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in biology, the biosynthesis of ribonucleic acid (RNA) on a deoxyribonucleic acid (DNA) template that takes place in living cells.
Transcription is a fundamental biological process, the first stage in the production of the genetic information contained in DNA in the form of a linear sequence of four types of monomer links, or nucleotides. Transcription is effected by enzymes that are DNA-dependent RNA polymers. It results in a polymer chain of RNA that also contains nucleotides. In this chain, the sequence of monomer links duplicates the sequence of the monomer units of one of the two complementary chains of the part of the DNA being copied.
The four types of RNA produced by transcription perform a variety of functions: (1) messenger RNA, which serves as a template for protein synthesis by ribosomes (translation); (2) ribosomal RNA, the structural constituents of ribosomes; (3) transfer RNA, the main elements that during protein synthesis recode the information contained in messenger RNA from the language of nucleotides into the language of amino acids; and (4) RNA that serves as a primer for the replication of DNA.
DNA is transcribed by operons containing one or more genes. The enzyme RNA polymerase recognizes the beginning of the operon (the promoter site), unites with the promoter site, unwinds the DNA double helix, and, beginning at that point, copies one of the chains of the helix, moving along the DNA chain and sequentially joining the monomer links, or nucleotides, to the resulting RNA according to the principle of complementarity. As the RNA polymerase moves, the lengthening RNA chain separates from the matrix, and the DNA double helix is restored behind the enzyme. When the RNA polymerase reaches the end of the part of the helix being copied (the terminator), the RNA separates from the matrix. The number of copies of the different parts of DNA depends on the cells’ need for the corresponding proteins. The number of copies may also be altered owing to environmental conditions or changes in the development of the given organism. The regulatory mechanism of transcription has been thoroughly studied in bacteria. The study of the mechanisms of transcription in higher organisms is one of the most important tasks of molecular biology.
Information can be transferred both from DNA to RNA and from RNA to DNA. Such reverse transcription takes place in RNA-containing oncogenic viruses. These viruses contain an enzyme that uses viral RNA as a template for synthesizing the complementary DNA strand after the cells are infected. The result is a double-strand RNA-DNA hybrid that is used to synthesize the second DNA strand, which is complementary to the first strand. The double-helix DNA that is formed and that contains all the information of the original RNA may be incorporated into the chromosomes of a cell infected by the virus and may cause the cell to become malignant.
The discovery of reverse transcription has confirmed the viral theory of the origin of cancer advanced by the Soviet microbiologist L. A. Zil’ber. Reverse transcription may play an important part in the systems used in normal cells for the production and accumulation of information—for example, in embryonic development. The enzyme that causes reverse transcription, RNA-dependent DNA polymerase (reverse transcriptase, or revértase), is similar to DNA-dependent DNA polymerases but considerably different from the DNA-dependent RNA polymerases that cause transcription.
REFERENCESTemin, G. “RNK napravliaiet sintez DNK.” Priroda, 1972, no. 9.
Gershenzon, S. M. “Obratnaia transkriptsiia i ee znachenie dlia obshchei genetiki i onkologii.” Uspekhi sovremennoi biologii, 1973, vol. 75, no. 3.
Stent, G. Molekuliarnaia genetika. Moscow, 1974. Chapter 16. (Translated from English.)
B. G. NIKIFOROV
in linguistics, the written representation of words and texts in terms of their pronunciation by means of a special system of letters and symbols. Transcription may be scholarly or orthographic.
Scholarly transcription is used in linguistic studies of speech and may be phonetic or phonemic. Phonetic transcription is the precise representation of a word’s sounds; stress and positional variants are also indicated. Phonemic transcription represents the phonemic composition of words without regard for positional changes of phonemes. Phonetic transcription is often used in bilingual dictionaries, where it is shown in square brackets, whereas phonemic transcription is shown within slashes or angle brackets. Scholarly transcription is usually based on the Latin alphabet supplemented by special symbols and diacritical marks. The most widely used transcription system is the universal alphabet of the International Phonetic Association (IPA), which was devised in 1886 and has been gradually improved since then.
There is also a Cyrillic transcription system, used for languages with a Cyrillic alphabet, and mainly for Russian. In this system, Russian podkhodit’ (“to approach”) is rendered p” tkhad’ it’ in Cyrillic phonetic transcription, and podkhod’it’ in Cyrillic phonemic transcription.
Analytic phonetic transcription is a type of transcription in which each symbol corresponds not to a single sound but to such distinctive elements in the sound’s articulation as labialization or occlusion. This type of transcription is sometimes used for specialized scholarly purposes. Its best-known variant is O. Jespersen’s system of analphabetic notation.
Orthographic transcription is the representation of untranslatable foreign words in the standard spelling of a given national alphabet. Orthographic transcription is used mainly in writing foreign personal names and surnames, as well as geographic names. This type of transcription is less precise than scholarly transcription and is different for each language; it uses only the standard alphabet of the given language. For example, Russian Pushkin is rendered in French as Pouchkine, in German as Puschkin and in Hungarian as Puskin. A successful orthographic transcription always reflects the native pronunciation of the given word. An example of an incorrect transcription, retained because of tradition, is Russian Gudzon for Hudson; a correct transcription would be Khadson. Transcription must be distinguished from transliteration and orthography.
REFERENCESAvanesov, R. I. Fonetika sovremennogo russkogo literaturnogo iazyka. Moscow, 1956.
Zinder, L. R. Obshchaia fonetika. Leningrad, 1960.
Shcherba, L. V. Fonetika frantsuzskogo iazyka, 7th ed. Moscow, 1963.
Reformatskii, A. A. Vvedenie v iazykovedenie, 4th ed. Moscow, 1967.
V. A. VINOGRADOV
in music, an adaptation or a performer’s free interpretation of a musical composition. Transcription held an important place in the development of instrumental music; in the 16th century many works for keyboard instruments were in fact transcriptions of vocal music. Famous transcriptions include those for violin by F. Kreisler and piano transcriptions by F. Liszt, F. Busoni, L. Godowski, M. A. Balakirev, S. V. Rachmaninoff, and K. Tausig. (See alsoPARAPHRASE.)