DNA molecules. Base stacking is caused by hydro- phobic interactions between purine and pyrimidine bases, and results in maximum hydrogen bonding between complementary base pairs. basic amino acids amino acids that have a net positive charge at neutral pH. Lysine and arginine bear positively charged side chains under most con- ditions. basic dye any organic cation that binds to and stains negatively charged macromolecules, such as nucleic acids.
See azure B. basic number the lowest haploid chromosome number in a polyploid series (symbolized by x). The monoploid number. basikaryotype the karyotype corresponding to the monoploid. basophilic designating an acidic compound that readily stains with basic dyes.
Basques a human population living in the western part of the Pyrenees mountains. About 900,000 live in Spain and another 80,000 in France. Their lan- guage is unrelated to any Indo-European language, and they are believed to be the direct descendents of tribes dating back to paleolithic times. See cystic fibrosis, Rh factor. Batesian mimicry a form of mimicry (q.v.) de- scribed by the British naturalist Henry Walter Bates in 1862. B cell B lymphocyte.
See lymphocyte. B chromosomes supernumerary chromosomes that are not duplicates of any of the members of the basic complement of the normal or “A” chromosomes. B chromosomes are devoid of structural genes. During meiosis, B chromosomes never pair with A chromo- somes, and Bs show an irregular and non-Mendelian pattern of inheritance. B chromosomes are extreme- ly widespread among flowering plants and have been extensively studied in rye and maize. B chromo- somes are believed to perpetuate and spread them- selves in populations because they replicate faster than A chromosomes. See Appendix C, 1928, Ran- dolph.
beads on a string the concept that genes in a chromosome resembled beads on a string and that recombination involved the string and not the beads. S. Benzer disproved this idea by showing that re- combination did occur within genes. In fact it could occur between any adjacent nucleotides within a gene. See rII, T phages. Becker muscular dystrophy (BMD) a less severe form of muscular dystrophy (q.v.). It was described in 1955 by Peter Emil Becker and shown to develop later in life and to progress more slowly than the common form. BMD constitutes about 10 percent of all cases of X-linked muscular dystrophy.
becquerel a unit of activity of a radioactive sub- stance. 1 Bq = 1 disintegration per second. bee dances circling and waggling movements (Rundtanzen and Schwanzeltanzen) performed by worker bees to give their hive mates information as to the location of a new source of food. beef thymus the gland from which DNA was iso- lated for early structural studies. See Appendix C, 1951, Wilkins and Gosling; 1952, Franklin and Gos- ling; nucleic acid. bees See Apis mellifera, African bees. Beer-Lambert law the law that the absorption of light by a solution is a function of the concentration of solute.
It yields a relation commonly used in pho- tometry: E = log10I0/I = kcb, where E = optical den- sity; I0 = the intensity of the incident monochro- matic light; I = the intensity of the transmitted light; k=a constant determined by the solvent, wave- length, and temperature; c = the concentration of absorbing material in moles per liter; and b = the thickness in centimeters of the solution traversed by the light. The Beer-Lambert law is based on discov- eries by the German mathematician, chemist, and physicist, August Beer (1825-1863) and the French mathematician, physicist, and astronomer, Johann Heinrich Lambert (1728-1777).
behavioral isolation a prezygotic (premating) iso- lating mechanism in which two allopatric species re- fuse to mate because of differences in courtship be- havior; ethological isolation. behavior genetics a branch of genetics that con- cerns the inheritance of forms of behavior such as courtship displays, nest building, etc., in lower ani- mals and intelligence and personality traits in hu- mans. Many traits that are of interest to behavioral scientists are quantitative characters (q.v.). Bence-Jones proteins proteins identified by the English physician Henry Bence-Jones in 1847. B-J proteins are excreted in the urine of patients suffer- ing from malignancies of antibody-secreting cells (plasmacytomas or myelomas). B-J proteins consist of dimers of immunoglobulin chains.
The proteins are synthesized by clones of identical cells, and there- fore each patient produces identical peptides in suf- ficient quantities for the amino acid sequences to be determined. B-J proteins played a critical role in in- vestigations of the chemical structure of the immu- noglobulins (q.v.). benign See neoplasm. benign subtertian malaria See malaria. benzene the simplest of the aromatic organic com- pounds. beta carotene See carotenoids, retinene.
beta chain one of the two polypeptides found in adult hemoglobin. beta galactosidase an enzyme that breaks lactose into glucose and galactose. In E. coli, the enzyme is a tetramer of about 500,000 daltons encoded by the lac Z gene. See homomeric protein, operon, lac op- eron, lactose. beta lactamase See penicillinases. beta-2 microglobulin See MHC molecules. beta particle a high-energy electron emitted from an atomic nucleus undergoing radioactive decay. beta pleated sheet one of two common, regularly repeating structures seen in proteins (compare with alpha helix).
