Transcription: Mechanism and Enzymes
Transcription is the fundamental process of synthesizing RNA from a DNA template. This is the first step in gene expression, where the genetic information encoded in DNA is copied into a complementary RNA molecule. This RNA molecule then serves various roles, most notably as messenger RNA (mRNA) carrying the code for protein synthesis.
The Central Dogma of Molecular Biology
Transcription is a key component of the central dogma, which describes the flow of genetic information within a biological system: DNA → RNA → Protein. Understanding transcription is crucial for grasping how genes are expressed and how cellular functions are regulated.
Transcription is the synthesis of RNA from a DNA template.
This process involves unwinding the DNA double helix and using one strand as a template to build a complementary RNA strand. It's a highly regulated process essential for gene expression.
The process begins when RNA polymerase binds to a specific region on the DNA called the promoter. This binding initiates the unwinding of the DNA double helix, exposing the nucleotide bases. RNA polymerase then moves along the template strand, adding complementary ribonucleotides (A, U, G, C) to the growing RNA chain. The process continues until RNA polymerase reaches a terminator sequence, signaling the end of transcription.
Key Enzymes in Transcription
The primary enzyme responsible for transcription is RNA polymerase. However, in eukaryotes, a complex set of proteins, including transcription factors, are also essential for initiating and regulating the process.
| Enzyme/Factor | Role in Transcription | Prokaryotes vs. Eukaryotes |
|---|---|---|
| RNA Polymerase | Synthesizes RNA by reading DNA template | Single type (core enzyme + sigma factor) in prokaryotes; Multiple types (I, II, III) in eukaryotes |
| Sigma Factor (Prokaryotes) | Recognizes and binds to promoter sequences | Essential for initiation in prokaryotes; absent in eukaryotes |
| Transcription Factors (Eukaryotes) | Bind to promoters and regulatory sequences to recruit RNA polymerase II | Crucial for regulating gene expression in eukaryotes |
Stages of Transcription
Loading diagram...
Transcription proceeds through three main stages: Initiation, Elongation, and Termination.
Initiation
This is the most regulated step. It involves the binding of RNA polymerase to the promoter region of the DNA. In prokaryotes, the sigma factor helps RNA polymerase recognize and bind to the promoter. In eukaryotes, general transcription factors bind to the promoter first, creating a platform for RNA polymerase II to bind.
Elongation
Once initiated, RNA polymerase moves along the DNA template strand, unwinding the helix ahead of it and rewinding it behind. It reads the DNA sequence and synthesizes a complementary RNA molecule by adding ribonucleotides. The RNA molecule is synthesized in the 5' to 3' direction.
The process of RNA synthesis by RNA polymerase involves the formation of a transcription bubble. Within this bubble, the DNA template strand is exposed, and incoming ribonucleoside triphosphates are matched to their complementary bases on the DNA. The polymerase catalyzes the formation of phosphodiester bonds, extending the RNA chain. The newly synthesized RNA strand detaches from the DNA template as the polymerase moves forward, and the DNA double helix reforms.
Text-based content
Library pages focus on text content
Termination
Transcription stops when RNA polymerase encounters a terminator sequence. In prokaryotes, this can occur via a Rho-dependent or Rho-independent mechanism. In eukaryotes, termination is more complex and involves specific sequences and protein factors that signal the release of the RNA transcript and the polymerase from the DNA.
Types of RNA Transcribed
Transcription produces various types of RNA, each with specific functions:
- Messenger RNA (mRNA): Carries genetic code from DNA to ribosomes for protein synthesis.
- Transfer RNA (tRNA): Involved in protein synthesis by bringing specific amino acids to the ribosome.
- Ribosomal RNA (rRNA): A structural and catalytic component of ribosomes.
- Small nuclear RNA (snRNA), microRNA (miRNA), small interfering RNA (siRNA): Involved in gene regulation and RNA processing.
Understanding the precise binding of RNA polymerase to the promoter and the role of transcription factors is crucial for comprehending gene regulation.
RNA polymerase.
Initiation, Elongation, and Termination.
Learning Resources
A clear and concise video explaining the overview of transcription, its stages, and key players.
An accessible article from Nature Scitable that breaks down the fundamental concepts of transcription.
A comprehensive overview of transcription, including its mechanism, regulation, and importance in genetics.
An excerpt from the renowned textbook 'Molecular Biology of the Cell' detailing the intricacies of transcription.
A detailed explanation of RNA polymerase, its structure, function, and different types.
Information on transcription factors, their role in gene regulation, and how they interact with DNA and RNA polymerase.
An engaging and visually rich video that explains transcription in a dynamic way.
A detailed animated video focusing on the step-by-step mechanism of transcription.
A focused video explaining the complex process of transcription initiation in eukaryotic cells.
A clear definition and explanation of transcription from the National Human Genome Research Institute.