and plants that can convert visible light into chemi- cal energy. In addition, chemoautotrophic bacteria can produce organic molecules from CO2 in the ab- sence of light. They use as energy sources for biosyn- thesis the oxidation of inorganic compounds such as molecular hydrogen, ammonia, and hydrogen sul- fide. Contrast with heterotrophs. autozygote an individual homozygous at a given locus whose two homologous genes are identical by descent, in that both are derived from the same gene in a common ancestor. See allozygote. auxesis growth in size by increase in cell volume without cell division. auxins a family of plant hormones that promote longitudinal growth and cell division. Natural auxins are indole derivatives biosynthesized from trypto- phan.
The most common auxin is indole acetic acid (q.v.), which is synthesized in all plants. See anti- auxin. auxocyte a cell whose nucleus is destined to enter meiotic prophase; a primary oocyte, primary sperm- atocyte, megasporocyte, or microsporocyte. auxotroph a mutant microorganism that can be grown only upon minimal medium that has been supplemented with growth factors not required by wild-type strains. Avena the genus to which the various species of oats belong.
The most commonly cultivated oat is A. sativa. Avena test a technique using the curvature of Avena coleoptiles as a bioassay for auxins. average life in nuclear physics, the average of the individual lives of all the atoms of a particular radio- active substance. It is 1.443 times the radioactive half-life (q.v.). avian leukosis See leukemia. avian myeloblastosis virus an oncogenic RNA virus. avidity the total combining power of an antibody with an antigen. It involves both the affinity of each binding site and the number of binding sites per an- tibody and antigen molecule. Compare with affinity.
Avogadro’s number the number of atoms (6.025 × 1023) in one gram atomic weight of an element; also the number of molecules in the gram molecular weight of a compound. awn a stiff, bristlelike appendage occurring on the flowering glumes of grasses and cereals. axenic growth of organisms of a given species in the complete absence of members of any other species. axolotl a salamander, Ambystoma mexicanum, that exhibits neoteny (q.v.).
It does not metamorphose, but mates and reproduces while a “juvenile,” and never leaves the water. axon the long process of a nerve cell, normally conducting impulses away from the nerve cell body. axoneme a shaft of microtubules extending the length of a cilium, flagellum, or pseudopod of a eu- karyotic cell. Axonemes from all cilia and flagella (including sperm tails) contain the same “9 + 2″ ar- rangement of microtubules. In the center of each ax- oneme are two singlet microtubules that run the length of the shaft. The central tubules are sur- rounded by a circle of doublet microtubules, each consisting of an A and B subfiber.
Each A subfiber has longitudinally repeating pairs of armlike projec- tions that contain dynein (q.v.). See Chlamydomonas reinhardi, flagellum, tektin, Y chromosome. axoplasm the cytoplasm contained in axons. axopodia rigid, linear cellular projections composed mostly of microtubules found in species belonging to the Actinopoda. See classification, Protoctista. 5-azacytidine an analog of cytidine in which a ni- trogen atom is substituted for a carbon in the num- ber 5 position of cytosine (q.v.). The analog is incor- porated into newly synthesized DNA, and such DNA is undermethylated. Since a reduction in the number of methyl groups attached to genes is associ- ated with an increase in their transcriptional activi- ties, 5-azacytidine can switch on certain genes.
For example, patients given the drug may start making fetal hemoglobin, which implies that their gamma genes have been switched on. See hemoglobin, he- moglobin genes. azaguanine a purine antagonist first synthesized in the laboratory and later shown to be identical to an antibiotic synthesized by Streptomyces spectabalis.
Azaguanine is incorporated into mRNA and causes errors in the translation.
azaserine a glutamine analog synthesized by vari-ous species of Streptomyces. Azaserine inhibits pu- rine biosynthesis and produces chromosome aber- rations. It is mutagenic and has anti-tumorigenic activity.
azoospermia absence of motile sperm in the ejac-ulate. Azotobacter a genus of free-living, rod-shaped, soil bacteria capable of nitrogen fixation (q.v.). Poly-nucleotide phosphorylase (q.v.) was isolated from Azotobacter vinelandii. See Appendix A, Bacteria, Pro- teobacteria; Appendix C, 1955, Grungerg-Manago and Ochoa. azure B a basic dye used in cytochemistry. See metachromasy.
