Molecular Biology
Transcription
DNA → RNA — the first step in gene expression
Transcription copies a gene's DNA sequence into mRNA (messenger RNA). Uses RNA polymerase. Three steps: (1) Initiation — RNA pol binds promoter (with help of transcription factors). (2) Elongation — adds RNA nucleotides 5' to 3', using one DNA strand as template. (3) Termination — RNA released. RNA differs from DNA: ribose (not deoxyribose), uracil (replaces thymine), single-stranded. Eukaryotes: mRNA processed (5' cap, 3' poly-A tail, splicing of introns). Drivers: gene regulation (when/which genes transcribed) defines cell type and response. Foundation of central dogma: DNA → RNA → protein.
- EnzymeRNA polymerase
- Direction5' to 3' (RNA synthesized)
- Three stepsInitiation, elongation, termination
- RNA differencesRibose, uracil, single-strand
- Eukaryote processing5' cap, 3' poly-A tail, splicing
- DiscoveryRoger Kornberg (Nobel 2006) for eukaryotic mechanism
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Why transcription matters
- Gene expression. First step in making proteins.
- Regulation. Determines what cell does.
- Development. Cell differentiation requires specific transcription.
- Disease. Mutations in TFs cause many diseases.
- Drug targets. Many drugs target transcription.
- Biotechnology. Recombinant protein production.
- Cancer. Dysregulated transcription is hallmark.
Common misconceptions
- Both strands transcribed. Only one strand at a time per gene.
- Transcription = translation. Different processes; transcription makes mRNA.
- All transcripts become protein. Many non-coding RNAs.
- One gene = one protein. Alternative splicing produces multiple.
- RNA polymerase same as DNA polymerase. Different enzymes.
- Transcription continuous. Highly regulated; on/off.
Frequently asked questions
How does transcription work?
RNA polymerase binds promoter (DNA sequence upstream of gene). Unwinds DNA; uses one strand as template (template strand). Adds RNA nucleotides complementary to template (A pairs with U; T pairs with A; G pairs with C; C pairs with G). Synthesis 5' to 3'. Termination signal stops transcription; RNA released.
What's a promoter?
DNA sequence upstream of gene that recruits RNA polymerase. Bacterial: -10 box (TATAAT) and -35 box (TTGACA) before transcription start. Eukaryotic: TATA box (TATAAA, ~25 bp upstream) bound by TBP (TATA binding protein) of TFIID. Plus enhancers (further away) and silencers regulate transcription. Different promoters → different expression levels.
What's RNA processing in eukaryotes?
Pre-mRNA modified before export. (1) 5' cap: 7-methylguanosine added to 5' end (protects from degradation). (2) 3' poly-A tail: ~200 adenines added (stability, translation). (3) Splicing: introns (non-coding) removed; exons (coding) joined. Done by spliceosome (RNA-protein complex). Alternative splicing: same gene → different mRNA → different protein. Major source of human protein diversity.
What's the difference between bacteria and eukaryotes?
Bacteria: one RNA polymerase; transcription and translation coupled (ribosomes start translating mRNA before transcription complete). Eukaryotes: three RNA polymerases (Pol I = rRNA, Pol II = mRNA, Pol III = tRNA + small RNAs). Transcription in nucleus; translation in cytoplasm. Pre-mRNA processed before export.
How is transcription regulated?
Multiple levels. (1) Transcription factors bind promoter/enhancers — activate or repress. (2) Chromatin structure: DNA wrapped around histones; tight chromatin (heterochromatin) — not transcribed; open (euchromatin) — accessible. (3) DNA methylation silences genes. (4) RNA stability: how long mRNA persists. Combined: cells turn genes on/off in response to signals.
What are transcription factors?
Proteins that bind DNA to regulate transcription. Activators: increase transcription. Repressors: decrease. Bind specific DNA sequences (e.g., enhancers). Combinatorial: many TFs work together. Important: cell differentiation (different TFs active in different cell types), development, signaling. Mutations in TFs: many cancers, developmental disorders.
What about non-coding RNAs?
Many RNAs not translated. tRNA (transfers amino acids during translation), rRNA (forms ribosomes), miRNA (microRNAs — regulate gene expression), lncRNA (long non-coding; various functions), siRNA, snRNA, etc. ~98% of human genome transcribed; only ~1.5% codes proteins. Non-coding RNAs do regulation, structure, catalysis, processing.