Flying spot cytometer
bacterial mutation is an event that is rare, discontin- uous, and random, then there should be a marked fluctuation in the number of resistant variants pres- ent, at a given time, in a large number of indepen- dent cultures, each of which was grown from a small inoculum. The number of variants per sample would fluctuate because some cultures would contain large numbers of variants arising from the division of an early mutant, while in other cultures mutants occur- ring later during the growth of the culture would produce clones of smaller size. Conversely, in sepa- rate samples taken from a single culture inoculated under identical conditions the variability in the number of mutants should be much less. However, if the agent used for selection induced the muta- tions, then the distribution of the number of mu- tants in any population of samples should be inde- pendent of the previous history of the culture.
Since it was found that the variance was much larger when samples came from independent cultures than when they were taken from the same culture, it was con- cluded that spontaneous mutation was the source of the variants. See Appendix C, 1943, Luria and Del- bru¨ck. fluid mosaic concept a model in which the cell membrane is considered to be a two-dimensional viscous solution consisting of a bilayer of highly ori- ented lipids. The layer is discontinuous, being inter- rupted by protein molecules that penetrate one or both layers. See Appendix C, 1972, Singer and Nich- olson; lipid bilayer model. fluke a common name for flatworms belonging to the class Trematoda. Flukes of medical importance are members of the order Digenea. These parasites have molluscs as intermediate hosts. See schistosomi- asis. fluourescein an orange-red compound that yields a bright-green fluorescence when exposed to ultravi- olet light. When conjugated to a specific antibody, this dye provides a means of localizing the antigen when the stained cell is viewed with a fluorescence microscope.
fluorescence See luminescence. fluorescence in situ hybridization (FISH) this technique uses synthetic polynucleotide strands that bear sequences known to be complementary to spe-cific target sequences at specific chromosomal sites. The polynucleotides are bound via a series of linking molecules to a fluorescent dye that can be detected by a fluorescence microscope. This probe is then in situ hybridized to the cells to be tested. The fluo- rescence signals observed under the microscope per- mit the number, size, and location of the target se- quences to be determined with speed and precision. See chromosome painting, in situ hybridization. fluorescence microscopy the usual methods of microscopical examination are based on observing the specimen in the light transmitted or reflected by it.
Fluorescence microscopical preparations are self- luminous. In most biological preparations the tissue sections are stained with a fluorochrome, a dye that emits light of longer wavelength when exposed to blue or ultraviolet light. The fluorescing parts of the stained object then appear bright against a dark background. The staining techniques are extremely sensitive and can often be used in living materials. See Appendix C, 1970, Caspersson, Zech, and Johan- sson.
fluorescent antibody technique a method for lo- calizing a specific protein or other antigen in a cell by staining a section of the tissue with an antibody specific for that antigen. The antibody is tagged di- rectly or indirectly by a fluorochrome for detection under a fluorescence microscope. See immunofluo- rescence. fluorescent screen a sheet of material coated with a substance such as calcium tungstate or zinc sulfide that will emit visible light when irradiated with ion- izing radiation. Such screens are used in TV sets and as the viewing screens of electron microscopes. fluorine a biological trace element. Atomic num- ber 9; atomic weight 18.9984; valence 1−; most abundant isotope 19F. fluorochrome a fluorescent dye that can be conju- gated to a compound that binds to a specific cell component.
An example would be a fluorescein- labeled antibody of rodaminylphalloidin (q.v.). flush end synonymous with blunt end. See restric- tion endonuclease. flying spot cytometer an instrument used in cyto- metric DNA measurements. The heterogenous dis- tribution of Feulgen stain within a nucleus leads to distributional errors when a single measurement is made of light transmission to estimate the amount of dye bound in the nucleus. Flying spot cytometers scan a defined microscopic area while making thou- sands of measurements with a minute measuring
spot. The sum of these point absorbance measure- ments as determined by a built-in computer is pro- portional to the true absorbance of the specimen. See Feulgen procedure, microspectrophotometer. F-mediated transduction sexduction. f-met N-formylmethionine (q.v.) f-met-tRNA the complex between N-formylme- thionine and its transfer RNA. FMN flavine mononucleotide, a coenzyme. FMR-1 gene See fragile X-associated mental retar- dation. foci 1. regions of growth of tumor cells appearing as raised clusters above a confluent monolayer of cells in tissue culture. 2. opaque pocks appearing on the chorioallantoic membrane of a developing chick embryo that has been infected with certain viruses such as herpes viruses. focus map a fate map (q.v.) for regions of the Dro- sophila blastoderm determined to become adult structures, inferred from the frequencies of specific kinds of mosaics. foldback DNA single-stranded regions of DNA that have renatured by intrastrand reassociation be- tween inverted repeats; hairpin DNA. folic acid the anti-pernicious-anemia vitamin. It is a compound made up of three components: a pteri- dine (q.v.), p-aminobenzoic acid, and glutamic acid.
The active form of folic acid is tetrahydrofolate. This compound contains hydrogen atoms attached to nitrogens 5 and 8 and carbons 6 and 7. The en- zyme dihydrofolate reductase catalyzes certain of these addition reactions. Tetrahydrofolate is an es- sential coenzyme in the biosynthesis of thymidylic acid. Thus, folic acid analogs like aminopterin and methotrexate block nucleic acid synthesis. Exposure to ultraviolet radiation causes the breakdown of fo- late. In light-skinned people, half the folate in the bloodstream can be lost during an hour’s exposure to sunlight. Women should take folic acid before and during pregnancy because too low folate levels can cause neural tube defects in fetuses.
follicle-stimulating hormone (FSH) a glycopro- tein hormone that stimulates the growth of ovarian follicles and estrogen secretion. It is produced by the adenohypophysis of vertebrates. Følling disease See phenylketonuria. footprinting a technique for identifying a segment of a DNA molecule that is bound to some protein of interest, on the principle that the phosphodiester bonds in the region covered by the protein are pro- tected against attack by endonucleases. A control sample of pure DNA and one of protein-bound DNA are subjected to endonuclease attack. The re- sulting fragments are electrophoresed on a gel that separates them according to their lengths. For every bond position that is susceptible, a band is found on the control gel.
The gel prepared from the protein- bound DNA will lack certain bands, and the missing bands identify the length of the site covered by the protein. Forbes disease a hereditary glycogen storage dis- ease in humans arising from a deficiency of the en- zyme amylo-1,6-glucosidase. Inherited as an auto- somal recessive. Prevalence 1/100,000. forensic DNA analysis See DNA forensics.
formaldehyde CH2O, a colorless gas readily solu- ble in water and having mutagenic properties. See Appendix C, 1946, Rapoport; formalin. formalin an aqueous solution of formaldehyde (q.v.) commonly used as a fixative, which functions through cross linking protein molecules. formamide a small organic molecule that com- bines with the free NH2 groups of adenine and prevents the formation of A-T base pairs, thereby causing denaturation of double-stranded DNA. See stringency.