Transcription is the first major step in gene expression. It is the process where the DNA sequence of a gene is copied into messenger RNA (mRNA). This mRNA later guides protein synthesis. The accuracy of transcription is essential because even a small mistake can alter the structure or function of a protein. Special proteins called transcription factors help regulate when and how transcription occurs.
What Is Transcription?
Transcription occurs inside the nucleus of eukaryotic cells. It involves converting the DNA template into a single-stranded RNA molecule. This RNA is complementary to the DNA template.
Key Enzyme: RNA Polymerase
RNA polymerase is the main enzyme that performs transcription. Humans have three major types:
- RNA Polymerase I – makes rRNA
- RNA Polymerase II – makes mRNA
- RNA Polymerase III – makes tRNA
Stages of Transcription
1. Initiation
The process begins when RNA polymerase binds to a specific region on DNA known as the promoter. This region contains the TATA box or similar sequences that signal the starting point.
Transcription factors help RNA polymerase bind correctly and open the DNA double helix.
2. Elongation
Once RNA polymerase attaches, it moves along the DNA strand and adds RNA nucleotides (A, U, C, G) to build the mRNA molecule. The DNA strand used is called the template strand.
3. Termination
Transcription ends when the enzyme reaches a terminator signal on DNA. The mRNA strand is released and processed to become mature mRNA.
Promoter and Regulatory Sequences
The promoter is the “start signal” for transcription. It contains short DNA sequences that help position RNA polymerase. Common promoter elements include:
- TATA box – helps align transcription machinery
- CAAT box – improves binding strength
- GC-rich regions – regulate transcription speed
Other DNA elements like enhancers and silencers also influence transcription.
What Are Transcription Factors?
Transcription factors (TFs) are proteins that control transcription. They help RNA polymerase bind to DNA, increase or decrease gene activity, and ensure transcription happens only when needed.
Types of Transcription Factors
1. General Transcription Factors
These are required for transcription of all genes. They assemble at the promoter and recruit RNA polymerase II.
2. Specific Transcription Factors
These regulate specific sets of genes.
- Activators – increase transcription by helping RNA polymerase bind better
- Repressors – reduce transcription by blocking promoter access
Functions of Transcription Factors
- Turn genes “on” or “off”
- Control response to hormones
- Help cells adapt to stress
- Guide development and cell differentiation
Role of Enhancers and Silencers
Enhancers are DNA regions that boost transcription. They may be far from the gene but communicate through DNA looping.
Silencers, on the other hand, decrease transcription by attracting repressors.
Post-Transcriptional Modifications
Before mRNA leaves the nucleus, it undergoes processing:
- 5′ cap addition
- Poly-A tail addition
- Removal of introns (splicing)
These steps ensure mRNA stability and proper translation.
Importance of Proper Transcription
Accurate transcription ensures that cells produce correct proteins. Errors in transcription or malfunctioning transcription factors can lead to:
- Cancer
- Developmental disorders
- Immune-related diseases
Detailed Notes:
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