A: abbreviation for adenine.
A form of DNA: the ‘crystalline’ form with a low water content, characterised by Rosalind Franklin.
Å: Ångström, atomic level unit of length. 1 Å = 10-10 metres, or one ten-millionth of a millimetre.
Adenine: a purine base in DNA.
Paired with thymine by hydrogen bonding in the double helix.
Alleles: the alternative forms of a particular gene, occupying the same location on the chromosome.
Alpha-helix: the spiral configuration commonly adopted by stretches of amino acids in proteins. Discovered by Linus Pauling in 1948.
Amino acids: a series of twenty organic compounds with common structural features, which are the building-blocks of proteins.
B form of DNA: the helical form with a higher water content, characterised by Rosalind Franklin.
Backbone of DNA: the spiral strands of phosphate alternating with deoxyribose which form the outside of the double helix.
Bacteriophage (also ‘phage’): viruses which prey on bacteria by injecting them with their DNA.
Base: nitrogen-containing organic molecules with a flat structure comprising either one ring (pyrimidines) or two rings (purines).
Base-pairing: pulled together by hydrogen bonding, the attraction of adenine for thymine, and of cytosine for guanine. Zips together the two DNA strands of the double helix.
Bausteine: German for ‘building-blocks’. The simple compounds that make up complicated biological molecules. In the case of DNA: phosphate, the sugar deoxyribose, and the bases adenine, guanine, cytosine and thymine.
Biophysics: see Molecular biology.
C: abbreviation for cytosine.
Capsule: the protective outer coating of pneumococci, containing specific antigens made of carbohydrate.
Cell, unit: basic building-block of a crystal structure.
Chargaff’s Rules: formulated by Erwin Chargaff. (1) The base composition of DNA is constant for all tissues of a given species but varies between species. (2) In DNA from all sources, the contents of adenine = thymine, and of cytosine = guanine (the consequence of base-pairing).
Chromatids: the two thread-like strands into which each chromosome divides longitudinally during cell division. This is after DNA replication; each chromatid contains a double helix of DNA.
Chromatin: the heavily stained substance of chromosomes, corresponding to DNA.
Chromosin: DNA with associated proteins, extracted from cell nuclei by Alfred Mirsky.
Chromosomes: thread-like structures in the nucleus, consisting of DNA and proteins and carrying the genes.
‘Crystalline’ DNA: the A form, with regular distortions introduced into the helical structure by the removal of water.
Cytidine: the nucleoside consisting of cytosine linked to desoxyribose.
Cytochemistry: the microscopical study of tissues and cells, using stains to highlight particular cellular components and substances, such as DNA.
Cytosine: a pyrimidine base in DNA. Paired with guanine by hydrogen bonding in the double helix.
Deoxyribose: the pentose sugar found in DNA and which gives it its name. Interlinked with phosphate groups, forms the spiral backbone of the double helix.
DNA: deoxyribonucleic acid (previously ‘desoxyribonucleic acid’).
DNase (deoxyribonuclease): enzyme which specifically breaks down DNA.
Dominant: an allele which masks the effect of another (the ‘recessive’).
Drosophila: the fruit fly, in which Thomas Hunt Morgan and others induced and mapped mutations, and defined their inheritance.
Eno: one of two interchangeable forms in which organic molecules such as the bases can occur. The other form, ‘keto’, prevails in vivo.
Eugenics: the belief that the genetic quality of a human population can be improved; and practical attempts to achieve this aim.
G: abbreviation for guanine.
Gene: a unit of heredity, carried on a chromosome and transferred from parent to offspring. Corresponds to a distinct sequence of DNA.
Genetic code: the sequence of DNA nucleotides which carry and transmit genetic information. Based on specific ‘triplets’, sequences of three consecutive nucleotides.
Genome: the complete set of genes, conveying the totality of genetic information, of a living organism.
Genotype: the genetic makeup of a particular inherited characteristic of a living organism, e.g. TT, Tt or tt for the height of pea plants. C.f. phenotype.
Guanine: a purine base in DNA. Paired with cytosine by hydrogen bonding in the double helix.
Heterozygote: possessing two different alleles of a particular gene, e.g. Tt for the height of pea plants. C.f. homozygote.
Histones: basic (alkaline) proteins in the nucleus that bind to and organise DNA.
