Describe DNA fingerprinting — Hinglish notes

6.2.9 · Biology › Genetic Engineering & CRISPR

DNA Fingerprinting Kya Hai?

Key principle: Hume genes (coding DNA) ki parwah nahi hai. Hume non-coding repetitive regions ki parwah hai jahan mutation rates zyada hote hain aur variation enormous hoti hai.

DNA Fingerprinting Kaise Kaam Karta Hai: Step-by-Step Derivation

Step 1: Sample Collection & DNA Extraction

YEH STEP KYUN? DNA cells ke andar hota hai, proteins aur membranes mein wrapped hota hai. Hume contaminants se mukht pure DNA chahiye.

KAISE:

Biological sample collect karo (blood, saliva, hair root, semen)
Detergent se cells lyse karo (membranes tod do)
Protease enzymes se proteins digest karo
Alcohol se DNA precipitate karo (DNA ethanol mein insoluble hota hai)
Solution mein purified DNA hasil karo

KYUN MAHATVAPOORN HAI: Chhote se chhote samples (single hair, dried blood spot) mein bhi kaafi DNA hota hai (~10 µg zaroori hota hai).

Step 2: PCR se DNA Amplification

First principles se Derivation:

Har cycle: Denature (95°C) → Anneal primers (55°C) → Extend (72°C)
Har template strand 2 daughter strands produce karta hai
Cycle 1 ke baad: $n_{0} \to 2 n_{0}$
Cycle 2 ke baad: $2 n_{0} \to 4 n_{0} = 2^{2} n_{0}$
Cycle $c$ ke baad: $n (c) = 2^{c} n_{0}$

EXPONENTIAL KYUN? Har product molecule agli cycle mein ek template ban jaata hai—yeh geometric growth hai, linear addition nahi.

Solution: $n (30) = 100 \times 2^{30} = 100 \times 1, 073, 741, 824 \approx 1.07 \times 1 0^{11} copies$

YEH STEP KYUN? Forensic samples (cigarette butt, envelope seal) mein picogram amounts hote hain. PCR sirf unhi VNTR/STR regions ko amplify karta hai jinmein humari interest hai, jisse unhe detectable banaya ja sake.

Step 3: Restriction Enzyme Digestion (RFLP Method)

YEH STEP KYUN? VNTRs ke aas-paas consistent sequences hote hain. Restriction endonucleases DNA ko specific recognition sites par kaatti hain, aur variable length ke fragments release karti hain jo is baat par depend karta hai ki cut sites ke beech kitne repeats hain.

Alag-alag individuals mein alag-alag $n$ hota hai → alag-alag fragment lengths.

Derivation:

Restriction site 1 (upstream): position $x_{1}$
VNTR region: position $x_{1}$ se $x_{1} + F /2 + n L + F /2$ tak
Restriction site 2 (downstream): position $x_{2} = x_{1} + F + n L$
Fragment length: $Δ x = x_{2} - x_{1} = F + n L$

Person A: $L_{A} = 200 + 20 \times 15 = 200 + 300 = 500 bp$

Person B: $L_{B} = 200 + 35 \times 15 = 200 + 525 = 725 bp$

ALAG KYUN? Repeats ki sankhya ( $n$ ) inherited hoti hai—tumhe ek allele har parent se milta hai. VNTR ek codominant marker hai: dono alleles bands ke roop mein dikhte hain.

Step 4: Gel Electrophoresis Separation

YEH STEP KYUN? DNA fragments length mein differ karte hain. Hum unhe electric field ka use karke size ke hisaab se separate karte hain.

Physics se Derivation:

DNA negatively charged hota hai (phosphate backbone)
Electric field $E$ force $F = q E$ lagata hai
Gel ek molecular sieve ki tarah kaam karta hai: chhote fragments pores mein se aasaani se nikal jaate hain, bade fragments phas jaate hain
Drag force $F_{d} \propto L$ (lambe molecules zyada friction feel karte hain)
Terminal velocity tab reach hoti hai jab $F = F_{d}$
Chhota $L$ → kam drag → tez migration → well se zyada door jaata hai

YEH KYUN KAAM KARTA HAI: ~2 ghante 100V par, fragments size ke hisaab se distinct bands mein separate ho jaate hain.

