Transcription means a written version of something. So, a DNA transcription means the formation of a copy of a DNA strand.
“The process by which the DNA sequence is read and the complementary RNA molecule is formed is called transcription.”
Thus the DNA codes for the RNA strand, which will then code for the protein.
Before transcription can start, the DNA double helix unwinds near the gene that will get transcribed. This opened up part of DNA is called the transcription bubble.
DNA Transcription of RNA
This occurs in a 3 step process as
The enzyme RNA polymerase binds to the DNA of the gene at the region called the promoter. The promoter is the site that tells the polymer is where the transcribing should start. The promoter has DNA sequences that lets the RNA polymerase and its helper proteins recognize and attach to the DNA. The promoter is located towards the 5′ end of the coding gene. The RNA polymerase binds transiently with the initiation factor to initiate the transcription.
The strand on which the promoter binds is the template strand. The other strand of the DNA is called the coding strand because the RNA produced will be complementary to the template strand and will have a similar sequence to the coding strand.
The terminator located at the 3′ end is the place where transcription stops.
The RNA polymerase starts forming the RNA chain by the complementarity rule. It uses nucleotide triphosphate as a substrate; it also opens the helix as it transcribes. The RNA strand remains bound to the enzyme while it transcribes.
The RNA polymerase binds transiently termination factor to terminate the transcription. Soon as the polymer reaches the termination region, it stops and the RNA strand it has formed falls off and so does the RNA polymerase.
RNA polymerase 1 transcribes rRNA. RNA polymerase 2 transcribes precursor of mRNA and RNA polymerase 3 transcribes tRNA, snRNA, etc.
In eukaryotes, the precursor of mRNA has specific coding sequences that are not expressed in mature RNA. These coding sequences are called introns. The sequences that are expressed in mature RNA are called exons. The introns are removed by splicing and the exons are joined. To facilitate splicing, capping (3 methyl guanosine triphosphate to the 5 ends) and tailing (adding adenylate residues at 3′ end) takes place. Thus we get the processed mRNA.