular dimensions equal to one millionth of a meter.


Electromagnetic radiation with wavelengths generally between 0.3 and 30 centimeters.


Small bodies found in all eukaryotic cells that play an essential role in the provision of energy.


Division of a cellular nucleus into two with exact duplication and separation of the chromosomes.

Molecular biology

That part of biology which attempts to interpret biological events in terms of the physico-chemical properties of the molecules (especially the macromolecules) in a cell.


A unit of length appropriate to molecular or crystalline dimensions; equal to one billionth of a meter.


Nucleic acids

Large linear molecules composed of nucleotide subunits joined by phosphate linkages between the sugar moieties. A string of nucleotides is called an oligonucleotide or polynucleotide . There are two general types:


In deoxyribonucleic acids (DNA), the sugar moiety of the nucleotide subunits is always deoxyribose. DNA molecules are composed of four principal nucleotide subunits: deoxyadenylic acid, deoxyguanylic acid, deoxycytidylic acid, and thymidylic acid. In some DNAs, a small amount of a fifth nucleotide, 5-methyldeoxcytidylic acid, replaces part of the deoxycytidylic acid.


In cells, DNA is invariably present as a double-helical structure, composed of two intertwined polynucleotide strands, in a paired fashion. DNA in some viruses is single-stranded.


Genes are composed of DNA, except in certain viruses which use RNA.


In ribonucleic acid (RNA), the sugar moiety of the nucleotide subunits is always ribose. RNA molecules are composed of four varieties of nucleotide subunits: adenylic acid, guanylic acid, cytidylic acid, and uridylic acid.


In cells, RNA is always single-stranded. Double-stranded RNA is found only in certain viruses and RNA virus infections


A subunit of nucleic acid. Each nucleotide is composed of an organic ring-shaped molecule (either a pyrimidine-type ring or a purine-type ring) attached to a five-carbon sugar attached in turn to a phosphate group.


See Nucleic acids.


A phenomenon in which some of the effects of ultraviolet radiation on cells can subsequently be reversed by exposure to visible light.


A small extrachromosomal genetic element in cells.


See Nucleic acids.


High molecular weight molecules composed of amino acids joined primarily by peptide linkages.


A celestial radio source emitting short bursts of radio emission at regular intervals.


A nine-membered double ring molecule composed of carbon and nitrogen atoms with varied side groups; the purines found in nucleic acids are adenine and guanine.



A six-membered ring composed of carbon and nitrogen atoms with varied side groups; the pyrimidines found in nucleic acids are cytosine, 5-methylcytosine, uracil, and thymine


A basic particle of physics with a charge of one or two thirds of the electron charge; presumed component of nucleons.


A quasi-stellar astronomical object of great brilliance and radio intensity, often with large red-shift.

Raman spectroscopy

See Spectroscopy.

Recombinant DNA

DNA produced by splicing together DNA molecules of different origin.

Recombinant, genetic

An organism in which genes from two sources have been recombined into one chromosome.

Refractive index

The ratio of the velocity of light in a vacuum to that in a medium; the difference in the refractive indices of two media is determinant of the amount of light reflected at their interface.

Restriction enzyme

An enzyme that can cleave DNA at a specific nucleotide sequence.

Ribonucleic acid

See Nucleic acids.

Ribosomal RNA

RNA molecules that form an essential part of the structure of the ribosomes.


Small complex particles, found in all cells, on which protein synthesis takes place.


See Nucleic acids.

Saturated hydrocarbon

A compound composed of carbon and hydrogen atoms with all carbon bonds filled.

Semiconservative replication

A term applied to replication of double-stranded DNA molecules, in which the two strands of the parental DNA remain intact but are separated, each passing to one of the two daughter DNA molecules.


The science of the production, measurement, and interpretation of electromagnetic radiation as a function of its wavelength. Infrared spectroscopy is concerned with radiation in the infrared region, typically between 0.7 and 25 microns in wavelength; ultraviolet spectroscopy is concerned with radiation in the ultraviolet region, typically between 0.2 and 0.4 microns in wavelength; Raman spectroscopy is concerned with radiation reemitted from a transparent medium with changed wavelength, indicative of molecular energy levels and structure.



The process of forming a messenger RNA molecule that conveys the information content of a portion of a DNA molecule.


A device that converts an input signal into an output signal of a different form, for example, a microphone.

Transfer RNA

Small RNA molecules that play an essential role in the translation via the genetic code from nucleotide sequence in nucleic acid to amino acid sequence in protein.

Transforming principle

A substance that effects specific genetic transformation in bacterial cells—known to be DNA.


A high-speed centrifuge capable of developing centrifugal force in excess of 100,000 times gravity.

Ultraviolet spectroscopy

See Spectroscopy.

X-ray diffraction

The scattering of X-rays by crystals with resultant interference effects dependent on the spacing and atomic composition of the components of the crystals.


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