Step 5: Visualization (RFLP ke liye Southern Blotting ya PCR ke liye Direct)

RFLP method:

Gel se DNA ko nylon membrane par transfer karo (Southern blot)
Radioactive ya fluorescent probe add karo (single-stranded DNA jo VNTR ka complementary ho)
Probe sirf VNTR fragments se hybridize karta hai
X-ray film se expose karo → bands probe locations par appear hote hain

PCR-STR method (modern):

PCR primers fluorescently labeled hote hain
Capillary electrophoresis fragments ko separate karta hai
Laser fluorescent peaks detect karta hai
Computer electropherogram generate karta hai (peaks = alleles)

PROBES/LABELS KYUN? Gel mein hazaaron DNA fragments hote hain; hum sirf polymorphic STR/VNTR regions dekhna chahte hain.

Step 6: Pattern Comparison

Derivation: Har locus independently segregate karta hai (Mendelian law). Agar locus 1 chance se $p_{1} = 1/10$ probability se match karta hai, aur locus 2 se $p_{2} = 1/15$ , to dono ka match: $P = p_{1} \times p_{2} = \frac{1}{10} \times \frac{1}{15} = \frac{1}{150}$

Modern forensic panels: 13-20 STR loci use karte hain → match probability $< 1 0^{- 15}$ (ek quadrillion mein ek).

Random match probability: $P = 0.1 \times 0.08 \times 0.12 \times 0.15 = 0.000144 = 1.44 \times 1 0^{- 4}$

Interpretation: 6,944 mein se 1 chance hai ki koi random insaan match kare. Real labs 13+ loci use karti hain → odds astronomically chhote ho jaate hain.

YEH STEP KYUN? Band positions ka pattern (ya electropherogram mein peak heights) samples ke beech compare kiya jaata hai. Saare loci par match → positive identification (forensic match, paternity confirmed).

Common Mistakes

Kyun galat hai:

3 billion bp padhna expensive aur unnecessary hai
Fingerprinting sirf genome ka 0.001% target karta hai (VNTRs/STRs)
Yeh regions highly polymorphic hote hain (har locus par bahut saare alleles)
13 STR loci $1 0^{- 15}$ discrimination dete hain—jo Earth ki population se kahin zyada hai

Fix: Hum hypervariable regions exploit karte hain jinka mutation rate zyada hota hai. Har locus par zyada alleles = kam loci se behtar discrimination.

Kyun galat hai:

Repeats ki sankhya individuals mein vary karti hai
Fragment length = $F + n L$ jahan $n$ repeat count hai
$n = 20$ wala person ek 500-bp fragment produce karta hai
$n = 35$ wala person ek 725-bp fragment produce karta hai
Yeh gel par alag positions par migrate karte hain

Fix: VNTRs/STRs polymorphic hote hain (bahut saare forms). Har insaan ko parents se alag alleles inherit hote hain.

Kyun galat hai:

PCR ko primers chahiye (short DNA sequences ~20 bp)
Primers STR ke aas-paas specific flanking sequences se bind karte hain
Sirf primers ke beech wala region amplify hota hai
Genome ka baaki hissa ignore ho jaata hai

Fix: PCR sequence-specific hai. Tum sirf unhi VNTRs/STRs ko target karne ke liye primers design karte ho jo tum analyze karna chahte ho.

