Balanced lethal system
like cell division. In males, bam gene product is thought to restrict the overproliferation of sperma- togonial cells undergoing incomplete cytokinesis. See cystocyte divisions, spermatogonia. balanced lethal system a strain of organisms bear- ing nonallelic recessive lethal genes, each in a differ- ent homologous chromosome. When interbred such organisms appear to breed true, because one-half of the progeny are homozygous for one or the other lethal gene and die prior to their detection. The sur- viving progeny, like their parents, are heterozygous for the lethal genes.
See Appendix C, 1918, Muller; 1930, Cleland and Blakeslee. balanced polymorphism genetic polymorphism maintained in a population because the heterozy- gotes for the alleles under consideration have a higher adaptive value than either homozygote. See Appendix C, 1954, Allison. balanced selection selection favoring heterozy- gotes that produces a balanced polymorphism (q.v.) balanced stock a genetic stock that, though het- erozygous, can be maintained generation after gener- ation without selection. Such stocks may contain balanced lethal genes, or a recessive lethal gene, which kills hemizygous males, combined with a non- allelic recessive gene, which confers sterility on ho- mozygous females.
See M5 technique. balanced translocation synonymous with recipro- cal translocation. See translocation. Balbiani body a transitory, membrane-less struc- ture consisting of cell organelles (e.g., mitochondria, centrioles, Golgi bodies, and endoplasmic reticu- lum) and macromolecules (e.g., RNAs, proteins, lip- ids, and ribonucleoproteins) found in the early ooplasm (q.v.) of a wide variety of vertebrate and invertebrate species.
A Balbiani body generally origi- nates near the nuclear envelope (q.v.) and differenti- ates into a well-defined mass often surrounded by mitochondria, before breaking down to release its constitutents into the ooplasm. It is thought to func- tion in the organization and transport of RNAs and organelles that later become localized in specific re- gions of the ooplasm. Balbiani bodies show hetero- geneity in number, morphology, and composition among different species and are named after the French biologist, Edouard-Ge´rard Balbiani (1823- 1899), who was among the first to describe them.
Also called Balbiani vitelline body. See mitochondrial cloud, sponge body, cytoplasmic localization. Balbiani chromosome a polytene chromosome. So called because such banded chromosomes were first discovered by E. G. Balbiani in Chironomus lar- vae in 1881. Balbiani ring a giant RNA puff present on a poly- tene chromosome of a salivary gland cell during a significant portion of larval development.
The largest and most extensively studied Balbiani rings are on chromosome 4 of Chironomus tentans (see illustra- tion). The transcription product of one of these puffs (BR 2) is a 75S RNA, which encodes the mes- sage for a giant polypeptide of the saliva. These se- cretory polypeptides are used by the larvae to build the tubes in which they reside. Balbiani rings contain thousands of DNA loops upon which mRNAs are being transcribed. These combine with proteins to form RNP granules (Balbiani ring granules) that eventually pass into the cytoplasm through nuclear pores. See Appendix C, 1881, Balbiani; 1952, Beer- mann; 1972, Daneholt; Chironomus, chromosomal puff.
Chromosome 4 in the salivary glands of Chironomus tentans with the BR 2 band in an unpuffed (above) and in a puffed stage (below). The position of the BR 2 band is indicated by an arrow. Bal 31 exonuclease a nuclease that digests linear, double-stranded DNA fragments from both ends. The enzyme is used in vitro to shorten restriction fragments. The shortened segment can then be reli- gated with a DNA ligase (q.v.) to generate deletion mutants.
See restriction endonucleases. Baltimore classification of viruses an organiza- tional scheme proposed by David Baltimore in 1971 that classified viruses according to the relationship between the viral genome and the messenger RNA used for viral protein synthesis. Class I viruses have double-stranded DNA genomes, and their mRNA is encoded by the template (negative) strand of the DNA. Class II viruses have a ssDNA genome and use a dsDNA intermediate during mRNA synthesis.
