Definition
Transcription is the process by which RNA is synthesized using a DNA segment as a template.
General Characteristics
- DNA stores genetic information.
- Genes are expressed via transcription into RNA.
- RNA is synthesized from the 5' to 3' direction.
- RNA and DNA strands are antiparallel and complementary.
- Specific promoter and terminator sequences define start and end.
Enzymes Involved
DNA-dependent RNA Polymerase:
- Separates DNA strands temporarily.
- Uses one DNA strand as a template.
- Catalyzes phosphodiester bond formation.
- Involved in gene regulation.
Structure: Complex quaternary protein.
Energy in Transcription
- Ribonucleotides (ATP, UTP, GTP, CTP) provide energy.
- Energy released from phosphate bond cleavage.
- Pyrophosphate hydrolyzed by pyrophosphatase, pushing reaction forward.
Stages of Transcription
1. Initiation
- Transcription factors bind TATA box (promoter).
- RNA Polymerase forms the transcription initiation complex.
- DNA unwinds, and phosphodiester bonds begin forming.
2. Elongation
- RNA Polymerase adds complementary nucleotides to the 3' end of RNA.
- DNA helix reforms after polymerase passes.
- Topoisomerases relieve supercoiling.
3. Termination
- RNA Polymerase halts at terminator sequence.
- Transcript is released.