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OVERVIEW RNA, TRANSCRIPTION, TRANSLATION The RNA Molecule RNA is structurally similar to DNA. Overview: The Central Dogma The central dogma of molecular biology describes the two-step process, transcription and translation, by which the information in genes flows into proteins: DNA to RNA to PROTEINS. Transcription is the synthesis of an RNA copy of a segment of DNA. RNA is synthesized by the enzyme RNA polymerase. Transcription and translation are spatially and temporally separated in eukaryotic cells; that is, transcription occurs in the nucleus to produce a pre-mRNA molecule. The pre-mRNA is typically processed (introns removed, exons kept) to produce the mature mRNA, which exits the nucleus and is translated in the cytoplasm. The Transcription Process RNA synthesis involves separation of the DNA strands and synthesis of an RNA molecule in the 5' to 3' direction by RNA polymerase, using one of the DNA strands as a template. In complementary base pairing, A, T, G, and C on the template DNA strand specify U, A, C, and G, respectively, on the RNA strand being synthesized. Complete Transcription of an RNA Molecule Transcription begins at the promoter, proceeds through the coding region, and ends at the terminator. RNA SPLICING Introns removed and exons spliced, then sent to ribosome Transcription copies the DNA code of a gene and converts it to messenger RNA (m RNA). The m RNA will be used at the ribosome to make polypeptides (proteins). However all of the code contained in the m RNA molecule is not needed to produce the polypeptide. The sections of m RNA which do not code for translation of polypeptide are called introns. As the m RNA readies itself to leave the nucleus, enzymes cut out and remove the introns. The remaining exons are spliced back together again by a different enzyme. This modified m RNA is what comes to the ribosome to be translated into polypeptides. TRANSLATION Of mRNA into a PROTEIN ISO During translation, a small ribosomal subunit attaches to a mRNA molecule. At the same time, an initiator tRNA molecule recognizes and binds to a specific codon sequence on the same mRNA molecule. A large ribosomal subunit then joins the newly formed complex. The initiator tRNA resides in one binding site of the ribosome called the P site, leaving the second binding site, the A site, open. When a new tRNA molecule recognizes the next codon sequence on the mRNA, it attaches to the open A binding site. A peptide bond forms connecting the amino acid attached to the tRNA in the P site to the amino acid attached to the tRNA in the A binding site. As the ribosome moves along the mRNA molecule, the tRNA in the P site is released and the tRNA in the A site is translocated to the P site. The A binding site becomes vacant again until another tRNA that recognizes the new mRNA codon takes the open position. This pattern continues as molecules of tRNA are released from the complex, new tRNA molecules attach, and the amino acid chain grows. The ribosome will translate the mRNA molecule until it reaches a termination codon on the mRNA.