Transcription is the process of copying genetic information from one strand of DNA into RNA. In this only one segment of DNA and only one strand is copied in RNA. The Adenosine forms base pairs with Uracil instead of Thymine.
Why both the strands are not copied during transcription?
Both the strands of DNA are not copied during transcription because:
- If both strands act as acts as a template, they would code for RNA molecule with different sequences, and in turn, if they code for proteins the sequence of amino acids in the proteins would be different. Hence, one segment of DNA would be coding for two different proteins.
- The two RNA molecules if produced simultaneously would be complementary to each other, hence would form dsRNA. This would prevent RNA from being translated into protein.
Transcription of DNA
Transcription of DNA includes:
- A Promoter
- A Structural Gene
- A terminator
DNA-dependent RNA polymerase also catalyzes the polymerization in only one direction i.e’ 5’ to 3’.
It is situated at the 5’side of the gene or upstream. RNA polymerase binds here to start the transcription.
The structural gene is the region between promoter and terminator.
It is situated at the 3’ side of the coding strand and marks the end of the transcription process.
Other terms of Transcription
The monocistronic structural gene has interrupted coding sequences, the gene is a eukaryotic gene in eukaryotes is called split genes.
Exons are coding sequences that or which appear in mature and processed RNA.
Introns are intervening sequences that do not appear in nature and processed RNA.
Difference between Template Strand and Coding Strand
It is a DNA with 3’ to 5’ polarity.
It is a DNA with 5’ to 3’ polarity.
Act as a template for transcription and codes for RNA
It does not code for any region of RNA during transcription.
Transcription in a Prokaryotic Cell
In bacteria, a single DNA-dependent RNA polymerase catalyzes the transcription of all the three RNA’s present:
Stages of Transcription :
RNA polymerase binds to the initiation factor, at the promoter site to start the process of transcription. Sigma factor helps in binding.
RNA polymerase is only capable of elongation. Ribonucleotides are added to the template strand that enables the growth of mRNA growth.
When the RNA polymerase reaches the terminator region, it binds with the terminator factor, to terminate the process and the nascent RNA falls off. Rho factor helps in binding.
In bacteria, mRNA does not require further processing and as the nucleus and cystol are not the separate translation is coupled with transcription and is initiated before the full transcription mRNA occurs.
Transcription in Eukaryotic Cell
There is three DNA dependent RNA polymerase to catalyze transcription of different RNA in a eukaryotic cell:
RNA Polymerase I: rRNA, 23s RNA.
RNA Polymerase II: mRNA, hnRNA, snRNA.
RNA Polymerase III: tRNA, snRNA.
mRNA provides the template
tRNA brings the amino acids and read the genetic code.
rRNA play a structural and catalytic role during the translation
The primary transcript contains both exon and intron and is non-functional.
Stages of Transcription:
The process of removing introns and joining together exons in a defined sequence.
Spliceosomes are responsible for the removal of introns. Spliceosomes are absent in prokaryotes.
hnRNA undergoes capping and tailing.
In capping a methyl guanosine triphosphate (MGppp) is added to the 5’ end of hnRNA. Methyl guanosine triphosphate (MGppp) is added to decrease the rate of mutation.
In tailing 200-300 adenylate residues are added at 3’end to transport mRNA outside the nucleus.