Biochemists refer to RNA synthesis as transcription . Transcription is the process of synthesizing ribonucleic acid (RNA). Synthesis takes place within the nucleus of eukaryotic cells or in the cytoplasm of prokaryotes and converts the genetic code from a gene in deoxyribonucleic acid ( DNA ) to a strand of RNA that then directs protein synthesis.
Three types of RNA are found in cells. Transfer RNA (tRNA) carries amino acids to the site of protein synthesis. Ribosomal RNA (rRNA) along with protein makes up ribosomes (the mechanism that synthesizes protein). Messenger RNA (mRNA) is the code or template for protein synthesis. Special enzymes synthesize the different forms of RNA.
DNA consists of a series of regions called operons, each containing one or more genes capable of coding for an mRNA strand. An operon consists of a number of segments, principal among which are a promoter region to which RNA polymerase , the enzyme that synthesizes RNA, readily attaches, an operator region that acts as an on/off switch for the operon, and one or more genes that code for mRNA production. For convenience, biochemists describe locations on chains of nucleotides by speaking of the ends of the sugar-phosphate chains as having 3′ (three-prime) and 5′ ends. Both DNA and RNA are synthesized by enzymes that start at the 5′ end of the strand being synthesized.
Transcription begins when RNA polymerase approaches the promoter gene, which often contains extended nucleotide sequences that help to match and bind the polymerase. After the polymerase binds, it is thought to move along the strand of DNA to the operator region. Protein repressor molecules that block transcription bind to operators; inducing agents may attach to the repressor molecules and pull them away from the operator, allowing the synthesis of mRNA. An example of this is the induction or turning on of the lac operon in Escherichia coli by the presence of lactose, producing mRNA that codes for enzymes that metabolize lactose.
RNA polymerase moves along the DNA molecule from the 3′ end of the operon to the 5′ end, copying only one strand. (Copying the complementary strand would result in useless or harmful nonsense mRNA.) DNA and RNA are similar in composition, but DNA contains deoxyribose instead of ribose and the pyrimidine base thymine instead of uracil . The newly formed RNA is complementary to the DNA code; adenine bases on one strand pair with thymine or uracil on the other strand, and guanine bases on one strand pair with cytosine on the complementary strand. The strands are said to be antiparallel; that is, the 3′ end of the DNA strand matches the 5′ end of the new mRNA strand. Since protein synthesis (translation) begins at the 5′ end of mRNA, protein synthesis in prokaryotic cells can begin while transcription is still under way, increasing the speed with which the organism responds to changes in its environment.
Devlin, Thomas M., ed. (2002). Textbook of Biochemistry: With Clinical Correlations, 5th edition. New York: Wiley-Liss.
Voet, Donald; Voet, Judith G.; and Pratt, Charlotte W. (2002). Fundamentals of Biochemistry, updated edition. New York: Wiley.