B1, B2, B3, etc. the first, second, third, etc., back- cross generations. The first backcross is made by mating an individual with one of its parents or with an individual of that identical genotype. The off- spring produced belong to the B1 generation. The second backcross is made by crossing B1 individuals again with individuals of genotype identical to the parent referred to in the first backcross, etc. BACs bacterial artificial chromosomes (q.v.).
Bacillus a genus of rod-shaped bacteria. B. subtilis is a Gram-positive, spore-forming, soil bacillus that grows readily in a chemically defined medium and undergoes genetic exchange by transformation and transduction (q.v.). Its genome contains 4,214,810 base pairs. There are about 4,100 protein-coding genes, and 53% of these are present as single copies. However, 25% of the genome is represented by fam- ilies of duplicated genes.
One family contains 77 genes for ATP-binding transport proteins. The ge- nome is also home for at least 10 prophages, and this suggests that bacteriophage infection may have allowed horizontal gene transfer during evolution. Bacillus megaterium is the species in which the lyso- genic cycle (q.v.) was deciphered. B. thuringiensis during sporulation produces an insecticidal crystal- line deposit that is innocuous to vertebrates.
The genes encoding the toxic proteins in these crystals have been identified and found to be carried on ex- trachromosomal plasmids. Some successful attempts have been made to splice these “insecticide genes” into the chromosomes of agriculturally important plant species.
See Appendix A, Bacteria, Endospora; Appendix C, 1950, Lwoff and Gutman; 1997, Kunst et al.; ABC transporters, Bt designer plants, horizontal transmission, prophage. Bacillus anthracis See anthrax. backbone in biochemistry, the supporting struc- ture of atoms in a polymer from which the side chains project. In a polynucleotide strand, alternat- ing sugar-phosphate molecules form such a back- bone. backcross a cross between an offspring and one of its parents or an individual genetically identical to one of its parents. backcross parent that parent of a hybrid with which it is again crossed or with which it is repeat- edly crossed.
A backcross may involve individuals of genotype identical to the parent rather than the par- ent itself. background constitutive synthesis the occasional transcription of genes in a repressed operon due to a momentary dissociation of the repressor that allows a molecule of RNA polymerase to bind to its pro- moter and initiate transcription. Sometimes called “sneak synthesis.”
background genotype the genotype of the organ- ism in addition to the genetic loci primarily responsi- ble for the phenotype under study. background radiation ionizing radiation arising from sources other than that under study. Back ground radiation due to cosmic rays and natural ra- dioactivity is always present, and there may also be background radiation due to man-made contaminat- ing radiation. back mutation reverse mutation (q.v.).
bacteria in the broader sense, all prokaryotes; more specifically, organisms belonging to the sub- kingdom Bacteria. See Appendix A, superkingdom Prokaryotes; Appendix C, 1677, van Leeuwenhoek; Appendix F, Genome sizes and gene numbers (of bacteria); Eubacteria. bacterial artificial chromosomes (BACs) cloning vectors derived from the naturally occuring F factor (q.v.) of Escherichia coli (q.v.) and designed to accept large inserts (i.e., those in the size range of 80-350 kilobases).
Insert-containing BACs are introduced into E. coli cells by electroporation (q.v.), where they can be maintained as circular plasmids. In contrast to yeast artificial chromosomes (YACs) (q.v.), which can show structural instability of inserts, DNA frag- ments closed in BACs remain structurally intact. This is because BACs contain F factor regulatory genes that control their replication and maintain their copy number to one or two per cell. The ab- sence of multiple artificial chromosomes in a single cell minimizes sequence rearrangement in inserts by reducing the likelihood of recombination between the inserted fragments. BACs are useful for cloning DNA from large genomes, chromosome walking (q.v.), physical mapping, and shotgun sequencing (q.v.) of complex genomes. See DNA vector, ge- nome, genomic library, kilobase, physical map, plas- mid cloning vector, P1 artificial chromosomes (PACs), regulator gene.
bacterial cell wall a structure forming a layer ex- ternal to the plasma membrane. It controls the shape of the bacterium and serves as a permeability bar- rier. In some species a capsule may be formed ex- ternal to the cell wall. The walls of Gram-positive bacteria are 30-100 nm thick and appear homoge- neous. They are composed mainly of peptidoglycans. Gram-negative cell walls are thinner (20-30 nm) and are layered.
