Breakage-fusion-bridge cycle

27 Mar

Breakage-fusion-bridge cycle
bonobo the pigmy chimpanzee. See Pan. border cells in Drosophila oogenesis, a group of anterior cells that detach from the follicular epithe- lium and migrate between the nurse cells in a poste- rior direction until they reach the anterior surface of the oocyte. Here they later participate in the forma- tion of the micropylar apparatus which allows the sperm to enter the egg.

Mutations in the sex-linked genes domeless, hopscotch, or unpaired disturb the migration of the border cells. Borrelia burgdorferi the spirochaete, transmitted by ticks, that causes Lyme disease in humans. Ge- nome sequencing has revealed that the bacterium contains a 910,725 bp megachromosome and 17 dif- ferent plasmids with a combined size of 533,000 bp. The main chromosome contains about 850 genes, and there are at least 430 genes on the plasmids. Un- like the chromosomes of most bacteria which are circular, the main chromosome and some of the plasmids are linear.

The DNA in the telomeres (q.v.) forms covalently closed hairpin structures. See Ap- pendix A, Prokaryotes, Bacteria, Spirochaetae; Ap- pendix C, 1997, Fraser et al. Bos the genus that includes the domestic cow, B. taurus, the Brahman, B. indicus, and the yak, B. grunniens. The haploid chromosome number for the domestic cow is 30, and about 500 genes have been mapped. See cattle for a listing of domestic breeds, beef thymus; Appendix E. bottleneck effect fluctuations in gene frequencies occurring when a large population passes through a contracted stage and then expands again with an al- tered gene pool (usually one with reduced variabil- ity) as a consequence of genetic drift (q.v.). botulism poisoning by an exotoxin (q.v.) synthe- sized by Clostridium botulinum (q.v.).

The poison is called the botulin toxin (botox), and when eaten it blocks nerve impulses and causes muscle paralysis. bouquet configuration a polarized arrangement of chromosome ends at the periphery while the re- maining chromatin fills the volume of the nucleus. This is the result of telomeres (q.v.) moving along the inner surface of the nuclear envelope during lep- tonema and eventually bunching together at the bouquet site.

The tethering of telomeres to the nu- clear periphery requires a specific meiotic telomere protein. In Saccharomyces cerevisiae this protein is encoded by a gene called Ndj1 (nondisjunction 1). Deletion of Ndj1 prevents bouquet formation and causes a delay in the pairing of homologues. See mei- osis. Boveri theory of cancer causation the proposal that a malignant tumor arises by the proliferation of a single cell which has acquired an excess or defi- ciency of chromosomes due to errors in the number of chromosomes it received during mitosis. See Ap- pendix C, 1914, Boveri.

bovine referring to members of the cattle family, especially to those of the domestic cattle species Bos taurus. bovine achondroplasia hereditary chondrodystro- phy seen in “bull-dog” calves of the Dexter breed. The condition is inherited as an autosomal recessive. See achondroplasia. bp abbreviation for “base pairs.” Bq becquerel (q.v.). brachydactyly abnormal shortness of fingers or toes or both. brachyury a short-tailed mutant phenotype in the mouse governed by a gene on chromosome 17. It was through this mutant that the T complex (q.v.) was discovered. See T box genes.

Bracon hebetor See Microbracon hebetor (also called Habrobracon juglandis). bradyauxesis See heterauxesis. bradytelic used to refer to a lower-than-average rate of evolution. See evolutionary rate. Brahman a breed of humped domestic cattle (Bos indicus). brain hormone prothoracicotropic hormone (q.v.). Branchiostoma a genus of lancelets, commonly called Amphioxus. Branchiostoma lanceolatum is the sole living representative of the Cephalochordata. See Appendix A, Animalia, Chordata, Hox genes. branch migration See Holliday model.

branch site See lariat. BRCA1, BRCA2 See breast cancer susceptibility genes. Brdu 5-bromodeoxyuridine (q.v.). breakage and reunion the classical and generally accepted model of crossing over by physical break- age and crossways reunion of broken chromatids during meiosis. See Holliday model. breakage-fusion-bridge cycle a cycle that begins with a dicentric chromosome forming a bridge as it is pulled toward both poles at once during anaphase.

