Describe DNA fingerprinting — Biology notes

What Is DNA Fingerprinting?

Key principle: We don't care about genes (coding DNA). We care about non-coding repetitive regions where mutation rates are high and variation is enormous.

How DNA Fingerprinting Works: Step-by-Step Derivation

Step 1: Sample Collection & DNA Extraction

WHY this step? DNA is inside cells, wrapped in proteins and membranes. We need pure DNA free of contaminants.

HOW:

Collect biological sample (blood, saliva, hair root, semen)
Lyse cells with detergent (breaks membranes)
Digest proteins with protease enzymes
Precipitate DNA with alcohol (DNA is insoluble in ethanol)
Obtain purified DNA in solution

WHY it matters: Even tiny samples (single hair, dried blood spot) contain enough DNA (~10 µg needed).

Step 2: DNA Amplification with PCR

Derivation from first principles:

Each cycle: Denature (95°C) → Anneal primers (55°C) → Extend (72
Each template strand produces2 daughter strands
After cycle1: $n_0 \rightarrow 2n_0$
After cycle 2: $2n_0 \rightarrow 4n_0 = 2^2 n_0$
After cycle $c$ : $n(c) = 2^c n_0$

WHY exponential? Each product molecule becomes a template in the next cycle—this is geometric growth, not linear addition.

Solution: $n(30) = 100 \times 2^{30} = 100 \times 1{,}073{,}741{,}824 \approx 1.07 \times 10^{11} \text{ copies}$

WHY this step? Forensic samples (cigarette bu, envelope seal) contain picogram amounts. PCR amplifies only the VNTR/STR regions we're interested in, making them detectable.

Step 3: Restriction Enzyme Digestion (RFLP Method)

WHY this step? VNTRs are flanked by consistent sequences. Restriction endonucleases cut DNA at specific recognition sites, releasing fragments of variable length depending on how many repeats exist between cut sites.

Different individuals have different $n$ → different fragment lengths.

Derivation:

Restriction site1 (upstream): position $x_1$
VNTR region: position $x_1$ to $x_1 + F/2 + nL + F/2$
Restriction site 2 (downstream): position $x_2 = x_1 + F + nL$
Fragment length: $\Delta x = x_2 - x_1 = F + nL$

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

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

WHY different? The number of repeats ( $n$ ) is inherited—you get one allele from each parent. The VNTR is a codominant marker: both alleles show up as bands.

Step 4: Gel Electrophoresis Separation

WHY this step? DNA fragments differ by length. We separate them by size using an electric field.

Derivation from physics:

DNA is negatively charged (phosphate backbone)
Electric field $E$ exerts force $F = qE$
Gel acts as a molecular sieve: small fragments slip through pores easily, large fragments get tangled
Drag force $F_d \propto L$ (longer molecules experience more friction)
Terminal velocity reached when $F = F_d$
Smaller $L$ → less drag → faster migration → travels farther from the well

WHY this works: After ~2 hours at 100V, fragments are separated into distinct bands by size.

Step 5: Visualization (Southern Blotting for RFLP or Direct for PCR)

RFLP method:

Transfer DNA from gel to nylon membrane (Southern blot)
Add radioactive or fluorescent probe (single-stranded DNA complementary to VNTR)
Probe hybridizes only to VNTR fragments
Expose to X-ray film → bands appear at probe locations

PCR-STR method (modern):

PCR primers are fluorescently labeled
Capillary electrophoresis separates fragments
Laser detects fluorescent peaks
Computer generates electropherogram (peaks = alleles)

WHY probes/labels? Gel has thousands of DNA fragments; we only want to see the polymorphic STR/VNTR regions.

Step 6: Pattern Comparison

Derivation: Each locus segregates independently (Mendelian law). If locus 1 matches by chance with probability $p_1 = 1/10$ , and locus 2 with $p_2 = 1/15$ , then both matching: $P = p_1 \times p_2 = \frac{1}{10} \times \frac{1}{15} = \frac{1}{150}$

Modern forensic panels: Use 13-20 STR loci → match probability $< 10^{-15}$ (one in a quadrillion).

Random match probability: $P = 0.1 \times 0.08 \times 0.12 \times 0.15 = 0.000144 = 1.44 \times 10^{-4}$

Interpretation: 1 in 6,944 chance of a random person matching. Real labs use 13+ loci → odds become astronomically small.

WHY this step? The pattern of band positions (or peak heights in electropherogram) is compared between samples. Match at all loci → positive identification (forensic match, paternity confirmed).

Common Mistakes

Why it's wrong:

Reading 3 billion bp is expensive and unnecessary
Fingerprinting targets only 0.001% of genome (VNTRs/STRs)
These regions are highly polymorphic (many alleles per locus)
13 STR loci give $10^{-15}$ discrimination—far exceding Earth's population

The fix: We exploit hypervariable regions with high mutation rates. More alleles per locus = better discrimination with fewer loci.

Why it's wrong:

Number of repeats varies between individuals
Fragment length = $F + nL$ where $n$ is the repeat count
Person with $n=20$ produces a500-bp fragment
Person with $n=35$ produces a 725-bp fragment
These migrate to different positions on the gel

The fix: VNTRs/STRs are polymorphic (many forms). Each person inherits different alleles from parents.