The inner layer (adjacent to the plas- ma membrane) contains peptidoglycans. The outer layer contains peptidoglycans cross-linked to lipo- proteins. Certain bacteria lack cell walls. See Gram- staining procedure, Mycoplasma, peptidoglycan. bacterial transformation See transformation. bacteriochlorophyll See chlorophyll. bacteriocins proteins synthesized by various bac- terial species that are toxic when absorbed by bacte- ria belonging to sensitive strains.
Resistance to and the ability to synthesize bacteriocins are controlled by plasmids. Escherichia coli strains produce bacte- riocins called colicins (q.v.). Bacteriocins from Pseu- domonas aeruginosa are called pycocins. bacteriophage a virus whose host is a bacterium; commonly called phage. Below are listed some com- mon bacteria and their viral parasites:
Bacterial viruses show extreme variations in com- plexity. For example, the RNA phage R17 has a ge- nome size of 1.1 × 106 daltons, whereas the DNA of the T4 phage weighs 130 × 106 daltons.
See Appen- dix C, 1915, Twort; 1917, d’Herelle; 1934, Schle- singer; 1934, Ellis and Delbruck; 1942, Luria and Anderson; 1945, Luria; 1949, Hershey and Rotman; 1952, Hershey and Chase; 1953, Visconti and Del- bru¨ck; 1966, Edgar and Wood; 1970, Alberts and Frey; 1973, Fiers et al.; Appendix F, bacteriophages; filamentous phage, lambda (λ) bacteriophage, MS2, phi X174, plaque, P1 phage, P22 phage, Q beta phage, temperate phage, T phages, transduction, vir- ulent phage. bacteroids intracellular, nitrogen-fixing symbionts found in the root nodules of leguminous plants. Bac- teroids are derived from free-living species of Rhizo- bium (q.v.). See leghemoglobin.
bacteriophage packaging insertion of recombi- nant bacteriophage lambda DNA into E. coli for rep- lication and encapsidation into plaque-forming bac- teriophage particles. bacteriostatic agent a substance that prevents the growth of bacteria without killing them.
baculoviruses a group of viruses that infect ar- thropods, especially insects. Baculoviruses utilize the synthetic machinery of the insect host cell to synthe- size polyhedrin, a protein that coats the virus parti- cle.
The gene for polyhedrin has a very strong tran- scriptional promoter to which foreign genes can be spliced to enhance their expression. These baculo- virus expression vectors (BEVs) have been used in basic research and by commercial biotechnology en- terprises aimed at the production of vaccines, ther- apeutics, and diagnostic reagents. By appropriate gene-splicing techniques baculoviruses have been engineered to synthesize foreign proteins, including the envelope protein of HIV (q.v.). bag of marbles (bam) an autosomal gene in Dro- sophila melanogaster that is thought to regulate early germ cell development in both sexes.
In the female, bam mutations produce ovarian tumors character- ized by mitotically active stem cells which fail to produce cystoblast cells and their progeny, the inter- connected cystocytes. In the male, bam mutations produce germ line cysts containing excessive num- bers of primary and secondary spermatogonial cells which fail to differentiate. The bam gene encodes a novel protein which is found in the spectrosome (q.v.), the fusome (q.v.), and cytoplasm of cysto- cytes and spermatogonial cells. Fusomes in bam mu- tant females are structurally abnormal, leading to the suggestion that normal fusome biogenesis is es- sential for a switch from stem cell-like to cystoblast.