Rucete ✏ Campbell Biology In a Nutshell
Unit 3 GENETICS — Concept 17.2 Transcription Is the DNA-Directed Synthesis of RNA: A Closer Look
Transcription is the process by which DNA is used as a template to synthesize messenger RNA (mRNA). This RNA serves as an intermediary, carrying genetic instructions from DNA to the cell’s protein-making machinery.
Molecular Components of Transcription
- Messenger RNA (mRNA) carries genetic information from DNA to ribosomes
- RNA polymerase separates DNA strands and synthesizes complementary RNA nucleotides (5′→3′)
- Unlike DNA polymerase, RNA polymerase doesn’t need a primer and can start an RNA chain from scratch
- Promoter: DNA sequence marking transcription initiation
- Terminator: DNA sequence signaling transcription end (in bacteria)
- Downstream: direction of transcription
- Upstream: opposite direction (promoter is upstream)
- The transcribed DNA region is called the transcription unit
- Bacteria use one RNA polymerase; eukaryotes have multiple types (RNA polymerase II synthesizes mRNA)
Stages of Transcription
1. Initiation
- RNA polymerase binds to the promoter and unwinds DNA strands
- In eukaryotes, transcription factors assist RNA polymerase in binding precisely
- The TATA box is a key promoter sequence that helps initiate transcription
- RNA polymerase and transcription factors form the transcription initiation complex
2. Elongation
- RNA polymerase moves along DNA, untwisting and adding RNA nucleotides at the 3′ end
- DNA rewinds behind the polymerase
- Multiple RNA polymerases can transcribe a gene simultaneously to increase mRNA output
3. Termination
- Bacteria: RNA polymerase stops at the terminator; the RNA transcript is ready for use
- Eukaryotes: RNA polymerase II transcribes a polyadenylation signal (AAUAAA)
- Proteins bind to this signal and cleave the RNA; RNA polymerase continues briefly then is released
- Resulting RNA (pre-mRNA) requires further processing
In a Nutshell
Transcription is the synthesis of RNA from a DNA template, involving initiation (promoter binding), elongation (RNA synthesis), and termination (RNA release). Eukaryotic transcription is more complex, involving multiple polymerases and RNA processing.