Breakage-reunion enzymes

Such dicentric chromosomes may arise from an ex- gene, change within a paracentric inversion or may be ra- diation-induced. Once the dicentric breaks, the bro- ken ends remain sticky, and these fuse subsequent to duplication.

The result is another dicentric that breaks at anaphase, and so the cycle continues, with the chromosomes being broken anew at every mito- sis (see illustration). Since each subsequent break is likely to be at a different place than the previous ones, there will be a repeated regrouping of the ge- netic loci to produce duplications and deficiencies. See Appendix C, 1938, McClintock; chromosome bridge, telomere.

breakage-reunion enzymes enzymes that use continuous stretches of DNA molecules, rather than preexisting termini, as substrates. The DNA duplex is broken and rejoined. The energy released by tion factor. breakage is stored in a covalent enzyme-DNA inter- mediate and utilized in rejoining the molecules. breakthrough an individual that escapes the dele- terious action of its genotype. In a population of in- dividuals homozygous for a given recessive lethal Such dicentric chromosomes may arise from an ex- “breakthroughs,” or “escapers.”

breast cancer susceptibility genes genes that when mutated greatly increase the susceptibility of heterozygous women to breast cancer. The first gene, BRCA1, resides at 17q21. It was cloned in 1994 and shown to encode a protein containing 1,863 amino acids. BRCA2 resides at 13q12-13. It was cloned in 1995 and is now known to encode a protein containing BRCA1 and BRCA2 are responsible for most cases of hereditary breast cancer.

BRCA1 also increases the risk of ovarian cancer, but BRCA2 does not. The continuous stretches of DNA molecules, rather than and is therefore believed to function as a transcrip- is broken and rejoined. The energy released by tion factor Wooster et al.; anti-oncogene, Mendelian Inheritance in Man (MIM), zinc finger protein. breathing in molecular genetics, the periodic, lo- terious action of its genotype. In a population of in- duce single-stranded “bubbles.”

breed an artificial mating group derived from a common ancestor for genetic study and domestica- tion. breeding the controlled propagation of plants and animals. breeding size the number of individuals in a pop- ulation that are actually involved in reproduction during that generation.

breeding true to produce offspring of phenotype identical to the parents; said of homozygotes. bridge migration synonymous with branch migra- tion. See Holliday model. bridging cross a mating made to transfer one or more genes between two reproductively isolated species by first transferring them to an intermediate species that is sexually compatible with the other two species.

bristle organ each insect bristle is an organ con- sisting of four cells: the cell that secretes the bristle, the socket cell that secretes the ring that encloses the bristle, a sensory nerve cell whose process ends near the base of the bristle, and the sheath cell that surrounds the nerve axon.

See trichogen cell. broad bean Vicia faba (q.v.). This is the European plant to which the term bean was originally applied. broad heritability the proportion of the total phe- notypic variance (for a polygenic trait in a given population) that is attributed to the total genetic variance (including additive, dominance, epistatic, and other types of gene action); symbolized H2. See heritability. 5-bromodeoxyuridine a thymidine analog that can be incorporated into DNA during its replication.

This substitution profoundly affects that structure of the DNA. When both strands are substituted with BUDR, a chromatid stains less intensely than when only one strand is so substituted. Thus when cells are grown in the presence of BUDR for two replica-


tion cycles, the two sister chromatids stain differen- tially and therefore are called harlequin chromo- somes. Consequently, the BUDR labeling method can be used to detect sister chromatid exchanges. BUDR causes breakage in chromosomal regions rich in heterochromatin. Additional acronyms are Budr and Brdu. See Appendix C, 1972, Zakharov, and Egolina.
5-bromouracil  a mutagenically active pyrimidine analog.

brood the offspring from a single birth or from a single clutch of eggs. broodiness the tendency of female birds to incu- bate eggs. Bruton tyrosine kinase See agammaglobulinemia. Bryophyta the plant phylum containing mosses, liverworts, and hornworts. Bryophytes lack a vascu- lar system. See Appendix A. Bt designer plants transgenic plants (q.v.) with the gene for the toxin produced by Bacillus thurin- giensis spliced into their genomes.

