Transcription is a layman term used in a variety of contexts. You might have heard or transcription in a classroom where teachers dictate the keynotes. At this point of time you will pen down the words in a rough note and you will later transcribe it to the original notes. So, transcription is generally called as copying something in a meaningful way. In genetics, transcription is a process through which a gene's DNA sequence is copied ( transcribed ) to form an RNA molecule. it is the first of various steps of DNA based gene expression in which a particular segment of DNA is copied into RNA with the help of the enzyme RNA polymerase. Both DNA and RNA are nucleic acids, which use base pairs of nucleotides as a complementary language. Transcription involves the activation of ribonucleotides, the formation of DNA template, base pairing, chain formation, separation of RNA and duplex formation. The whole idea of these steps is to synthesize proteins and this post will take you through the steps involved in the transcription.
Steps of Transcription
1. Activation of Ribo-nucleotides
It is the first phase of transcription. During the activation of ribonucleotides, zyme phosphorylase along with the energy and phosphorylated Ribonucleotides are activated. The activated Ribonucleotides includes adenosine triphosphate (ATP), guanosine triphosphate (GTP), uridine triphosphate (UTP) and cytidine triphosphate (CTP).
2. Formation Of DNA Template
In order to form a template, 4 major regions are required, namely, initiation site, a promoter region, coding region, and a terminator region. As the name indicates, the initiation site help in starting the transcription process. A promoter region has an RNA polymerase recognition site and binding site. In a coding region, the chain opening occurs. Chain opening needs unwindases, gyrases, and single-stranded binding proteins. In a terminator region, the two strands of DNA uncoil progressively from the site of polymerase binding. One of the two strands of DNA (3’—» 5′) functions as a template for transcription of RNA. It is called a master template or antisense strand.
3. Base Pairing
The Ribonucleoside triphosphates come closer to lie just opposite to the nitrogenous bases of the DNA template (Antisense strand). This results in the formation of complementary pairs, I,e U goes opposite A, A goes opposite T, С goes opposite G, and G goes opposite C. A pyrophosphate is released from each ribonucleoside triphosphate to form Ribonucleotides. The pyrophosphate is hydrolyzed with the help of enzyme pyrophosphatase. It releases energy.
4. Chain Formation
With the help of RNA polymerase, the adjacent ribonucleotides are held tightly against the DNA template to form an RNA chain.
5. Separation of RNA
Separation is enabled by ATPase activity. This helps in the release of the completed RNA chain from the previous phase of chain formation. The released RNA is called primary transcript which will further get processed to form a functional RNA.
Once the primary transcript is released, the 2 strands of DNA form linkages amongst complementary base pairs. At this stage, Gyrases, unwindases and SSB proteins are released, consequently, there is double-helical DNA formation.
7. Post-Transcription Processing
The primary transcript is often larger than the functional RNAs. It is called heterogeneous or hnRNA especially in case of mRNA. Post-transcription processing helps to convert the primary transcript into functional RNAs.