Applications

Forensics: Crime scene DNA ko suspect se match karo (blood, semen, hair, cigarette par saliva)
Paternity testing: Bacha har locus par har parent se ek allele inherit karta hai. Agar alleged father ke alleles child mein absent hain → excluded
Victims ki identification: Disaster victim identification (plane crashes, mass graves)
Wildlife forensics: Poaching track karo (ivory DNA → elephant herd), illegal trade
Plant breeding: Seed purity verify karo, plant variety rights protect karo

Modern PCR-STR vs. Classical RFLP

Feature	RFLP (Old)	PCR-STR (Modern)
DNA required	100+ ng (large sample)	1 ng (trace amounts)
Time	6-8 weeks	24-48 hours
Probe type	Radioactive ( $^{32}$ P)	Fluorescent dyes
Discrimination	High (VNTRs, many alleles)	Very high (13-20 STR loci)
Degraded DNA	Fails (high MW DNA chahiye)	Works (short 100-400 bp target karta hai)

PCR-STR KYUN JEETA: Forensic samples aksar degraded hote hain (heat, moisture, UV se expose). STRs chhote hote hain (2-6 bp repeats, total fragment 100-400 bp), isliye fragmented DNA bhi kaam karta hai.

Connections

Polymerase Chain Reaction (PCR) – exponential amplification technique
Restriction Endonucleases – Type II enzymes for cutting DNA
Gel Electrophoresis – size-based separation principle
Southern Blotting – DNA transfer and hybridization
Mendelian Inheritance – codominant alleles, independent assortment
Variable Number Tandem Repeats (VNTRs) – minisatellites, 10-100 bp units
Short Tandem Repeats (STRs) – microsatellites, 2-6 bp units
Probability in Genetics – product rule for independent events

Recall Feynman Technique: Ek 12-Saal-Ke Bachche Ko Explain Karo

Socho ki har insaan ke cells mein ek secret barcode chhupa hua hai. Yeh un genes ke baare mein nahi hai jo tumhe lamba banate hain ya brown eyes dete hain—yeh "junk DNA" regions mein hai jahan short sequences baar baar repeat hoti hain. Tumhari maa ke 10 repeats ho sakte hain, tumhare papa ke 20 repeats ho sakte hain, toh tumhe dono versions milte hain.

Scientists ek tiny sample se DNA extract karte hain (jaise ek single hair se!), phir ek copying machine (PCR) ka use karke sirf unhi repeat regions ki millions copies banate hain. Woh special molecular scissors (restriction enzymes) add karte hain jo DNA ko pieces mein kaatti hain. Kyunki tumhare paas kisi bhi dusre insaan se alag number of repeats hain, tumhare pieces alag lengths ke hote hain.

Woh DNA pieces ko jelly-jaisi gel mein rakhte hain aur electricity se zap karte hain. Chhote pieces tez bhagte hain (jaise race mein chhote bachche), bade pieces dheere bhagte hain (jaise backpack uthaye adults). Yeh unhe lines ke pattern mein separate kar deta hai—tumhara unique DNA fingerprint. Yeh grocery store par barcode ki tarah hai, lekin price batane ki jagah yeh batata hai ki tum kaun ho.

Police isko crime scene ke blood ko suspect se match karne ke liye use karti hai. Agar patterns 13 alag-alag jagahon par match hote hain, toh kisi aur ke hone ke chances ek trillion mein ek hain—basically impossible!

Alternate: "Princess Can Really Eat Vegetables Properly" (PCR, Cut, Run, Expose, Visualize, Probabilities)

#flashcards/biology

DNA fingerprinting kya hai aur yeh DNA ke kaun se regions analyze karta hai? :: DNA fingerprinting ek molecular technique hai jo Variable Number Tandem Repeats (VNTRs) aur Short Tandem Repeats (STRs)—polymorphic non-coding regions jahan short sequences alag-alag baar repeat hoti hain—ko analyze karke individual identification ke liye ek unique profile banati hai.

DNA fingerprinting ke liye hum coding genes ki jagah VNTRs/STRs ko kyun target karte hain?

