Step by Step
Op
Operons — prokaryotic gene regulation
An operon is a cluster of genes under a single promoter, regulated together. The classic example is the lac operon in E. coli: three genes for lactose metabolism (lacZ, lacY, lacA) are controlled by one promoter and one repressor. When lactose is absent, the repressor binds the operator and blocks transcription. When lactose is present, it binds the repressor, changing its shape — repressor falls off, genes are transcribed.
Memory trick: lac operon = light switch. Repressor = the hand on the switch. Lactose = knocks the hand away → light (genes) turns ON.
TF
Transcription factors — eukaryotic gene regulation
Eukaryotes don't use operons. Instead, transcription factors (proteins) bind to specific DNA sequences (promoters, enhancers, silencers) to activate or repress transcription. Activators increase transcription. Repressors decrease it. Multiple transcription factors work together — combinatorial control.
Epi
Epigenetic regulation — DNA methylation and histone modification
Gene expression can be regulated without changing the DNA sequence. DNA methylation (adding –CH₃ to cytosine) typically silences genes. Histone acetylation loosens chromatin → gene activation. Histone methylation can activate or repress depending on location. These changes can be heritable.
Memory trick: Methylation = muffles the gene. Acetylation = activates (opens chromatin).
miR
Post-transcriptional regulation — miRNA
MicroRNA (miRNA) are small non-coding RNA molecules (~22 nucleotides) that bind to complementary sequences on mRNA and either degrade it or block translation. This provides another layer of gene expression control after transcription is complete.
Applied Walkthrough
1
E. coli has no lactose available. The lac repressor binds the operator sequence, blocking RNA polymerase. lacZ, lacY, lacA genes are OFF.
2
Lactose is added to the environment. Allolactose (a lactose derivative) binds the repressor, changing its shape. Repressor can no longer bind the operator.
3
RNA polymerase proceeds — lacZ, lacY, lacA are transcribed and translated. Lactose-digesting enzymes are produced.
4
When lactose is depleted, the repressor is free again and re-binds the operator — genes turn OFF. Efficient and elegant.
Exam Application
Exams heavily test the lac operon — know the components (promoter, operator, repressor, structural genes), what happens with and without lactose, and what happens with and without glucose (CAP/cAMP system). Also know: DNA methylation silences genes, histone acetylation activates genes, miRNA silences post-transcriptionally.
⚠ Common Trap
Students confuse the operator and the promoter — the promoter is where RNA polymerase binds; the operator is where the repressor binds (downstream of the promoter). Also: "inducible" operons (like lac) are normally OFF and turned ON by an inducer. "Repressible" operons (like trp) are normally ON and turned OFF by a corepressor. Don't mix them up.
✓ Quick Self-Check
1. What is an operon?
A cluster of prokaryotic genes under a single promoter, regulated together as a unit.
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2. What happens to the lac operon when lactose is present?
Allolactose binds the repressor, preventing it from binding the operator — RNA polymerase can proceed and transcribe the lac genes.
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3. What does DNA methylation generally do to gene expression?
It silences gene expression — methylated regions of DNA are typically transcriptionally inactive.
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4. What does histone acetylation do to chromatin and gene expression?
Acetylation loosens chromatin structure, making DNA more accessible to transcription factors — generally activates gene expression.
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5. How does miRNA regulate gene expression?
miRNA binds complementary sequences on mRNA, causing its degradation or blocking its translation — post-transcriptional gene silencing.
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