Formation of RNA from DNA is called transcription in which only a segment of DNA and only one of the strands is copied into RNA. A transcription unit in DNA is defined primarily by the three regions in the DNA.
(i) A Promoter
(ii) The Structural gene
(iii) A Terminator
Since the two strands have opposite polarity and the DNA dependent RNA polymerase also catalyse the polymerization in only one direction, that is 5`®3`, the strand that has the polarity 3`®5` acts as a temple and is also referred to as template strand. The other strand which has the polarity (5~®3`) and the sequence same as RNA (except Thymine at the place of Uracil), is displaced during transcription. Strangely, this strand (which does not code for anything) is referred to as coding strand.
Type of RNA and the Process of Transcription
In bacteria, there are three major types of RNAs-mRNA (messenger RNA), tRNA (transfer RNA), and rRNA (ribosomal RNA). All three RNAs are needed to synthesized a protein in a cell. The mRNA provides the template, tRNA brings amino acids and reads the genetic code, and rRNAs play structural and catalytic role during translation.
There is single DNA dependent RNA polymerase that catalyses transcription of all types of RNA in bacteria. RNa polymerase binds to promoter and initiates transcription (Initiation). It somehow also facilitates opening of the helix and continues elongation. Only a short stretch of RNA remains bound to the enzyme. Once the polymerases reaches the terminator region. The nascent RNA fall off, so also the RNA polymerase. This results in termination of transcription.
An intriguing question is that how is the RNA polymerases able to catalyse all the three steps, which are initiation, elongation and termination. The RNA polymerase is only capable of catalyzing the process of elongation. It associates transiently with initiation-factor (s)and termination-factor (r) to initiate and terminate the transcription, respectively. Association with these factors alter the specificity of the RNA polymerase to either initiate or terminate.
In bacteria, since the mRNA does not require any processing to become active and also since transcription and translation take place in the same compartment (there is no separation of cytosol and nucleus in bacteria).
In eukaryotes, there are two additional complexities :
(i) There are at least three RNA polymerases in the nucleus (in addition to the RNA polymerase found in the organelles). There is a clear cut division of labour. The RNA polymerase I transcribes rRNAs (28S, 18S and 5.8S), whereas the RNA polymerase III is responsible for transcription of tRNA, 5srRNA, and snRNAs (small nuclear RNAs). The RNA polymerase II transcribes precursor of mRNA, the heterogeneous nuclear RNA (hnRNA).
(ii) The second complexity is that the primary transcripts contain both the exons and the introns and are non-functional. Hence, it is subjected to a process called splicing where the introns processing called as capping and tailing. In capping an unusual nucleotide (methyl guanosine triphosphate) is added to the 5`-end of hnRNA. In tailing, adenylate residues (200-300) are added at 3`-end in a template independent manner. It is the fully processed hnRNA, now called mRNA, that is transported out of the nucleus for translation.