triiodothyronine See thyroid hormones.
trilobites the earliest of the arthropods, a group found throughout the Paleozoic era. A dorsal view of a Cambrian trilobite, Cedaria, is shown here. See ec- dysis.
trimesters traditional terms used in medicine which divide the 9-month period of normal preg- nancy in women into three stages, each lasting 3 months. The differentiation of cells and tissues in the embryo take place during the first trimester. The mother feels fetal movements early in the second tri- mester, and there is a rapid growth and maturation of the organ systems. During the third trimester the organ systems are completed, and fat accumulates under the skin of the fetus. It moves into position for birth which brings the third trimester to an end. trimethylpsoralen a low-molecular-weight, planar molecule that undergoes photochemical reactions with pyrimidines. Trimethylpsoralen molecules in- tercalate into double-stranded DNA. Upon expo- sure to ultraviolet light, the molecules attach cova- lently to pyrimidines forming both monoadducts and interstrand cross links.
Trimethylpsoralen has no effects on proteins. trinucleotide repeats unstable DNA sequences found in several human genes. Normally the triplets are repeated in tandem 5-50 times. When the num- ber rises above the normal range, mutant disease syndromes appear. The triplets subject to expansion start with C and end with G. Examples are given in the table below. Also called triplet repeats. See micro- satellites. trioecy a sexual trimorphism in plants that can ex- ist as either male, female, or bisexual individuals. triparental recombinant a progeny phage contain- ing marker genes derived from each of three differ- ent phages that simultaneously infected the host cell. The observation of triparental recombinants demonstrates that repeated recombinational events occur in the infected cell between replicating nucleic acid molecules derived from all parent viruses.
See Visconti-Delbru¨ck hypothesis. tripartite ribbons synaptonemal complexes (q.v.). triplet a unit of three successive bases in DNA or RNA that codes for a specific amino acid. See amino acid, genetic code, translation. triplet code a code in which a given amino acid is specified by a set of three nucleotides.
See Appendix C, 1961, Crick, Brenner et al.; amino acid, genetic code, proflavin, reading frame, translation. triplet repeats See trinucleotide repeats. triplex See autotetraploid. triploid an organism having three haploid sets of chromosomes in each nucleus. In those situations where triploid tissues have two paternal and one ma- ternal complement, the triploid is said to be android, whereas gynoid triploids have two maternal and one paternal set of chromosomes. triskelion See receptor-mediated endocytosis. trisomic an organism that is diploid but contains one extra chromosome, homologous with one of the existing pairs, so that one kind of chromosome is present in triplicate.
See Appendix C, 1920, Blakeslee et al.; Down syndrome, Edwards syndrome, metafem- ale, Patau syndrome.