Transcription is the first step in gene expression, where DNA is converted into RNA. This process involves unwinding the DNA double helix, RNA polymerase binding to the promoter, and nucleotides adding to the growing RNA strand following base-pairing rules.
The transcription process has three main stages: initiation, elongation, and termination. Each stage involves specific molecular events and proteins, ensuring accurate gene expression and RNA production. Post-transcriptional modifications further refine the RNA molecule.
Transcription Process
DNA to RNA Conversion
- Transcription copies genetic information from DNA to RNA as the first step in gene expression
- Process begins with unwinding of DNA double helix at the gene to be transcribed, exposing the template strand
- RNA polymerase binds to the promoter region of the gene to initiate transcription
- Nucleotides add to growing RNA strand complementary to DNA template strand following base-pairing rules (A-U, C-G)
- Transcription proceeds 5' to 3' direction as RNA polymerase moves along DNA template strand
Termination and Post-Transcriptional Modifications
- Process terminates when RNA polymerase encounters termination sequence, releasing newly synthesized RNA molecule
- In eukaryotes, primary transcript undergoes post-transcriptional modifications
- Addition of 5' cap protects mRNA from degradation and aids in ribosome binding
- Poly-A tail added to 3' end enhances mRNA stability and facilitates export from nucleus
- Splicing removes introns and joins exons in eukaryotic pre-mRNA to produce mature mRNA
Template vs Coding Strands
Strand Identification and Function
- DNA double-stranded with one strand serving as template for RNA synthesis during transcription
- Template strand (antisense strand) complementary to RNA transcript being produced
- Coding strand (sense strand) has same sequence as RNA transcript except thymine (T) replaced by uracil (U) in RNA
- Template strand read 3' to 5' direction by RNA polymerase while RNA synthesized 5' to 3' direction
- Identify template strand by comparing DNA sequence to resulting RNA sequence accounting for complementary base pairing
Coding Strand Applications
- Coding strand directly determines amino acid sequence of protein product
- Matches sequence of mRNA with T replaced by U
- Used in molecular biology techniques (PCR primers, gene synthesis)
- Helps predict potential regulatory sequences (promoters, enhancers) on same strand as gene
Role of RNA Polymerase
Enzyme Function and Characteristics
- RNA polymerase catalyzes synthesis of RNA during transcription
- Recognizes and binds to specific DNA sequences called promoters initiating transcription process
- Catalyzes formation of phosphodiester bonds between ribonucleotides elongating RNA chain
- Unwinds DNA double helix as it moves along template strand creating transcription bubble
- Ensures accuracy through proofreading function removing and replacing incorrectly added nucleotides
RNA Polymerase Types and Regulation
- Prokaryotes use single RNA polymerase to transcribe all types of RNA
- Eukaryotes have multiple RNA polymerases for different RNA species
- RNA Polymerase I transcribes rRNA genes
- RNA Polymerase II transcribes mRNA and some snRNAs
- RNA Polymerase III transcribes tRNA and 5S rRNA
- Interacts with transcription factors and regulatory proteins to modulate gene expression
- General transcription factors (TFIIA, TFIIB, TFIID, etc.) in eukaryotes
- Sigma factors in prokaryotes
Stages of Transcription
Initiation
- First stage involving binding of RNA polymerase to promoter region of gene
- Eukaryotes require assembly of general transcription factors to form preinitiation complex
- TATA-binding protein (TBP) recognizes TATA box in promoter
- Other factors (TFIIA, TFIIB, TFIID, etc.) recruit RNA polymerase II
- DNA double helix unwound exposing template strand for transcription
- Formation of open complex allows RNA polymerase to begin RNA synthesis
Elongation
- Second stage where RNA polymerase moves along template strand synthesizing RNA transcript
- Nucleotides added to growing RNA chain complementary to DNA template
- Transcription bubble moves along DNA as RNA polymerase progresses
- Elongation factors (TFIIS, TFIIF) enhance processivity and fidelity of transcription
- RNA-DNA hybrid forms temporarily behind RNA polymerase
Termination
- Final stage where transcription ends and RNA transcript released
- Prokaryotic termination occurs through two mechanisms:
- Rho-dependent termination involves Rho protein
- Rho-independent termination uses hairpin structures in RNA
- Eukaryotic termination involves recognition of specific sequences and action of termination factors
- Poly(A) signal sequence in pre-mRNA triggers termination
- Cleavage and polyadenylation specificity factor (CPSF) and other proteins facilitate process
- Each stage has distinct molecular events and requires different sets of proteins and factors
- Transitions between stages regulated to ensure proper gene expression and RNA production