Transcription is the process by which the chemical information encoded in DNA is copied into RNA. In the synthesis of RNA during transcription, the enzyme RNA polymerase moves along the DNA helix, unwinding small portions of the helix as it goes. Once RNA for a particular region has been made, the double helix quickly reforms, thereby displacing the growing single strand of RNA.
Generally, only one strand of the double-stranded DNA is transcribed. The segment of the DNA molecule from which an RNA molecule is transcribed is in a sense equivalent to a gene, or a segment of DNA that (through RNA) specifies a protein. Most genes are composed of two general parts: a coding region to specify that portion of a gene or DNA molecule that is transcribed into RNA, and a regulatory region to regulate transcription of the coding region.
The enzymes involved in transcription are RNA polymerases. RNA polymerase can begin assembling chains of new RNA bases only after it identifies and binds to a specific DNA sequence in a regulatory region known as a promoter. Following the binding of RNA polymerase to the promoter, the giant enzyme begins to break the hydrogen bonds of the complementary bases, unwinding the DNA and exposing the segment of unpaired DNA bases. Transcription begins here and the base-pairing of DNA and RNA occurs. As a segment is transcribed, the opened helix rewinds while a new segment ahead unwinds. Uracil is substituted for thymine, while guanine pairs with cytosine, and adenine with thymine. Transcription of a DNA strand continues until the moving polymerase encounters a termination signal.
Information in an mRNA is converted into a protein through translation, which is divided into initiation, elongation, and termination. Initiation requires the smaller ribosomal subunit, an initiation tRNA (with a 5'-CAU-3' anticodon), and the mRNA initiator codon (5'-AUG-3'), all of which form the ribosomal initiation complex. When each component is in place, the larger ribosomal subunit joins the complex, and a second amino acid can be inserted. Polypeptides in eukaryotes all begin with methionine, and each has an N-terminal (NH2) end and a C-terminal carboxyl (COO-) end. Elongation occurs through translocation, or the formation of the peptide bond and the movement of a tRNA from the A to the P site. As a polypeptide grows, a charged tRNA whose anticodon matches the mRNA codon in the tRNA whose anticodon matches the mRNA codon in the A pocket becomes attached. A peptide bond forms between its amino acid and the last one in the polypeptide above, and translocation occurs again. When the ribosome reaches a chain termination (stop) codon, proteins block the pockets, and the final tRNA is released along with the completed polypeptide.
Proteins called elongation factors are involved in the following 3 step cycle: