Transcription
By Aarav Ghate | 7 September 2024
Transcription is the first step in the production of proteins using information present in DNA. It involves transferring the genetic information encoded in DNA to a primary RNA transcript. It is carried out by an enzyme called DNA Dependent RNA Polymerase and modulated by entities called transcription factors.Transcription factors bind to certain segments of DNA, known as promoter sites, to activate or inhibit the transcription of those genes. Transcription factors and RNA Polymerase together form the transcription initiation complex.
DNA is normally present in a double helix, but for the purpose of transcription, part of the DNA helix is unwound using an enzyme called helicase. This gives us two strands of DNA, which are known as template strand and coding strand. The template strand runs 3'-5' and is acted on by DNA Dependent RNA Polymerase (DNRP) and other enzymes, whereas the RNA transcript has nucleotides equivalent (thymine being replaced by uracil) to the coding strand.
A primer molecule attaches complementary nucleotides to the template strand to begin the transcription. RNA Polymerase continues to attach complementary bases and the transcribed mRNA begins to form. The bases are added on the 3’ end of the transcript, that is, polymerisation proceeds in the 5'-3' direction. The 5’ end is formed by the primer nucleotides. The process continues until the RNA Polymerase encounters a segment of DNA called the terminator sequence. This triggers the release of the RNA Polymerase from the transcription initiation complex and the process of transcription ends.
Other than its role in gene expression, transcription also plays an important role in gene regulation. Regulation is the control of quantity, location and appropriate timing for the production of genes. Transcription factors play an important role here since they are responsible for choosing which sequences should be transcribed. The transcription process does not begin without the binding of a transcription factor, hence they control the timing of the process. There are other factors which play a role in gene expression regulation, such as the operon system in prokaryotes.
Transcription is just the initial step of gene expression and where the major part of gene regulation takes place. It is followed by translation.
References
- Crick F, On Protein Synthesis, Symposia for the Society of Experimental Biology, (1958)
- Latchman D, Transcription Factors: an Overview, Int J Exp Pathol. (1993)