Transcription involves following steps
Pre-Initiation
Transcription does not need a primer to start, it does require a promoter like sequence.
Initiation
Transcription begins at a specific site called as promoter (In eukaryotes, a TATA box, a nucleotide sequence like TATAAAA…, is typically present in the promoter.) The TATA box aids in the recognition of the promoter. Promoter itself is not transcribed.
First transcription factor locate and attach to the TATA box. Then RNA polymerase II binds to the promoter. This complete assembly of transcription factors and RNA polymerase along with promoter is called as transcription initiation complex. RNA polymerase II begins to unwind the DNA helix and transcribing the DNA sequence. Once the complex has been opened, Initiation starts and the first phosphodiester bond is formed. This is the end of step initiation.
RNA chain starts with triphosphate (PPP) and the first base is always purine, adenine or guanine.
Abortive Initiation
After synthesize of first bond, RNA polymerase must clear the promoter. During this time RNA transcript is released and truncated transcripts produced. This is called abortive initiation When the transcript reaches length of approximately 23 nucleotides it no longer slips and elongation can occur.
Elongation
RNA polymerase II reads the DNA template strand in a 3’-5’ direction, while building the new RNA strand in a 5’-3’ direction. It adds nucleotides to the 3’-OH group of the existing strand.
RNA polymerase continues moving along the template, as the DNA strands join back together in its wake and the newly synthesized RNA strand separates from the template. During this process a phosphate bond is formed and energy is released. In RNA the nitrogenous base thymine is replaced by uracil. Also, nucleotides are composed of a ribose (5-carbon) sugar while the DNA template has deoxyribose sugar. Transcription can involve multiple RNA polymerases on a single DNA template and multiple rounds of transcription, so a single gene may be transcribed thousands of times.
In Eukaryotic transcription the polymerase can experience pauses, to allow appropriate RNA editing factors to bind.
Termination
RNA chain is terminated when the enzyme encounters a terminator or a sequence of nucleotides that signals the end of transcription. The enzyme RNA polymerase transcribe the terminator sequence and then continues for about 10-15 nucleotides before the pre-mRNA strand is released. Then the pre-mRNA is released and DNA helix reforms.
Before the pre-mRNA strand or mRNA leaves the nucleus, it must undergo RNA processing.
RNA processing
Both ends of mRNA are protected by two different ways.
5’ end – the 5’end is capped with a methylated guanosine during the early stages of transcription. Sometimes, additional methyl groups may be added to the nucleotides at this end. This cap protects the mRNA from degradation in cytoplasm and helps in recognition by the ribosome.
3’ end – poly (A) polymerase adds a tail of 100-200 adenine nucleotides to 3’ end. It also performs functions like protection of mRNA and act as a signal for the attachment of the ribosome.
Then, introns (non-coding sequence) are removed from the primary transcript. At the end intron contain nucleotide sequences that are recognizable as splice sites, these indicate where the strand is to be cut. An assembly, the spliceosome, locates the splice sites and cuts out the intron. After the introns are cut out, the spliceosome attach the exons together. The intron bonds to itself and floats away. Introns are removed by multiple spliceosomes along the length of mRNA strand, and the leftover exons join together. Now the mRNA strand can leave the nucleus and encounter a ribosome, to start further process called translation.
Reverse Transcription
This concept is given on the page Reverse Transcription.