Each of the component polypeptide chains in the beta pleated sheet is fully extended, and the sheets are stabilized by hydrogen bonds be- tween the NH and CO groups of the same or differ- ent polypeptide chains. Adjacent chains can run in the same or in opposite directions (parallel versus antiparallel beta sheets).
For example, silk fibroin is composed of antiparallel beta pleated sheets. See Ap- pendix C, 1951, Pauling and Corey; silk. BEV the acronym for baculovirus expression vec- tor. See baculoviruses. bicoid (bcd) a gene that is essential for normal axis formation in Drosophila. The transcription of bcd takes place in the nurse cells (q.v.), and these are of maternal genotype.
This mRNA is transported in a cytoplasmic stream that is pumped by the nurse cells (q.v.) into the oocyte, and here it is localized at the anterior pole. The sequence required for this local- ization resides in the trailer portion of the mRNA. This bcd mRNA is not translated until after the egg is fertilized. The product is a homeodomain protein that functions as a morphogen for anterior struc- tures. Mothers who lack a functional allele of bcd produce embryos with aberrant or missing head and thoracic structures and anterior abdominal seg- ments.
Transplantation of anterior polar cytoplasm from wild type embryos or injection of purified bcd mRNA into bcd-deficient embryos can rescue (q.v.) the mutant phenotype. bcd protein is distributed in an anterioposterior concentration gradient in the zy- gote (q.v.) and controls the expression of several zy- gotic genes, depending on its local concentration. The bcd gene is a member of the anterior class of maternal polarity genes, and it lies within the Anten- napedia (q.v.) complex.
See Appendix C, 1988, Mac- donald and Struhl; 1988, 1989, Driever and Nu¨ssl- ein-Volhard; cytoplasmic localization, cytoplasmic determinants, goosecoid, homeobox, hunchback, ma- ternal polarity mutants, nanos (nos), trailer sequence, zygotic gene. bidirectional genes a pair of open reading frames (q.v.), one on the plus strand and the other on the minus strand of the same DNA double helix and overlapping to a certain degree. Compare with over- lapping genes. bidirectional replication a mechanism of DNA replication involving two replication forks moving in opposite directions away from the same origin. biennial designating a plant that requires two years to complete its life cycle, from seed germina- tion to seed production and death. The plant devel- ops vegetatively during the first growing season and flowers in the second.
bifunctional vector See shuttle vector. bilateral symmetry a form of symmetry in which the body can be divided by a longitudinal plane into two parts that are mirror images of each other. Bilateria animals with bilateral symmetry. See classification. bilharziasis See schistosomiasis. bilirubin an orange pigment formed as a break- down product of the heme component of hemepro-
teins, especially the hemoglobin released during the normal destruction of erythrocytes by the reticulo- endothelial system. Bilirubin released into the circu- lation by the reticuloendothelial system is taken up by the liver and excreted into the bile. The accumu- lation of bilirubin in plasma and tissues results in jaundice. See Crigler-Najjar syndrome.
bimodal population a population in which the measurements of a given character are clustered around two values. binary fission an amitotic, asexual division pro- cess by which a parent prokaryote cell splits trans- versely into daughter cells of approximately equal size. Compare with septal fission. Binet-Simon classification See intelligence quo- tient classification. binomial distribution a probability function so named because the probabilities that an event will or will not occur, n, n − 1, n − 2, . . . , 0 times are given by the successive coefficients in the binomial expansion (a + b)n. Since a and b are the probabili- ties of occurrence and non-occurrence, respectively, their sum equals 1. The coefficients in a given bino- mial expansion can be found by referring to Pascal’s pyramid (see illustration on page 49). Here each ho- rizontal row consists of the coefficients in question for consecutive values of n. The expansions for n
equaling 1, 2, 3, 4, or 5 are shown below:
(a + b)1 = 1a + 1b
(a + b)2 = 1a2+2ab + 1b2
(a + b)3 = 1a3 + 3a2b + 3ab2 + 1b3
(a + b)4 = 1a4 + 4a3b + 6a2b2 + 4ab3 + 1b4
(a + b)5 = 1a5 + 5a4b + 10a3b2 + 10a2b3 + 5ab4 + 1b5
Note that each term of the triangle is obtained by adding together the numbers to the immediate left and right on the line above. Such a binomial distri- bution can be used for calculating the frequency of families in which a certain proportion of individuals show a given phenotype.
If we ask, for example, what will be the distribution of girls and boys in families numbering four children and let the fre- quency of boys = a, and that of girls = b; then using the formula (a + b)4, we conclude that the distribu- tion would be 1/16 all boys; 4/16 3 boys and 1 girl; 6/16 2 boys and 2 girls; 4/16 1 boy and 3 girls; and 1/16 all girls. Pascal’s pyramid is named after its in- ventor, the French mathematician Blaise Pascal (1623-1662). binomial nomenclature See Linnean system of bi- nomial nomenclature.