3.3.1
~1950 se pehle, biologists jaante the ki genes exist karti hain, lekin kaun sa molecule unhe carry karta hai? Chromosomes mein DONO protein aur DNA hote hain. Protein sabse obvious candidate lagta tha (20 amino acids → itna "complex" lagta hai ki information store kar sake), jabki DNA (sirf 4 bases) bahut boring lagta tha. Neeche ki story yeh hai ki kaise teen experiments ne scientists ko dheere-dheere majboor kiya ki woh us boring molecule ko answer maanein.
Definition TransformationTransformation tab hota hai jab ek cell apne aas-paas se free DNA uthaa leti hai aur iske result mein uske inherited traits badal jaate hain .
USNE KYA USE KIYA: Streptococcus pneumoniae ke do strains (bacteria jo chuhon mein pneumonia cause karte hain):
S strain (Smooth): ek polysaccharide capsule hota hai → virulent (chuhon ko maar deta hai).R strain (Rough): capsule nahi → harmless (chuha zinda rehta hai).
EXPERIMENT KAISE CHALA (chuhon mein 4 injections):
Injection
Result
Kyun?
Live R
Chuha zinda rehta hai
harmless
Live S
Chuha mar jaata hai
virulent
Heat-killed S Chuha zinda rehta hai
dead bacteria infect nahi kar sakta
Heat-killed S + live R
Chuha MAR JAATA HAI 😱
surprise!
Yeh step kyun matter karta hai: Last injection mein, koi bhi akela ingredient nahi maarta. Phir bhi chuha mar jaata hai, aur Griffith ne usme se living S bacteria recover kiye.
Dead S cells se kuch cheez physically live R cells mein gayi aur unhe permanently S mein badal diya. Griffith ne ise transforming principle kaha. Usse pata nahi tha ki yeh DNA tha — usne sirf yeh prove kiya ki koi cheez heritable ek cell se doosri mein ja sakti hai.
Intuition Strategy: ek-ek karke suspects ko destroy karoHeat-killed S cells ka extract lo. Har batch ko ek aisa enzyme do jo exactly ek type ke molecule ko destroy kare . Jiski destruction transformation ko rokti hai = wahi genetic molecule hai.
KAISE:
Enzyme add kiya
Destroy karta hai
Transformation abhi bhi hoti hai?
Protease
proteins
✅ Haan
RNase
RNA
✅ Haan
DNase DNA ❌ Nahi
Yeh kyun key hai: Sirf DNA ko destroy karne se R→S transformation ruki. Isliye transforming principle = DNA .
Common mistake "Lekin logon ne Avery ko believe nahi kiya!"Shak kyun reasonable lagta tha: Critics ne argue kiya ki DNA prep mein contaminating protein ka ek chhota sa trace ho sakta hai jo asli kaam kar raha ho. DNA ki 4-base "monotony" ne bhi ise gene hone ke liye bahut simple dikhaya. Fix: Avery ke controls achhe the, lekin ek cleaner, zyada visual experiment (Hershey-Chase) sabko convince karne ke liye zaroori tha.
Ek bacteriophage (phage) ek aisa virus hai jo bacteria ko infect karta hai. Yeh sirf do cheezein se bana hota hai: ek protein coat + andar DNA. Yeh perfect tool hai — koi aur cheez nahi hai jo result ko confuse kare.
Intuition Clever trick: har molecule ko alag se label karo
Protein mein sulfur (S) hota hai lekin phosphorus nahi → radioactive ==35 S DNA mein phosphorus (P) hota hai lekin sulfur nahi → radioactive ==32 P tag karta hai. Phir hum poochte hain: kaun sa tag bacteria ke ANDAR jaata hai?
KAISE (steps):
Do phage batches ugao: ek 35 S 32 P
Dono ko E. coli se infect karne do.
Blend karo (baahir chipke huye empty phage coats ko hilaao) → "Yeh step kyun?" — yeh separate karne ke liye ki cell mein kya gaya aur kya baahir raha.Centrifuge karo: Heavy bacteria neeche pellet banaate hain; light phage coats supernatant liquid mein rehte hain.Mapo ki radioactivity kahaan hai.
Label
Pellet mein mila (cells ke andar)?
Conclusion
35 S zyaadatar supernatant mein
protein BAAHIR raha
32 P zyaadatar pellet mein
DNA ANDAR gaya
Yeh isse prove kyun karta hai: Jo molecule bacteria ke andar jaata hai aur naye phage ki production direct karta hai = wahi genetic material hai. Woh molecule DNA tha.
Worked example Example 1 — Griffith ka missing control predict karoQ: Agar aap heat-killed S akele ko DNase se treat karke inject karo toh kya hoga?
