Transcription
Principles & Regulation
Explore how RNA polymerase reads DNA to synthesize mRNA, and discover the regulatory mechanisms that control gene expression at the transcriptional level.
Biochemistry
What Is Transcription?
Transcription is the first step of gene expression: the process by which the information encoded in a segment of DNA is copied into a complementary strand of messenger RNA (mRNA). This mRNA then serves as a template for protein synthesis during translation.
The Key Players in Transcription
RNA Polymerase
The enzyme that synthesizes RNA from a DNA template, reading \(3' \to 5'\) and building \(5' \to 3'\).
Promoter
A specific DNA sequence upstream of a gene where RNA polymerase and transcription factors bind to initiate transcription.
Template Strand
The \(3' \to 5'\) DNA strand that is read by RNA polymerase. The resulting mRNA is complementary to this strand.
The Central Dogma in Context
Three Stages of Transcription
1
Initiation: RNA polymerase binds to the promoter region. In eukaryotes, transcription factors help recruit RNA Pol II to the TATA box.
2
Elongation: RNA polymerase unwinds DNA and synthesizes the mRNA strand in the \(5' \to 3'\) direction, adding complementary ribonucleotides.
3
Termination: RNA polymerase reaches a terminator sequence, the mRNA transcript is released, and the DNA double helix re-forms.
The Transcription Bubble
Prokaryotic vs Eukaryotic Transcription
| Feature | Prokaryotic | Eukaryotic |
|---|---|---|
| RNA Polymerase | One type | Three types (I, II, III) |
| Promoter | -10 and -35 regions | TATA box, enhancers |
| Transcription Factors | Sigma factor (σ) | General TFs (TFIIA-H) |
| mRNA Processing | None (coupled with translation) | 5' cap, 3' poly-A, splicing |
| Location | Cytoplasm | Nucleus |
Eukaryotic mRNA Processing
Why Regulate Transcription?
Every cell in your body contains the same DNA, yet a neuron looks and functions entirely differently from a muscle cell. The difference lies in which genes are transcribed. Transcriptional regulation is the primary mechanism cells use to control gene expression.
Cell Differentiation
Different cell types express different subsets of genes, creating specialized functions.
Environmental Response
Cells adjust gene expression in response to signals like hormones, nutrients, or stress.
Disease Prevention
Misregulation of transcription can lead to cancer, developmental disorders, and other diseases.
Regulatory Elements on DNA
Activators and Repressors
Activators
Bind to enhancers and help recruit RNA polymerase to the promoter. They increase the rate of transcription, often by facilitating the assembly of the pre-initiation complex.
Repressors
Bind to silencers or directly to the promoter to block RNA polymerase. Some repressors recruit histone deacetylases (HDACs) to compact chromatin and silence genes.
The Lac Operon: A Model System
Epigenetic Regulation
Beyond DNA sequence-specific regulation, cells control transcription through epigenetic modifications: chemical changes to DNA or histone proteins that alter chromatin structure without changing the nucleotide sequence.
DNA Methylation
Adding methyl groups (\(-CH_3\)) to cytosine bases (CpG islands) typically silences gene expression.
Histone Acetylation
Adding acetyl groups loosens chromatin (euchromatin), making DNA more accessible to RNA polymerase.
Histone Methylation
Can either activate or silence genes depending on which histone residue is methylated (e.g., H3K4 vs H3K27).
How to Analyze a Transcription Problem
1
Identify the template strand. It runs \(3' \to 5'\). The mRNA will be complementary and antiparallel to it.
2
Write the mRNA sequence. Match A→U, T→A, G→C, C→G. The mRNA reads \(5' \to 3'\).
3
Verify with the coding strand. The mRNA should match the coding strand exactly, with U replacing T.
4
Consider regulation. Ask: Is the gene activated or repressed? What signals control its transcription?
Example 1: Writing an mRNA Sequence
1
Read the template \(3' \to 5'\): T-A-C-G-G-A-T-C
2
Apply base pairing (DNA → RNA): T→A, A→U, C→G, G→C
3
mRNA: \(5'\)-A U G C C U A G-\(3'\)
Example 2: Lac Operon Logic
1
Lactose present: Allolactose binds the lac repressor, removing it from the operator. Operon is derepressed.
2
Glucose also present: cAMP levels are low. CAP (catabolite activator protein) cannot bind the promoter.
3
Result: Without CAP activation, transcription occurs at low basal levels only. Glucose is the preferred carbon source.
Common Mistakes
✕ Confusing Strands
Writing mRNA from the coding strand instead of the template strand. The mRNA is complementary to the template (\(3' \to 5'\)) strand.
✕ Using T Instead of U
Forgetting that RNA uses uracil (U) in place of thymine (T). DNA → RNA: wherever you see A on the template, write U in the mRNA.
✕ Ignoring Directionality
mRNA is always synthesized \(5' \to 3'\). Writing it backwards produces an incorrect sequence that won't translate properly.
Regulation Summary
| Mechanism | Effect | Example |
|---|---|---|
| Activator + Enhancer | Increases transcription | CAP in lac operon |
| Repressor + Operator | Decreases transcription | Lac repressor |
| DNA Methylation | Silences genes | CpG island methylation |
| Histone Acetylation | Opens chromatin | HATs at active genes |
| Histone Deacetylation | Compacts chromatin | HDACs at silenced genes |
Question 1 of 5
RNA polymerase reads the DNA template strand in which direction?
RNA polymerase moves along the template strand in the \(3' \to 5'\) direction, synthesizing the new mRNA strand in the \(5' \to 3'\) direction.
Question 2 of 5
If the template DNA strand reads \(3'\)-A T G C C A-\(5'\), what is the mRNA sequence?
Apply complementary base pairing to the template strand: A→U, T→A, G→C, C→G. The mRNA is \(5'\)-U A C G G U-\(3'\). Remember: RNA uses U instead of T.
Question 3 of 5
In the lac operon, what happens when both glucose and lactose are present in the medium?
When glucose is present, cAMP levels are low, so CAP cannot bind the promoter. Lactose removes the repressor, but without CAP activation, only low basal transcription occurs. Maximal transcription requires lactose and no glucose.
Question 4 of 5
Which eukaryotic mRNA modification involves the removal of non-coding sequences?
RNA splicing removes introns (non-coding sequences) and joins exons (coding sequences) together. This is carried out by the spliceosome, a complex of snRNPs. The 5' cap and poly-A tail are separate modifications for stability and export.
Question 5 of 5
A researcher observes that a gene becomes silenced after heavy methylation of CpG islands in its promoter region. Which statement best explains this?
DNA methylation at CpG islands recruits methyl-CpG-binding proteins, which in turn recruit histone deacetylases (HDACs). HDACs remove acetyl groups from histones, causing chromatin to condense into heterochromatin, physically blocking RNA polymerase access.