VNTRs/STRs hypervariable hote hain (har locus par bahut saare alleles) kyunki non-coding regions mein mutation rates zyada hote hain, jo kam analyze kiye gaye loci se behtar discrimination provide karte hain. Coding genes bahut zyada conserved hote hain (individuals mein similar) isliye identification ke liye useful nahi hote.

n_{0}

copies se shuru karke

c

PCR cycles ke baad DNA copies ki sankhya ka formula kya hai?

n (c) = n_{0} \cdot 2^{c}

. Har cycle copies ki sankhya double kar deta hai kyunki exponential (geometric) growth hoti hai—har product molecule agli cycle mein ek template ban jaati hai.

Agar ek VNTR mein

L

bp ki unit ke

n

repeats hain, aur flanking regions ka total

F

bp hai, toh restriction digestion ke baad fragment length kya hogi?

Fragment length = F + n L

. Alag-alag individuals mein alag-alag

n

(repeats ki sankhya) hota hai, jo alag lengths ke fragments produce karte hain jo electrophoresis ke dauran separate hote hain.

Gel electrophoresis ke dauran chhote DNA fragments bade fragments se zyada door kyun migrate karte hain?

Chhote fragments gel ke pores se guzarte waqt kam drag force feel karte hain. Gel ek molecular sieve ki tarah kaam karta hai: chhote fragments aasaani se nikal jaate hain, jabki bade fragments phas jaate hain aur slow ho jaate hain. Migration distance

d \propto 1/ L

Match probabilities

p_{1}, p_{2}, \dots, p_{n}

wale

n

independent STR loci ke liye, random match ki probability kya hai?

P_{total} = \prod_{i = 1}^{n} p_{i} = p_{1} \times p_{2} \times \dots \times p_{n}

. Har locus independently segregate karta hai (product rule), isliye probabilities multiply hoti hain. 13-20 loci ke saath, match probability

1 0^{- 15}

se neeche aa jaati hai.

DNA fingerprinting ke liye PCR-STR ke RFLP par key advantages kya hain?

PCR-STR ko bahut kam DNA chahiye (1 ng vs. 100+ ng), degraded DNA ke saath kaam karta hai (short 100-400 bp fragments target karta hai), zyada tez hai (24-48 ghante vs. 6-8 hafte), aur radioactive probes ki jagah fluorescent labels use karta hai.

RFLP DNA fingerprinting mein Southern blotting ka kya purpose hai?

Southern blotting DNA ko gel se nylon membrane par transfer karta hai, phir ek labeled probe (radioactive ya fluorescent DNA jo VNTR ka complementary ho) use karta hai jo sirf target fragments se hybridize karta hai. Yeh hazaaron dusre DNA fragments ke beech specific VNTR bands ko visible banata hai.

Degraded forensic samples ko analyze karne ke liye VNTRs ki jagah STRs (Short Tandem Repeats) kyun behtar hain?

STRs short sequences hote hain (2-6 bp repeats, total fragment 100-400 bp), isliye agar DNA heat, moisture, ya UV exposure se fragmented bhi ho, STR regions intact rehte hain aur PCR se amplifiable hote hain. VNTRs lambe hote hain aur degraded samples mein tootne ki zyada sambhavna hoti hai.

Paternity testing mein tum kaise determine karte ho ki alleged father excluded hai?

Har STR locus par check karo ki bacha kya aise alleles carry karta hai jo alleged father se aa sakte the. Har bacha har parent se ek allele inherit karta hai. Agar kisi bhi locus par bache mein aise alleles hain jo naa alleged father mein hain naa mother mein, toh alleged father excluded hai.

Identical twins ka DNA fingerprint same kyun hota hai?

Identical twins ek single fertilized egg se bante hain jo split ho jaata hai, isliye unka exact same DNA sequence share hota hai, including har VNTR/STR locus par repeats ki sankhya. Unke fingerprints indistinguishable hote hain (lekin epigenetics jaisi dusri forensic methods kabhi kabhi unhe differentiate kar sakti hain).