Forecast: Chuha zinda rehta hai (koi live virulent cells nahi; koi functional DNA nahi).
Verify: Correct — kuch bhi virulent nahi hai aur koi kaam karne wala transforming principle nahi hai. Yeh step kyun? Yeh isolate karta hai ki DNA, sirf "dead cell debris" nahi, woh cheez hai jo matter karti hai.
Worked example Example 2 — Hershey-Chase tweakQ: Aapne galti se phage ko ek saath 32 P aur 35 S Forecast: 32 P 35 S Verify: Haan. Yeh step kyun? Kyunki dono atoms do alag molecules ko alag fates ke saath tag karte hain; experiment phir bhi kaam karta hai kyunki tags independent hain.
Worked example Example 3 — Bacteriophage kyun, mice nahi?Q: Hershey-Chase Avery se "cleaner" kyun hai?
Forecast: Phage = sirf protein + DNA, toh kisi teesre molecule ko blame nahi kiya ja sakta.
Verify: Correct. Avery ke extract ko "contaminated" claim kiya ja sakta tha; phage mein aur kuch nahi hota, woh loophole hat jaata hai.
Recall Feynman: ek 12-saal ke bacche ko explain karoSocho tum jaanna chahte ho ki cake mein kaun si ingredient use sweet banati hai: maida, ande, ya cheeni. Griffith ne ek "dead sweet cake" ko ek "plain cake" ke saath mix kiya aur plain wala magically sweet ho gaya — toh kuch sweet cheez udhar se gayi. Avery ne phir ek-ek ingredient hataayi; sirf jab usne cheeni hatayi tab sweetness gayi — toh cheeni (DNA) hi sweet-maker hai. Hershey-Chase ne cheeni ko ek rang se aur maida ko doosre rang se paint kiya, cake banaaya, aur dekha ki sirf cheeni ka rang naye cakes ke andar gaya. Toh DNA woh "recipe" hai jo aage pass hoti hai, protein nahi.
Mnemonic Order aur roles yaad karo"Great Aunt Helen" → G riffith (transformation exist karti hai), A very (yeh DNA hai), H ershey-Chase (proves it with radioactive tags).
Labels ke liye: "DNA Pays, Protein has Sulfur" → D NA = P (32 P ), Protein = S (35 S ).
What did Griffith's experiment demonstrate? Yeh ki ek heritable "transforming principle" dead virulent (S) bacteria se live harmless (R) bacteria mein ja sakti hai, unhe virulent bana deti hai.
Why did Griffith NOT identify DNA as the genetic material? Usne sirf yeh dikhaya ki kuch cheez transfer hui; uske paas koi method nahi tha ki woh identify kar sake kaunsa molecule tha.
What is "transformation" in bacteria? Baahri free DNA ka uptake jo ek cell ke inherited traits ko permanently badal deta hai.
In Avery's experiment, which enzyme abolished transformation? DNase (jo DNA ko destroy karta hai), yeh prove karta hai ki DNA transforming principle tha.
Why were people skeptical of Avery's conclusion? Unhe darr tha ki protein contamination ka trace hoga, aur DNA ke 4 bases genetic information store karne ke liye bahut simple lagte the.
What two molecules make up a bacteriophage? Ek protein coat aur DNA — aur kuch nahi.
Which radioactive isotope labeled protein in Hershey-Chase, and why? 35 S
Which isotope labeled DNA, and why? 32 P
Why is the blending step needed in Hershey-Chase? Bacterial surface se empty phage coats ko hilaane ke liye taaki hum bata sakein ki asal mein cell mein kya gaya.
After centrifugation, where were bacteria vs phage coats? Bacteria (heavy) pellet mein; phage coats (light) supernatant mein.
What was found in the pellet in Hershey-Chase? 32 P
Why is Hershey-Chase considered more convincing than Avery? Ek phage mein sirf protein + DNA hota hai, "contaminating third molecule" wala loophole hat jaata hai.
DNA Structure — jab DNA genetic sabit ho gaya, scientists uski structure dhundhne mein lag gaye (Watson-Crick).DNA Replication — explain karta hai ki yeh genetic material kaise copy hota hai.Bacterial Transformation in Biotech — Griffith ki discovery modern gene cloning ki neev hai.Central Dogma — DNA → RNA → Protein; yeh note DNA ko start point establish karta hai.Chargaff's Rules — base-pairing evidence jo DNA ke information role ko support karti hai.
heat-killed S + live R kills mouse
heritable factor moves between cells
destroy suspects with enzymes
label protein 35S, DNA 32P
What molecule carries genes?
Only DNase stops transformation
Protein contamination concern
Bacteriophage protein + DNA