Homozygote: possessing the same two alleles of a particular gene, e.g. TT or tt for height in pea plants.
Hydrogen bonding: attractive force which pulls together hydrogen and another atom in the same or different molecules.
Keto: one of two interchangeable forms in which organic molecules such as the bases can occur, and the form that prevails in vivo. Cf. Eno.
Meiosis: the special process of cell division in the formation of eggs and spermatozoa, resulting in four daughter cells each with half the normal number of chromosomes. Cf. Mitosis.
Mitosis: the process of cell division, yielding two daughter cells with a complete set of chromosomes. Cf. Meiosis.
Molecular biology: the interface between biology and physics, covering the structure and function of the complex cellular molecules essential for life. Synonymous with ‘biophysics’.
Mutation: a change in the DNA sequence of a gene, either occurring spontaneously or induced artificially with X-rays, chemicals or other means.
Nucleic acid: term that replaced ‘nuclein’ and originally embraced both DNA and RNA.
Nuclein: original name for DNA, isolated from pus cells by Friedrich Miescher in 1868.
Nucleoside: a nucleotide without the phosphate, i.e. deoxyribose joined to one of the bases (adenine, guanine, cytosine or thymine).
Nucleotide: the basic structural unit of DNA, consisting of phosphate joined to deoxyribose (forming part of the outer backbone), and an inwardly projecting base (one of adenine, guanine, cytosine or thymine).
Nucleus: the membrane-bound organelle which contains the chromosomes and controls all the activities of the cell by directing the synthesis of RNA and thus of proteins.
Paper chromatography: method for separating and measuring amounts of particular substances, such as the bases in samples of DNA.
Pentoses: simple sugars with a five-cornered ring structure. They include ribose and deoxyribose, found in RNA and DNA, respectively.
Phagocytosis: the engulfing of bacteria and other tiny particles, notably by white blood cells.
Phenotype: an observable physical characteristic of a living organism, e.g. tallness or shortness in pea plants.
Pneumococcus: bacterium which causes lobar pneumonia. The subject of Griffith’s ‘transformation’ experiments which led to the first evidence that DNA is the genetic material.
Protamines: basic (alkaline) proteins, tightly bound to DNA in the heads of spermatozoa. They replace the histones in the later stages of sperm formation.
Purines: two-ringed bases, such as adenine and guanine, occurring in both DNA and RNA.
Recessive: an allele which is masked by another (the ‘dominant’).
Ribose: a pentose sugar found in RNA, and which gives it its name. RNA: ribonucleic acid.
RNase (ribonuclease): enzyme which specifically breaks down RNA.
Sepia: the cuttlefish, which provided sperm for studies of DNA in living cells.
Sex-linked: a gene which is associated with one sex or the other, e.g. white eye colour, only in male fruit flies.
Sodium thymonucleate: alternative name for DNA.
SSS (Soluble Specific Substance): the carbohydrate antigen in the capsule which gives each pneumococcus its immunological identity.
Tetranucleotide: a hypothetical structure for DNA, proposing a short, boring molecule that contains just one of each of the bases and is therefore incapable of carrying complex information such as heredity.
Thymine: a pyrimidine base in DNA. Paired with adenine by hydrogen bonding in the double helix.
Thymus: organ in the neck or thorax of mammals that is a gastronomic delicacy as well as a rich source of DNA.
Thymus nucleic acid (also ‘thymonucleic acid’): old name for DNA.
Transformation (of pneumococci): changing the genetic characteristics of a bacterium with extracts of dead bacteria, first achieved by Fred Griffith in 1928.
Transforming principle (also, ‘transforming factor’): the chemical substance mediating transformation in bacteria, which Avery’s team showed in 1944 to be DNA.
Type, of pneumococcus: classification of pneumococci based on the immune reactions of the antigens in the capsule.
Uracil: a pyrimidine base not found in DNA but in RNA, in which it replaces thymine. Paired with adenine by hydrogen bonding.
X-ray crystallography: X-ray diffraction used to elucidate the structure of molecules that can form crystals.
X-ray diffraction: the scattering of X-rays by regularly spaced obstructions, exploited to define the structures of mineral crystals, virus particles and biological fibres such as DNA.
Yeast nucleic acid: old name for RNA.