Regulation of Gene Expression 🧬

Cells selectively switch genes on or off so they make the right proteins at the right time. Eukaryotes regulate genes at four checkpoints: transcription, RNA processing, mRNA transport, and translation. :contentReference[oaicite:10]{index=10}

1 Levels of Control in Eukaryotes 🏗️

• Transcriptional control – the cell decides whether to start the primary transcript.
• Processing control – splicing choices decide the final mRNA recipe.
• mRNA transport – only exported transcripts reach cytoplasmic ribosomes.
• Translational control – the cell can pause or speed-up protein synthesis from an existing mRNA. :contentReference[oaicite:11]{index=11}

2 Why genes need switches 🔄

Escherichia coli makes the enzyme β-galactosidase only when lactose is around, because the enzyme splits lactose into glucose + galactose for energy. Without lactose, making the enzyme would waste energy. :contentReference[oaicite:12]{index=12}

3 Regulatory parts in Prokaryotes 🦠

• Promoter – landing pad for RNA polymerase.
• Operator – DNA gatekeeper next to the promoter; a repressor can sit here and block RNA polymerase. :contentReference[oaicite:13]{index=13}
• Regulatory proteins
  • Activators (positive regulators) help RNA polymerase start.
  • Repressors (negative regulators) block the start. :contentReference[oaicite:14]{index=14}

4 The Classic lac Operon 🥛

4.1 Players

These three structural genes lie downstream of one promoter and one operator, so they behave as a polycistronic unit. :contentReference[oaicite:15]{index=15}

4.2 “Off” state 😴

The repressor (from i) binds the operator and blocks RNA polymerase. Lactose cannot be metabolised, and only a trickle of permease is made. :contentReference[oaicite:16]{index=16}

4.3 “On” state ⚡

1. Lactose (or its isomer allolactose) enters via basal permease.
2. Lactose binds the repressor, changing its shape so it falls off the operator.
3. RNA polymerase now transcribes the z-y-a cassette → a single mRNA → three enzymes.

This is negative regulation because removing the repressor “releases the brake.” (It also faces positive control mechanisms not covered here.) :contentReference[oaicite:17]{index=17}

4.4 Key take-aways 💡

• The substrate (lactose) acts as an inducer — the cell makes the enzymes only when it can actually use them. :contentReference[oaicite:18]{index=18}
• Glucose or galactose cannot induce the operon, so the switch is specific.

5 High-Yield Ideas for NEET 🎯

1. Four regulation levels in eukaryotes — transcription, processing, export, translation.
2. Operon concept — one promoter can drive several structural genes (example = lac operon).
3. Inducible vs. constitutive synthesis — lactose removes the lac repressor; i gene expresses constitutively.
4. Role of operator + repressor — precise DNA–protein pairing decides whether transcription begins.
5. Metabolic control example — β-galactosidase produced only when its substrate is available.