translocated chromosome) and one translocated chromosome. A translocation heterozygote (illus- trated below) forms a quadrivalent chromosomal as- sociation during pachynema (see meiosis), and the subsequent segregation of the four chromosomes is determined by their centromere orientations. In the case of adjacent disjunction, (or adjacent segregation), each daughter cell receives a normal and a translo- cated chromosome. The gametes produced from such cells are inviable because they contain certain genes in duplicate and are deficient for others. Two kinds of adjacent disjunction are recognized. In adja- cent-1 segregation, homologous centromeres go to op- posite anaphase poles. In adjacent-2 segregation, ho- mologous centromeres go to the same anaphase pole. In the case of alternate disjunction, both trans- located chromosomes go to one cell and both normal ones to the other. The gametes that result are viable because they contain all the genetic material. translocation of proteins See receptor-mediated translocation. translocation mapping gene mapping by the use of translocation chromosomes as markers. The semi- sterility that is usually associated with structural translocation heterozygotes is a phenotypic marker that can be used to locate the position of the break- point of the translocation relative to other more con- ventional gene markers. translocon an organelle in the endoplasmic reticu- lum (q.v.) that allows a protein manufactured on a ribosome to pass from the cytoplasm to the ER lu-
men. The mammalian translocons are composed of a small number of specific proteins, and some of these are homologous to proteins that were identi- fied in early genetic screens for secretion mutants in Saccharomyces. When bound to a ribosome, the aqueous pore that spans the translocon is estimated to be about 5 nm in diameter. The translocon does not disassemble when the ribosome leaves. How- ever, it does change its conformation, and the pore diameter is reduced to about 2 nm. See Appendix C, 1975, Blobel and Dobberstein; 1991, Simon and Blobel; receptor-mediated translocation. 7 transmembrane domain (7TM) receptor an in- tegral protein (q.v.) belonging to a large superfamily of transmembrane receptors that contain seven membrane-spanning domains.
Most of the members of 7 TM receptors are G protein-coupled receptors (q.v.), each of which binds a signaling molecule on the extracellular side of the membrane and trans- duces a signal on the cytoplasmic side to initiate or inhibit biochemical reactions within the cell. Exam- ples of 7 TM receptors are sensory and neurotrans- mitter receptors. transmission electron microscope See electron microscope. transmission genetics that part of genetics con- cerning the mechanisms involved in the transfer of genes from parents to offspring. transmutation the transformation of one element into another accompanying radioactive decay (q.v.). transplantation transfer of a part of an organism to another organism or to another position in the same organism. In zoology, the term is used inter- changeably with graft (q.v.). In botany, graft is used in the above sense, and transplantation is used in the sense of planting again in a different place.
See rejec- tion. transplantation antigen a protein coded by genes in the major (e.g., human HLA, mouse H-2) or mi- nor histocompatibility loci and present on most ver- tebrate cells. Transplantation antigens are targets for T lymphocytes if the graft bears antigens different from that of the host. See histocompatibility mole- cules. transposable elements DNA sequences that move from one chromosomal site to another. They were discovered in maize by McClintock during her anal- ysis of the Activator-Dissociation system (q.v.). Trans- posable elements were subsequently detected in bac- teria as insertion sequences (symbolized IS1, IS2, etc.). Later bacterial transposable elements were dis-