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Home >>Biology Dictionary >> Trachea - Transcription
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Topsoil. The surface layer of soil to plough depth.
Tormogen. A cell that in insects. Secretes the socket for a bristle.
Torus.
(1) Receptacle of flower;
(2) Thickened portion of closing membrane of bordered pits.
Trace element. An element which must be available to an organism for its normal health though it is necessary only in minute amounts e.g. higher plants need traces of at least the elements zinc, boron, manganese, molybdenum, and copper; absence of these may produce economically serous disease, such as ‘heart-rot’ of sugar beet (boron deficiency). In animals such deficiency disease is also known, e.g. coast disease’ of cattle and sheep.
In man, thyroid deficiency (goiter, cretinism) may be due to lack of sei-trace element, iodine. Trace elements are probably constituents of enzyme systems; and also, in animals, of hormones.
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Tracer.
The atoms of most chemical elements are not all alike, but are of a few different kinds, called isotopes, differing in atomic weight but not in chemical properties. Some isotopes are very rare naturally, and these can be concentrated; Many radioactive ones can be prepared artificially. These isotopes are used experimentally as ‘tracers’ i.e. they can be incorporated into compounds of biological importance, administered to an organism, and their movements and changes of chemical combinatio0n determined by analysis of the organism, or of its products.
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The radioactive isotopes are particularly easy to follow by their radiation; for instance by autoradiographs.
Trachea. (1) (Bot.). Conducting element of xylem. (2) (Zool.). A tube which is part of the breathing apparatus of air breathing animals. In land-living vertebrates the ‘wind-pipe’, a single tube which lead from the throat, starting at the glottis, through the neck to the point where it bifurcates into two bronchi. In insects, tracheae ramify throughout the body and conduct air from a few openings at the surface directly to the tissues; made of epidermis and cuticle; finest branches and are called tracheoles.
Tracheole. The small branching tubes from the Tracheaf (2) of insects.
Trachymedusa. A medusa of the hydrozoan coelenterate order Trachylina.
Tract. Spatially delimited bundle of nerve fibres, usually all with similar connections, in the central nervous system. Tracts connect nuclei with each other and with peripheral nervous system.
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Transaminase. An enzyme that transfers amino (NH2) groups.
Transcription. Synthesis of RNA, made up of a particular sequence of nucleotides, by matching with DNA, made up of corresponding sequence of nucleotides. Synthesis of DNA by matching with RNA reverse transcription.
Transduction. Transfer of genetic material from one bacterium to another through the agency of bacteriophage. Gene or genes of one (host) bacterial cell become incorporated in phage particle which after release from dead host cell act as vectors in the transport of this genetic material to another bacterial cells.
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Transfer cells. Plant cells characterized by irregular wall in growth and usually high surface to volume protoplast ratios. Mitochondria numerous and endoplasmic reticulum conspicuous. Present in various locations in most major plant groups, e.g. gland cells of insectivorous plants: absorbing cells of dodder haustoria Minor veins of leaves, linking xylem vessels and phloem sieve-tubes.
Transformation.
(1) Phenomenon in which certain bacteria, when growth in the presence of killed cells, culture filtrates or extracts from other related strains, acquire some of the genetic characters of these strains. Transforming principle is DNA.
(2) Inherited change produced in cultured animal cells by infection with certain viruses, or by action of other agents.
Transfusion tissue. Tissue of empty cells pitted and with occasionally, internally thickened walls, and protein-containing, parenchymatous cells, accompanying vascular tissue in leaves of most gymnosperms. Lying on either side of the vascular bundles of the single vein.
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Translation.
The process in living cells in which the genetic information encoded in messenger RNA (mRNA) in the form of a sequence of base triplets (codons) is translated into a sequence of amino acids in a polypeptide chain during protein synthesis. Translation takes place on ribosomes in the cell cytoplasm. The ribosomes move among the mRNA reading’ each codon in turn.
Molecules of transfer RNA (tRAN), each bearing a particular amino acid, are brought to their correct positions along the mRNA molecule; base pairing occurs between the bases of the codons and the complementary base triplets of TRNA. In this way amino acids are assembled in the correct sequence to form the polypetide chain.
Toxin. A non-enzymic metabolite of one organism which is injurious.
Trans. Denoting across or over.
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