The anticodon occupies positions 36-38. In the drawing, A, U, C, and G have their usual meanings.
The rare bases are symbolized as follows: ψ, pseudo- uridylic acid; Tr, ribothymidylic acid; Ud, dihydrouri- dylic acid; Gm, methylguanylic acid; Gd, dimethyl- guanylic acid; I, inosinic acid; and Im, methylinosinic acid. See Appendix C, 1958, Crick, Zamecnik; 1965, Holley et al.; 1969, 1971, Dudock et al.; 1970, Khorana el al.; 1973, Kim et al.; codon bias, initiator tRNA, isoacceptor RNA, rare bases, ribonucleic acid (RNA), tRNA genes. trans-filter induction in vitro inductions using or- ganizer tissues separated from reactive cells by a Millipore filter (q.v.). The trans-filter induction sys- tem allows one to interrupt induction at any time by removing the inducing cells from the surface of the filter. transformants cells or multicellular organisms that show an inherited modification after exposure to a transforming principle or after incorporating ex- ogenous DNA. See transformation. transformation 1. in microbial genetics, the phe- nomenon by which genes are transmitted from one bacterial strain to another in the form of soluble frag- ments of DNA. These may originate from live or dead cells.
The DNA fragments dissolved in the ex- ternal medium can penetrate cells only if they have receptor sites for the DNA on their surfaces. Once inside, a fragment usually replaces, by recombina- tion, a short section of the DNA of the receptor cell that contains a zone of homology. Also called bacte- rial transformation. See Appendix C, 1928, Griffith; 1944, Avery et al.; 1964, Fox and Allen; 1970, Man- del and Higa; 1972, Cohen et al.; Pneumoccus Trans- forming Principle (PTP). 2. in genetic engineering of bacterial and eukaryotic cells, the acquisition of a new genetic characteristic through the uptake of ex- ogenous DNA from the surrounding medium, DNA microinjection directly into the cell, or other means. For example, ampicillin-sensitive E. coli cells that take up a plasmid (q.v.) containing the ampicillin- resistant gene ampR (q.v.) are transformed to ampi- cillin resistance. See competence, plasmid cloning vector. 3. in multicellular organisms, the acquisition of a new genetic makeup through experimental transfer and incorporation of exogenous DNA into the genome (q.v.). For example, microinjection of purified recombinant DNA (q.v.) into a Drosophila embryo produces a transformed adult fly which con- tains the exogenous DNA in its cellular DNA.
The insertion of exogenous DNA into the germ cell DNA of the transformant (q.v.) is called germ line transformation, which ensures that the transforming DNA is transmitted to the next generation. See transgenic animals, P element, P element transforma- tion. 4. in cultured cells, the conversion of normal animal cells to a state of unregulated growth by on- cogenic viruses (q.v.) or by mutations in anti-onco- genes (q.v.). Such transformations are generally ac- companied by alterations in cell shape, changed antigenic properties, and loss of contact inhibition (q.v.). Also called cellular transformation or transfec- tion. See Appendix C, 1980, Capecchi. transformation rescue the suppression of a mu- tant phenotype by introducing a specific wild-type nucleotide sequence into an embryo. A transposable element is generally used as a vector. The introduced wild-type gene sequences are said to “rescue” the mutant phenotype to normality.
Transformation res- cue is commonly used in germ line transformation experiments to identify functional components of a gene or to identify genomic sequences that corre- spond to a genetic locus. Also called plasmid rescue. See Appendix C, 1982, Spradling and Rubin; P ele- ments. transformation series the various expressions of a character ordered in a hypothesized sequence from the most primitive, plesiomorphic state to the most derived, apomorphic state.
This sequence may be linear (e.g., A0 → A1 → A2), or it may be branched. transforming growth factor (TGF- ) a large family of intercellular signaling molecules. These proteins are serine/threonine kinases, and they act by binding receptors on cell surfaces. The factors act as dimers, and their receptors are also dimers. TGF- βs play a role in embryonic development, cellular differentiation, hormone secretion, and immune suppression. See activin, cellular signal transduction, decapentaplegic. transgene a foreign gene that is introduced into an organism by injecting the gene into newly fertilized eggs.
Some of the animals that develop from the in- jected eggs will carry the foreign gene in their ge- nomes and will transmit it to their progeny. transgenic animals animals into which cloned ge- netic material has been experimentally transferred. In the case of laboratory mice, one-celled embryos have been injected with plasmid solutions, and some of the transferred sequences were retained through- out embryonic development. Some sequences be- came integrated into the host genome and were transmitted through the germ line to succeeding generations. A subset of these foreign genes ex- pressed themselves in the offspring. See Appendix C,