1980, Gordon et al.; 1986, Costantini et al.; 1987, Keuhn; Sus scrofa. transgenic plants genetically engineered plants that contain useful genes from other species. See Ap- pendix C, 1981, Kemp and Hall; Bt designer plants, promoter 35S, Roundup. transgressive variation progeny phenotypes out- side the range of that which occurs in the parents; usually attributed to polygene segregation. transient diploid a relatively short stage in the life cycles of predominantly haploid fungi or algae dur- ing which meiosis occurs. transient polymorphism polymorphism existing in a population during the period when an allele is being replaced by a superior one. transit cells in descriptions of stem cell differenti- ation, transitional, proliferating cells that are descen- dants of stem cells and that give rise to one or more mature, differentiated cell lineage(s). Transit cells have properties intermediate between stem cells (q.v.) and differentiated cells, as well as the plasticity to behave like one or the other of these cell types in response to specific environmental signals. Also called transit amplifying cells. transition See base-pair substitution. translation the formation of a protein directed by a specific messenger RNA (mRNA) molecule (see il- lustration on page 453). Translation occurs in a ribo- some (q.v.).
A ribosome begins protein synthesis once the 5′ end of an mRNA tape is inserted into it. As the mRNA molecule moves through the ribo- some, much like a tape through the head of a tape recorder, a lengthening polypeptide chain is pro- duced. Once the leading (5′) end of the messenger tape emerges from the first ribosome, it can attach to a second ribosome, and so a second identical poly- peptide can start to form. When the 3′ end of the mRNA molecule has moved through the first ribo- some, the newly formed protein is released and the vacant ribosome is available for a new set of taped instructions. The assembly of amino acids into a peptide starts at the amino end (N terminus) and finishes at the carboxyl end (C terminus). There are two binding sites for transfer RNA (tRNA) in the ribosome. The P site (peptidyl-tRNA binding site) holds the tRNA molecule that is attached to the growing end of the nascent polypeptide. The A site (aminoacyl-tRNA binding site) holds the incoming tRNA molecule charged with the next amino acid. The tRNAs are held so that their anticodons form base pairs with adjacent complementary codons of the mRNA moving through the ribosome. In the di- agram above, an mRNA molecule is shown progress- ing through a ribosome.
At time T0, codon 5 of the messenger tape occupies the P site and codon 6 the A site. About half a second later, the mRNA has ad- vanced to the left by one codon. The bond between tRNA I and the 5th amino acid of the nascent poly- peptide has been split, and it has been linked to amino acid 6. The atoms involved in the rearrange- ments are in boldface type. This reaction is catalyzed by peptidyl transferase, an enzyme that is bound tightly to the ribosome. Transfer RNA molecule I is discharged from the ribosome, tRNA II has entered the P site and is now attached to a polypeptide chain six amino acids long.
A new tRNA (III) carrying an appropriate anticodon enters the A site. Note that, in the diagram, letters represent atoms, whereas cir- cled letters represent molecules (i.e., nucleotides or amino acid residues). See Appendix C, 1959, Mc- Quillen et al.; 1961, Dintzis; 1963, Okamoto and Takanami, Noll et al.; 1964, Gilbert; 1974, Shine and Dalgarno; 1976, Pelham and Jackson; amino acid, elongation factors, initiation factors, leader se- quence peptide, N-formylmethionine, peptide bond, receptor-mediated translocation, start codon. translational amplification a mechanism for pro- ducing large amounts of a polypeptide based upon prolonged mRNA lifetime. Since there is only a sin- gle copy of the ovalbumin gene per genome, large numbers of ovalbumin molecules produced by the cells of the chicken oviduct are generated by transla- tional amplification. translational control the regulation of gene ex- pression through determining the rate at which a specific RNA message is translated. translation elongation factors ubiquitous pro- teins that transport aminoacyl tRNAs to the ribo- somes and participate in their selection by the ribo- somes. Translation elongation factors are symbolized Ef-Tu in prokaryotes and Ef-1 in eukaryotes.
Com- parisons of the amino acid sequences in specific seg- ments of Efs from a variety of prokaryotes and eu- karyotes have shown that these molecules are the slowest evolving proteins discovered so far. There- fore, certain sequences from Efs have been used to construct phylogenetic trees to determine critical steps in the early evolution of life. See endokaryotic hypothesis, opisthokonta. translation rate the speed at which amino acids are polymerized into polypeptide chains on ribo- somes. In bacteria at 37°C, the translation rate is ap- proximately 15 amino acids per second. The transla-