Transcription is the first stage in the process of gene expression, as the genetic information in a gene is transferred to an mRNA which encodes a protein. Here, the DNA acts as a template for the synthesis of mRNA and is based on the principle of complementarity, with one exception that adenine base pairs with uracil instead of thymine.
The transcription process is different from DNA replication. In that, only one DNA strand gets copied into RNA in transcription, while in replication both DNA strands get copied. In case both DNA strands act as templates in transcription, two RNA molecules complementary to each other are produced and form double-stranded RNA. As a result, RNA does not translate into protein and the transcription gets nullified.
The segment of DNA taking part in the transcription is called the transcription unit and it encodes for at least one gene. Transcription unit consists of mainly the promoter, the structural gene and the terminator. The promoter is present upstream at the five prime end of the structural gene, and RNA polymerase binds to this region and moves along the DNA strand from three prime to five prime direction and codes for the DNA template. The structural gene consists of two DNA strands, one with three-prime to five-prime polarity, is called the template strand while the DNA strand with five-prime to three-prime polarity is called the coding strand and is displaced during transcription.
The structural gene can be defined in terms of a cistron that codes for a polypeptide. The structural gene in a transcription unit can be either monocistronic or polycistronic. A monocistronic structural gene found in eukaryotes translates only a single protein, while a polycistronic structural gene found in prokaryotes translates into several proteins. The monocistronic structural gene has a split gene arrangement, where sequences called introns and exons are present. The transcription helps in removing non coding regions called introns and joining coding sequences called exons into functional mRNA. It is the exons along with the promoter in DNA that determine the inheritance of a character. The structural gene also consists of regulatory genes, which regulate the functions of other genes but do not code for any RNA or protein. At the three-prime end of the structural gene is the terminator, which ends the transcription.