hollow tube through which DNA is transferred dur- ing conjugation. See circular linkage map, F-prime fac- tor, Hfr strain, MS2, pilus. F′ factor See F-prime factor. fibrillin a 2,871 amino acid glycoprotein associ- ated with microfibrils about 10 nanometers in diam- eter. The protein is found in skin, tendon, bone, muscle, lung, kidney, blood vessels, and the suspen- sory ligament of the lens. Fibrillin contains 49 EGF domains. The protein is encoded by a gene symbol- ized by FBN1, and mutations in it are responsible for an inherited disease called Marfan syndrome (q.v.). See epidermal growth factor. fibrin See blood clotting. fibrinogen See blood clotting. fibroblasts spindle-shaped cells responsible for the formation of extracellular fibers such as collagen (q.v.) in connective tissues. fibroin the major protein component of silk (q.v.). fibronectin a dimer made up of two similar pro- tein subunits. Each has an Mr of 250,000, and the two are joined at one end by disulfide bonds.
The proteins are modular in the sense that they are di- vided into a series of domains, each with specific binding properties. For example, there are different domains that bind specifically to actin, to collagen, and to certain receptor proteins embedded in the plasma membranes of cells. Fibronectin mediates the attachment of cells to collagenous substrates, participates in the organization of stress fibers, and facilitates cell-to-cell adhesions. The fibronectin gene contains a series of exons, and there is one-to- one correspondence of exons to the protein-binding domains. Fibronectins exist in a variety of isoforms, many of which result from alternative splicing (q.v.). See peripheral protein. fiducial limits See confidence limits. field See prepattern. filaform thread-shaped. filamentous phage a bacterial virus (e.g., M13, fd) that specifically infects male (donor) cells and carries a single strand of DNA within a filamentous protein coat. A filamentous phage forms a double- stranded replicative form (q.v.) during its life cycle. filial generations any generation following the pa- rental generation. Symbolized F1, F2, etc.
filopodia very thin, fingerlike extensions of the plasma membrane; used by cells that move by amoe- boid locomotion. single strands and immobilized on a nitrocellulose filter, to a solution of radioactively labeled RNA or DNA; only hybrid double-stranded molecules re- main on the filter after washing. Compare with liquid hybridization. See Appendix C, 1975, Benton and Davis. filter route a migration path along which only a few species can easily disperse. filtration enrichment a method for the isolation of nutritional mutants in fungal genetics. Mutagen- ized spores are placed upon a minimal medium. Normal spores germinate and send out an exten- sive mycelial network. These colonies are then fil- tered off, and the remaining germinated spores that show poor mycelial development are grown upon a supplemented medium, where each produces enough mycelia to allow further propagation and study. fimbria (plural fimbriae) a thin filament that ex- tends from the surface of a microorganism and func- tions to facilitate the adhesion of the cell to other cells or to the substratum.
Fimbriae occur in large numbers on a given cell, and they are not to be con- fused with conjugative pili. Fimbriae occur on Gram- negative bacteria and certain fungi. Each fimbria consists of linear repeating molecules of a protein called fimbrillin. See P blood group, pilus. fimbrillins adhesive proteins of fimbriae (q.v.). Bacteria that are able to synthesize fimbrillin H avoid destruction by phagolysosomes (q.v.).
This is because endocytosis by caveolae (q.v.) is triggered when FimH binds to a receptor in the caveolae in the host cell. finalism a philosophy that views evolution as be- ing directed (by some rational force) toward an ulti- mate goal. See teleology (of which this is a special case); see also orthogenesis. fine-structure genetic mapping the high-resolu- tion analysis of intragenic recombination down to the nucleotide level. fingerprinting technique in biochemistry, a meth- od employed to determine differences in amino acid sequences between related proteins.