Bt corn is an ex- ample of such a genetically engineered crop, and the Bt toxin kills off its major enemy, the European corn borer. One third of the corn sold in the U.S. is Bt corn. Farmers plant even more Bt soybeans and Bt cotton. See Bacillus, GMO. bubble a structure formed in a duplex DNA at the site of initiation and strand separation during repli- cation. bud 1. A sibling cell produced during the division of a budding yeast, like Saccharomyces cerevisiae. The daughter cell develops as a protrusion of the cell wall of the mother cell. The mother retains its old cell wall components while the bud gets newly syn- thesized wall material (contrast with septal fission).

The nucleus migrates to the neck of the bud. Here mitosis occurs with the nuclear envelope remaining intact, and a set of telophase chromosomes is deliv- ered to each cell. 2. an underdeveloped plant shoot, consisting of a short stem bearing crowded, overlap- ping, immature leaves. budding 1. in bacteria, yeast, and plants, the pro- cess by which a bud (q.v.) is produced. 2. in enve- loped viruses such as influenza virus and Sindbis vi- rus, a mode of release from the host cell in which a portion of the cell membrane forms an envelope around the nucleocapsid. The envelope contains vi- ral proteins, but no cellular proteins. BUDR 5-bromodeoxyuridine (q.v.).

buffer a compound that, in solution, tends to pre- vent or resist rapid changes in pH upon the addition of small quantities of acid or base. buffering the resistance of a system to change by outside forces. Bufo a genus of toads. Wild populations of species of this genus have been extensively studied by popu- lation geneticists. bulb a modified shoot consisting of a very much shortened underground stem enclosed by fleshy scalelike leaves. It serves as an organ of vegetative reproduction. The onion, daffodil, tulip, and hya- cinth produce bulbs. bull the adult male of various animals including domesticated cattle, elephants, moose, and elk.

bull-dog calf See bovine achondroplasia. buoyant density the equilibrium density at which a molecule under study comes to rest within a den- sity gradient. See centrifugation separation. Burkitt lymphoma a monoclonal malignant prolif- eration of B lymphocytes primarily affecting the jaw and associated facial bones. The cancer is named after Denis Burkitt, who first described it in central African children in 1958. Most Burkitt tumors oc- curring in Africans contain Epstein-Barr virus (EBV) (q.v.), and this virus is believed to be mosquito borne. Burkitt lymphomas from United States and European patients lack EBV. Burkitt lymphoma cells always contain a reciprocal translocation involving the long arm of chromosome 8 and chromosome 14, or less frequently 22 or 2.

The break point on chro- mosome 8 is always near the myc oncogene (q.v.). The break point on the other chromosome is always near an immunoglobulin gene, namely, 14 (heavy chains), 22 (lambda light chains), or 2 (kappa light chains). In its translocated state myc is activated and the cancer ensues. See immunoglobulin chains, Phila- delphia (Ph1) chromosome. bursa of Fabricius a saclike structure connected to the posterior alimentary canal in birds. The bursa is the major site where B lymphocytes become mature immunoglobulin (antibody)-secreting plasma cells.

The equivalent organ in mammals has not been defi- nitely identified. Most evidence suggests the bone marrow. The organ bears the Latinized name of the Italian anatomist Girolamo Fabrizio (1578-1657) who first described it. bursicon an insect hormone that appears in the blood after molting and is required for the tanning and hardening of new cuticle. burst size the average number of bacteriophages released from a lysed host. See Appendix C, 1939, Ellis and Delbru¨ck. busulfan a mutagenic, alkylating agent.

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