Describe the lac operon (inducible system)

3.5.8 · Biology › Mutations & Gene Regulation

lac operon kya hai?

Yeh kyun zaroori hai?

• Protein synthesis mein cell ke ATP budget ka ~20% kharch hota hai
• Lactose ke bina lactose enzymes banana = energy ki barbaadi
• Selection pressure un bacteria ko favor karta hai jo efficiently regulate karte hain
• Yeh mechanism explain karta hai ki 1,000+ operons apne metabolism ko kaise coordinate karte hain

Molecular Players

1. Structural Genes (the workers)

• lacZ: β-galactosidase encode karta hai → lactose ko glucose + galactose mein cleave karta hai
• lacY: permease encode karta hai → lactose ko cell ke andar pump karta hai
• lacA: transacetylase encode karta hai → lactose metabolites ko modify karta hai (kam critical)

Ek hi operon mein teen genes kyun? Teeno lactose metabolism mein saath kaam karte hain. Inhe ek promoter ke under link karna matlab hai inhe ek polycistronic mRNA ke roop mein transcribe kiya jaata hai → saath translate hote hain → coordinated response milta hai.

2. Regulatory Elements (the control panel)

• Promoter (P): RNA polymerase binding site
• Operator (O): Repressor protein binding site (promoter ke saath overlap karta hai)
• CAP-cAMP binding site: Promoter ke upstream enhancer region

3. Regulatory Gene

• lacI gene: lac repressor protein ko code karta hai (hamesha low levels par express hota hai)

Kaise Kaam Karta Hai: The Mechanism

State 1: Lactose ABSENT (Operon OFF)

Step-by-step:

1. lacI constitutively thodi si repressor protein banata hai
2. Repressor (ek tetramer) tightly operator sequence se bind karta hai
3. RNA polymerase promoter access nahi kar sakta (physically blocked hai)
4. Structural genes transcribe nahi hote → koi enzyme nahi banta

Yeh step kyun?

• Repressor ka binding site promoter ke saath overlap karta hai
• Agar RNA polymerase weakly bhi bind kare, repressor usse downstream jaane se rokta hai
• Default state = OFF se energy bachti hai

State 2: Lactose PRESENT (Operon ON)

Step-by-step:

1. Lactose cell mein enter karta hai (basal permease ke zariye jo pehle se present hai)
2. Thodi si maatra allolactose mein convert hoti hai (residual β-galactosidase ke zariye)
3. Allolactose repressor se bind karta hai → conformational change → repressor operator se release ho jaata hai
4. RNA polymerase ab structural genes transcribe kar sakta hai
5. β-galactosidase, permease, transacetylase synthesize hote hain
6. Zyada lactose import hota hai → positive feedback loop

Allolactose kyun, lactose kyun nahi?

• Allolactose hi asli inducer hai (structural isomer: glucose-β-1,6-galactose)
• Yeh β-galactosidase action ka byproduct hai
• Yeh repressor ke allosteric site par lactose se zyada achhi tarah fit hota hai

Glucose Override: CAP-cAMP Positive Control

Lactose present hone par bhi, glucose depletion ke bina operon sirf ~2% efficiency par run karta hai. Kyun?

The mechanism:

1. Low glucose → high cAMP (cyclic AMP)
2. cAMP CAP (catabolite activator protein) se bind karta hai
3. CAP-cAMP complex promoter ke upstream bind karta hai
4. RNA polymerase ko recruit karta hai → transcription mein 50× increase

Yeh hierarchy kyun?

• Glucose preferred carbon source hai (faster metabolism)
• Chahe lactose available ho, cell kehta hai "pehle glucose use karo"
• cAMP glucose ka inversely proportional hai (low glucose → high cAMP)

lac Operon ke Char States

| Glucose | Lactose | cAMP | Repressor | CAP-cAMP | Transcription | |------|---------|------|-----------|------------| | High | Absent | Low | Bound | Absent | OFF (0%) | | High | Present | Low | Released | Absent | ON (2%) | | Low | Absent | High | Bound | Bound | OFF (0%) | | Low | Present | High | Released | Bound | ON (100%) |

KEY INSIGHT: Dono conditions chahiye:

• Repressor release ho (lactose present ho)
• CAP-cAMP bound ho (glucose absent ho)

Common Mutations aur Predictions

"Inducible" vs "Repressible" kyun?

lac operon inducible hai kyunki:

• Default state = OFF
• Substrate (lactose) ise ON induce karta hai
• Catabolic pathways ke liye sahi hai (nutrients break down karna)

Repressible operons se compare karo (jaise trp operon):

• Default state = ON
• Product (tryptophan) ise OFF repress karta hai
• Anabolic pathways ke liye sahi hai (molecules synthesize karna)

Difference kyun?

• Amino acids tab tak banao jab tak kafi na ho jaayein → band karo (feedback inhibition)
• Nutrients sirf available hone par digest karo → on karo (feedforward activation)

Connections

• Gene regulation in prokaryotes: lac model system hai
• Negative feedback loops: allolactose → enzymes → zyada allolactose (short-term positive, long-term negative jab lactose deplete ho)
• CAP-cAMP and catabolite repression: glucose preference hierarchy
• Operon theory (Jacob & Monod): lac studies se Nobel-winning model
• Bacterial adaptation to environment: kaise operons metabolic flexibility dete hain
• trp operon (repressible system): contrasting regulatory logic
• Allosteric regulation: repressor conformational change
• Mutations as genetic tools: lacI⁻, lacOᶜ, lacZ⁻ mechanisms reveal karte hain

#flashcards/biology

Inducible operon kya hota hai? :: Ek operon jo normally OFF hota hai aur apne substrate ki presence se ON ho jaata hai (jaise lac operon lactose se induce hota hai). Default state repressed hoti hai; substrate transcription induce karta hai.

lac operon ko "repressible" ki jagah "inducible" kyun kaha jaata hai? :: Default state OFF hai; substrate (lactose) ise ON induce karta hai. Catabolic pathways ke liye suitable hai jahan enzymes sirf tab banao jab substrate available ho. Compare karo: repressible operons (trp) by default ON hote hain, product unhe OFF karta hai.

Concept Map