The protein un- der study is enzymatically cleaved into a group of polypeptide fragments. These are separated in two dimensions: first by paper electrophoresis that sepa- rates peptides on the basis of net charge and second by partition chromatography that separates peptides on the basis of their degree of polarity (affinity for the hydrated cellulose support, which is highly po-
lar). The result will be a two-dimensional array of spots, the “fingerprint.” This is compared to the stan- dard fingerprint. The difference in the position of one spot in the case of the HbS and HbA fingerprints led to the discovery that the normal and mutant he- moglobins differed in a single amino acid substitu- tion. See Appendix C, 1957, Ingram; nucleic acid fin- gerprinting. finished sequence in genome sequencing, a high- quality DNA sequence that is contiguous (or nearly contiguous) and has a high rate of sequence accu- racy.
With an error rate of 0.01%, i.e., just one error per 10,000 bases, The Human Genome Project (q.v.) produced data with 99.99% accuracy. The finished sequence of the human genome was ready in 2003. Compare with draft sequence. See Appendix C, 2003, The International Human Genome Se- quencing Consortium. fireflies beetles belonging to the family Lampyri- dae. They are unique in being able to flash their ab- dominal luminescent organs on and off. The rhythm of the flashes is species specific. Males fly about in the evening flashing their signals. Receptive females on the ground respond with a delayed signal which guides males to them. See luciferase, mimicry.
first-arriver principle a theory proposing that the first individuals to colonize a new environment or to become adapted to a specific niche acquire thereby a selective advantage over later arrivals, merely be- cause they got there first; also known as “king-of- the-mountain” principle. first cousin See cousin. first-degree relative when referring to a specific individual in a pedigree (q.v.), any relative who is only one meiosis away from that individual (a par- ent, a sibling, or an offspring). Any relative with whom one half of one’s genes are shared. Contrast with second-degree relative. first-division segregation ascus pattern in asco- mycetes, a 4-4 linear order of spore phenotypes within an ascus. This pattern indicates that a pair of alleles (e.g., those controlling spore pigmentation) separated at the first meiotic division, because no crossovers occurred between the locus and the cen- tromere. See ordered tetrad.
first-order kinetics the progression of an enzy- matic reaction in which the rate at which the prod- uct is formed is proportional to the prevailing sub- strate concentration, with the result that the rate slows gradually, and the reaction never goes to com- pletion. See zero-order kinetics. FISH fluorescence in situ hybridization (q.v.). fission 1. binary fission (q.v.). 2. nuclear fission (q.v.). fitness the relative ability of an organism to sur- vive and transmit its genes to the next generation. fixation the first step in making permanent prepa- rations of tissues for microscopic study. The proce- dure aims at killing cells and preventing subsequent decay with the least distortion of structure. See fixa- tive, genetic fixation, nitrogen fixation. fixative a solution used for the preparation of tis- sues for cytological or histological study. It precipi- tates the proteinaceous enzymes of tissues and so prevents autolysis, destroys bacteria that might pro- duce decay of the tissue, and causes many of the cel- lular constituents to become insoluble. fixing in photography, the removal of the un- changed halide after the image is developed.
An aqueous solution of sodium thiosulfate (hypo) is used. flagellate a protoctist belonging to the Zoomasti- gina or Euglenophyta. See Appendix A. flagellin a member of a family of proteins that are a major component of the flagellae of prokaryotes. The flagellins found in the archaea are distinct in composition and assembly from those of bacteria. flagellum (plural flagella) 1. in prokaryotes, a whip-like motility appendage present on the surface of some species. Bacterial flagellae range in length from 2 to 20 nanometers. Bacteria having a single flagellum are called monotrichous; those with a tuft of flagella at one pole are called lophotrichous; and those with flagella covering the entire surface are called peritrichous. Antigens associated with flagella are called H antigens. Compare with pilus. 2. in eu- karyotes, flagellum refers to a threadlike protoplas- mic extension containing a microtubular axoneme (q.v.) used to propel flagellates and sperm. Flagella have the same basic structure as cilia (q.v.), but are longer in proportion to the cell bearing them and are pres- ent in much smaller numbers (most sperm are mo- notrichous).
In recent literature, flagellum is restricted to prokaryotic mobility appendages, and the term flagellum in eukaryotes has been replaced by unduli- podium (q.v.). flanking DNA nucleotide sequences on either side of the region under consideration. For example, the hallmarks of a transposon (q.v.) are (1) it is flanked by inverted repeats at each end, and (2) the inverted repeats are flanked by direct repeats.