Transcription: RNA Synthesis from DNA | NEET Biology Notes
Transcription: Process of RNA Synthesis from DNA | NEET Biology Notes
Transcription is the process of synthesis of RNA from DNA. It is an important step of gene expression in which genetic information present in DNA is copied into RNA.
1. Principle of Complementarity
Transcription is based on the principle of complementarity.
- Non-template / Coding / Sense strand: 5′ → 3′
- Template / Non-coding / Antisense / Master strand: 3′ → 5′
- RNA is synthesized complementary to the template DNA strand.
During transcription, only a part of DNA containing information for a particular protein is transcribed.
Only one strand of DNA, generally the 3′ → 5′ template strand, acts as the template for RNA synthesis.
If both strands of DNA acted as templates, two complementary RNA molecules would be produced. These RNA molecules could pair with each other to form double-stranded RNA, which could interfere with translation.
2. Transcription Unit
A part of double-stranded DNA that takes part in transcription is called a transcription unit.
Components of a Transcription Unit
- Promoter
- Structural gene
- Terminator
Promoter
- The promoter is located upstream of the structural gene.
- It is located at the 5′ end with respect to the coding strand.
- The promoter provides a site for attachment of RNA polymerase.
Terminator
- The terminator is located downstream of the structural gene.
- It is located at the 3′ end with respect to the coding strand.
- It signals the end of transcription.
Structural Gene
Structural genes contain information required for the synthesis of RNA or protein.
- In eukaryotes, structural genes are generally monocistronic, carrying information for one polypeptide.
- In prokaryotes, structural genes may be polycistronic, carrying information for more than one polypeptide.
Cistron
A cistron is a functional unit of a gene containing information for a particular polypeptide.
3. Recognition Sites of Promoters
In Prokaryotes
Important promoter recognition sequences include:
- Pribnow box: approximately TATAAT
- −35 region: approximately TTGACA
In Eukaryotes
- Hogness box / TATA box: approximately TATA
- CAAT box
Promoter sequences are important for recognition and initiation of transcription.
4. RNA Polymerase
RNA Polymerase in Prokaryotes
Prokaryotes possess one major RNA polymerase responsible for transcription.
It consists of:
- Core enzyme
- Sigma (σ) factor
The sigma factor helps RNA polymerase recognize the promoter region and initiate transcription.
The core enzyme performs the catalytic activity required for RNA chain elongation.
5. RNA Polymerases in Eukaryotes
| RNA Polymerase | Major Products |
|---|---|
| RNA Polymerase I | 5.8S rRNA, 18S rRNA and 28S rRNA |
| RNA Polymerase II | hnRNA / precursor mRNA and mRNA |
| RNA Polymerase III | 5S rRNA, tRNA and other small RNAs |
NEET Tip: Remember: Pol I → rRNA, Pol II → mRNA, Pol III → tRNA + 5S rRNA.
6. Mechanism of Transcription in Prokaryotes
The mechanism of transcription can be divided into three major stages:
I. Chain Initiation
The promoter region is recognized by the sigma (σ) factor. RNA polymerase then attaches to the promoter and initiates RNA synthesis.
II. Chain Elongation
The RNA chain is elongated continuously in the 5′ → 3′ direction.
III. Chain Termination
Termination occurs when RNA polymerase reaches the termination region. The newly synthesized RNA is then released.
7. Transcription in Eukaryotes
The basic mechanism of transcription is similar in prokaryotes and eukaryotes, but eukaryotic transcription is followed by extensive RNA processing.
The initial RNA transcript is called hnRNA (heterogeneous nuclear RNA).
hnRNA contains both:
- Exons – expressed/coding sequences retained in mature mRNA.
- Introns – intervening/non-coding sequences removed during RNA processing.
8. Processing of hnRNA
Processing of hnRNA takes place in the nucleus.
The major processing steps are:
- 5′ Capping
- 3′ Poly-A tailing
- Splicing
5′ Capping
A 7-methylguanosine (7-mG) cap is added at the 5′ end of hnRNA.
The 7-mG cap:
- Helps in recognition of mRNA.
- Provides a site involved in ribosome binding during translation.
- Contributes to mRNA stability.
3′ Poly-A Tail
A poly-A tail is added at the 3′ end of hnRNA. It consists of many adenine residues and contributes to the stability and proper functioning of mature mRNA.
Splicing
Splicing is the removal of introns from hnRNA followed by joining of exons.
Splicing is carried out by the spliceosome.
hnRNA → Capping → Poly-A tailing → Splicing → Mature mRNA
9. Transcription Unit: Simple Diagram
Promoter → Structural Gene → Terminator
RNA polymerase binds → RNA synthesis → Transcription terminates
10. Differences Between Prokaryotic and Eukaryotic Transcription
| Feature | Prokaryotes | Eukaryotes |
|---|---|---|
| Site | Cytoplasm | Nucleus |
| RNA polymerase | One major type | Three major types |
| Promoter recognition | Sigma factor | Transcription factors |
| RNA processing | Limited | Extensive |
| Introns | Usually absent | Common in many genes |
| 5′ Capping | Generally absent | Present |
| Poly-A tail | Generally absent in bacterial mRNA | Present |
| Splicing | Generally absent | Common |
11. Central Dogma and Transcription
The flow of genetic information during normal gene expression can be represented as:
DNA → Transcription → RNA → Translation → Protein
Thus, transcription represents the transfer of genetic information from DNA to RNA.
12. Important NEET Biology Points
- Template DNA strand runs in the 3′ → 5′ direction.
- RNA is synthesized in the 5′ → 3′ direction.
- The promoter is located upstream of the structural gene.
- The terminator is located downstream of the structural gene.
- Sigma factor helps bacterial RNA polymerase recognize the promoter.
- RNA polymerase II synthesizes hnRNA/precursor mRNA.
- hnRNA undergoes capping, polyadenylation and splicing.
- Introns are removed and exons are joined.
- Mature eukaryotic mRNA contains a 5′ cap and 3′ poly-A tail.
Frequently Asked Questions
What is transcription?
Transcription is the synthesis of RNA from a DNA template.
Which DNA strand acts as the template during transcription?
The DNA strand running in the 3′ → 5′ direction acts as the template for RNA synthesis.
What is the direction of RNA synthesis?
RNA is synthesized in the 5′ → 3′ direction.
What is a transcription unit?
A transcription unit is a segment of DNA containing a promoter, structural gene and terminator.
Which RNA polymerase synthesizes mRNA in eukaryotes?
RNA polymerase II synthesizes hnRNA, which is processed to form mature mRNA.
What is splicing?
Splicing is the removal of introns and joining of exons during RNA processing.
Related Topics:
Molecular Basis of Inheritance | DNA Replication | Genetic Code | Translation | NEET Biology
Conclusion: Transcription is a fundamental process of gene expression in which genetic information encoded in DNA is transferred to RNA. Understanding the template strand, promoter, RNA polymerase, transcription unit and RNA processing is essential for mastering the Molecular Basis of Inheritance chapter for NEET and CBSE Biology.
