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Transcription is the first step of gene expression, in which a particular segment of DNA is copied into RNA (mRNA) by the enzyme RNA polymerase. Both RNA and DNA are nucleic acids, which use base pairs of nucleotides as a complementary language. The two can be converted back and forth from DNA to RNA by the action of the correct enzymes. During transcription, a DNA sequence is read by an RNA polymerase, which produces a complementary, antiparallel RNA strand called a primary transcript. Transcription proceeds in the following general steps: #One or more sigma factor protein binds to the RNA polymerase holoenzyme, allowing it to bind to promoter DNA. #RNA polymerase creates a transcription bubble, which separates the two strands of the DNA helix. This is done by breaking the hydrogen bonds between complementary DNA nucleotides. #RNA polymerase adds matching RNA nucleotides to the complementary nucleotides of one DNA strand. #RNA sugar-phosphate backbone forms with assistance from RNA polymerase to form an RNA strand. #Hydrogen bonds of the untwisted RNA-DNA helix break, freeing the newly synthesized RNA strand. #If the cell has a nucleus, the RNA may be further processed. This may include polyadenylation, capping, and splicing. # The RNA may remain in the nucleus or exit to the cytoplasm through the nuclear pore complex. The stretch of DNA transcribed into an RNA molecule is called a ''transcription unit'' and encodes at least one gene. If the gene transcribed encodes a protein, messenger RNA (mRNA) will be transcribed; the mRNA will in turn serve as a template for the protein's synthesis through translation. Alternatively, the transcribed gene may encode for either non-coding RNA (such as microRNA), ribosomal RNA (rRNA), transfer RNA (tRNA), or other enzymatic RNA molecules called ribozymes.〔Eldra P. Solomon, Linda R. Berg, Diana W. Martin. ''Biology, 8th Edition, International Student Edition''. Thomson Brooks/Cole. ISBN 978-0495317142〕 Overall, RNA helps synthesize, regulate, and process proteins; it therefore plays a fundamental role in performing functions within a cell. In virology, the term may also be used when referring to mRNA synthesis from an RNA molecule (i.e., RNA replication). For instance, the genome of a negative-sense single-stranded RNA (ssRNA -) virus may be template for a positive-sense single-stranded RNA (ssRNA +). This is because the positive-sense strand contains the information needed to translate the viral proteins for viral replication afterwards. This process is catalysed by a viral RNA replicase. == Background == A DNA transcription unit encoding for a protein may contain both a ''coding sequence'', which will be translated into the protein, and ''regulatory sequences'', which direct and regulate the synthesis of that protein. The regulatory sequence before ("upstream" from) the coding sequence is called the five prime untranslated region (5'UTR); the sequence after ("downstream" from) the coding sequence is called the three prime untranslated region (3'UTR).〔 As opposed to DNA replication, transcription results in an RNA complement that includes the nucleotide uracil (U) in all instances where thymine (T) would have occurred in a DNA complement. Only one of the two DNA strands serve as a template for transcription. The antisense strand of DNA is read by RNA polymerase from the 3' end to the 5' end during transcription (3' → 5'). The complementary RNA is created in the opposite direction, in the 5' → 3' direction, matching the sequence of the sense strand with the exception of switching uracil for thymine. This directionality is because RNA polymerase can only add nucleotides to the 3' end of the growing mRNA chain. This use of only the 3' → 5' DNA strand eliminates the need for the Okazaki fragments that are seen in DNA replication.〔 This removes the need for an RNA primer to initiate RNA synthesis, as is the case in DNA replication. The ''non''-template sense strand of DNA is called the coding strand, because its sequence is the same as the newly created RNA transcript (except for the substitution of uracil for thymine). This is the strand that is used by convention when presenting a DNA sequence. Transcription has some proofreading mechanisms, but they are fewer and less effective than the controls for copying DNA; therefore, transcription has a lower copying fidelity than DNA replication. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Transcription (genetics)」の詳細全文を読む スポンサード リンク
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