Class III viruses have a dsRNA genome and one of the strands is equivalent to its mRNA. Class IV vi- ruses are positive-strand RNA viruses. Therefore the genomic RNA is equivalent to mRNA. Class V vi- ruses are negative-strand RNA viruses. Here the neg- ative strand can serve as a template for mRNA syn- thesis. Class VI viruses are positive-strand RNA viruses that undergo reverse transcription (q.v.). See messenger RNA, virus. Balzer freeze-fracture apparatus See freeze etch- ing. bam See bag of marbles. BamH1 See restriction endonuclease. bananas perennial giant herbs of the tropics.
The diploid fertile species Musa acuminata and M. bal- bisiana have genomes that may be symbolized AA and BB. Most cultivated bananas are sterile triploids that are vegetatively propagated. The triploid (AAA, ABB, or AAB) cannot form pollen, but the unpolli- nated ovary grows into a seedless fruit with edible pulp. The stimulus for parthenocarpy depends on the presence of at least three dominant complemen- tary genes in the triploid. See Musaceae, partheno- carpy. band 1. in an electropherogram (q.v.), a region of the gel that contains clustered molecules of a par- ticular size class visualized by staining, autoradi- ography, immunofluorescence, and so forth. 2. in chromosome studies, a vertical stripe on a polytene chromosome that results from the specific associa- tion of a large number of homologous chromomeres at the same level in the somatically paired bundle of chromosomes. See Balbiani ring, deficiency loop, Drosophila salivary gland chromosomes, human chro- mosome band designations. BAP 6-benzylaminopurine (q.v.).
Bar a sex-linked dominant mutation in Drosophila melanogaster which results in a reduction in the number of facets in the compound eye. This muta- tion, symbolized by B, is commonly used as a marker when constructing balancer X chromosomes. The original mutation contained a tandem duplication of chromosome segment 16A. Unequal crossing over (q.v.) can cause this mutation to revert to wild type. The 16A duplication is now thought to be a transpo- son-induced rearrangement. Analysis of the Bar phe- nomenon led to the discovery of position effects (q.v.).
See Appendix C, 1925, Sturtevant; M5 tech- nique, transposable elements. barley See Hordeum vulgare. Barr body the condensed single X chromosome seen in the nuclei of somatic cells of female mam- mals. See Appendix C, 1949, Barr and Bertram; dos- age compensation, drumstick, late replicating X chro- mosome, Lyon hypothesis, sex chromatin. basal body (granule) a structure generally com- posed of a ring of nine triplet microtubules sur- rounding a central cavity, found at the base of cilia. See axoneme, centriole, flagellum, kinetosome.
Basc chromosome See M5 technique. base analog a purine or pyrimidine base that dif- fers slightly in structure from the normal base. Some analogs may be incorporated into nucleic acids in place of the normal constituent. See aminopurine, azaguanine, mercaptopurine. Analogs of nucleosides behave similarly. See 5-bromodeoxyuridine. basement membrane a delicate acellular mem- brane that underlies most animal epithelia.
See epi- dermolysis bullosa, laminin. base pair a pair of hydrogen-bonded nitrogenous bases (one purine and one pyrimidine) that join the component strands of the DNA double helix. Abbre- viated bp. See deoxyribonucleic acid, genome size, gigabase, Hoogsteen base pairs, hydrogen bond, kilo- base, megabase, nucleotide pair. base-pairing rules the rule that adenine forms a base pair with thymine (or uracil) and guanine with cytosine in a double-stranded nucleic acid molecule. base-pair ratio See (A + T)/(G + C) ratio. base-pair substitution a type of lesion in a DNA molecule that results in a mutation.
There are two subtypes. In the case of transitions, one purine is substituted by the other or one pyrimidine by the other, and so the purine-pyrimidine axis is pre- served. In the case of transversions, a purine is substi- tuted by a pyrimidine or vice versa, and the purine- pyrimidine axis is reversed. See Appendix C, 1959, Freese. bases of nucleic acids the organic bases univer- sally found in DNA and RNA (see page 46).
In a nucleotide sequence, a purine is often symbolized R, while a pyrimidine is symbolized by Y. The purines adenine and guanine occur in both DNA and RNA. The pyrimidine cytosine also occurs in both classes of nucleic acid. Thymine is found only in DNA, and uracil occurs only in RNA. See rare bases. base stacking the orientation of adjacent base pairs with their planes parallel and with their sur- faces nearly in contact, as occurs in double-stranded