Transcription — Mechanism and RNA Polymerase

Transcription is the process by which the genetic information encoded in a specific segment of DNA (a gene) is copied into a complementary strand of RNA. Unlike DNA replication, which copies an entire DNA molecule, transcription copies only a specific, defined segment of DNA — the particular gene, or stretch of genes, being expressed at that time — and, unlike replication, transcription copies only ONE of the two DNA strands at any given gene, not both. Of the two strands of DNA at a gene, only one — called the template strand (or antisense strand) — is actually used as the template against which the complementary RNA is synthesised; the other strand, called the coding strand (or sense strand), is not directly copied, but has a sequence identical to the newly made RNA (except that the RNA has uracil where the coding DNA strand has thymine).

The stretch of DNA that is transcribed to produce one complete RNA molecule, together with the regulatory sequences that control its transcription, is called a transcription unit. A transcription unit has three defining parts. The promoter is a specific DNA sequence located at the 5' end of the template strand (upstream of the actual gene sequence to be transcribed); it is the sequence that the enzyme RNA polymerase recognises and binds to, and it determines both which of the two DNA strands will serve as the template and the direction in which transcription will proceed. The structural gene itself is the DNA sequence that is actually transcribed into RNA. The terminator is a specific DNA sequence located at the 3' end of the coding strand, marking the point at which RNA polymerase stops transcription and releases the newly synthesised RNA.

The enzyme responsible for carrying out transcription is RNA polymerase, which — unlike DNA polymerase — is able to initiate RNA synthesis without requiring a primer, though it still synthesises the new RNA strand only in the 5' to 3' direction, reading the DNA template strand in its 3' to 5' direction, exactly as DNA polymerase does during replication. In bacteria, a single type of RNA polymerase carries out the transcription of all three main types of RNA (mRNA, tRNA and rRNA); eukaryotic cells, by contrast, possess three distinct, specialised RNA polymerases — RNA polymerase I transcribes rRNA, RNA polymerase II transcribes the precursor of mRNA (called heterogeneous nuclear RNA, or hnRNA), and RNA polymerase III transcribes tRNA and certain small RNA molecules. …