Why it's wrong:

PCR requires primers (short DNA sequences ~20 bp)
Primers bind to specific flanking sequences around the STR
Only the region between primers gets amplified
Rest of the genome is ignored

The fix: PCR is sequence-specific. You design primers to target only the VNTRs/STRs you want to analyze.

Applications

Forensics: Match crime scene DNA to suspect (blood, semen, hair, saliva on cigarette)
Paternity testing: Child inherits one allele from each parent at every locus. If alleged father's alleles are absent in child → excluded
Identification of victims: Disaster victim identification (plane crashes, mass graves)
Wildlife forensics: Track poaching (ivory DNA → elephant herd), illegal trade
Plant breeding: Verify seed purity, protect plant variety rights

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 (needs high MW DNA) | Works (targets short100-400 bp) |

WHY PCR-STR won: Forensic samples are often degraded (exposed to heat, moisture, UV). STRs are short (2-6 bp repeats, total fragment100-400 bp), so even fragmented DNA works.

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: Explain to a 12-Year-Old

Imagine every person has a secret barcode hidden in their cells. It's not about the genes that make you tall or give you brown eyes—it's in the "junk DNA" regions where short sequences repeat over and over. Your mom might have 10 repeats, your dad might have 20 repeats, so you get both versions.

Scientists extract DNA from a tiny sample (like a single hair!), then use a copying machine (PCR) to make millions of copies of just those repeat regions. They add special molecular scissors (restriction enzymes) that cut the DNA into pieces. Because you have a different number of repeats than anyone else, your pieces are different lengths.

They put the DNA pieces in jello-like gel and zap it with electricity. Small pieces run fast (like small kids in a race), big pieces run slow (like adults carrying backpacks). This separates them into a pattern of lines—your unique DNA fingerprint. It's like a barcode at a grocery store, but instead of telling the price, it tells who you are.

Police use this to match blood at crime scene to a suspect. If the patterns match at13 different spots, the odds of it being someone else are one in a trillion—basically impossible!

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

#flashcards/biology

What is DNA fingerprinting and what regions of DNA does it analyze? :: DNA fingerprinting is a molecular technique that analyzes Variable Number Tandem Repeats (VNTRs) and Short Tandem Repeats (STRs)—polymorphic non-coding regions where short sequences repeat different numbers of times—to create a unique profile for individual identification.

Why do we target VNTRs/STRs instead of coding genes for DNA fingerprinting?

VNTRs/STRs are hypervariable (many alleles per locus) due to high mutation rates in non-coding regions, providing better discrimination with fewer loci analyzed. Coding genes are too conserved (similar across individuals) to be useful for identification.

What is the formula for the number of DNA copies after

c

PCR cycles, starting with

n_0

copies?

n(c) = n_0 \cdot 2^c

. Each cycle doubles the number of copies due to exponential (geometric) growth—every product molecule becomes a template in the next cycle.

If a VNTR has

n

repeats of a unit of length

L

bp, with flanking regions totaling

F

bp, what is the fragment length after restriction digestion?

\text{Fragment length} = F + nL

. Different individuals have different

n

(number of repeats), producing fragments of different lengths that separate during electrophoresis.

Why do smaller DNA fragments migrate farther than larger fragments during gel electrophoresis?

Smaller fragments experience less drag force as they move through the gel's pores. The gel acts as a molecular sieve: small fragments slip through easily, while large fragments get tangled and slow down. Migration distance

d \propto 1/L

For

n

independent STR loci with match probabilities

p_1, p_2, \ldots, p_n

, what is the probability of a random match?

P_{\text{total}} = \prod_{i=1}^{n} p_i = p_1 \times p_2 \times \cdots \times p_n

. Each locus segregates independently (product rule), so probabilities multiply. With13-20 loci, match probability drops below

10^{-15}

What are the key advantages of PCR-STR over RFLP for DNA fingerprinting?

PCR-STR requires much less DNA (1 ng vs. 100+ ng), works with degraded DNA (targets short 100-400 bp fragments), is faster (24-48 hours vs. 6-8 weeks), and uses fluorescent labels instead of radioactive probes.

What is the purpose of Southern blotting in RFLP DNA fingerprinting?

Southern blotting transfers DNA from the gel to a nylon membrane, then uses a labeled probe (radioactive or fluorescent DNA complementary to the VNTR) that hybridizes only to the target fragments. This makes the specific VNTR bands visible among thousands of other DNA fragments.

Why are STRs (Short Tandem Repeats) better than VNTRs for analyzing degraded forensic samples?

STRs are short sequences (2-6 bp repeats, total fragment 100-400 bp), so even if DNA is fragmented by heat, moisture, or UV exposure, the STR regions remain intact and amplifiable by PCR. VNTRs are longer and more likely to be broken in degraded samples.

In paternity testing, how do you determine if an alleged father is excluded?

Check if the child carries alleles at each STR locus that could have come from the alleged father. Each child inherits one allele from each parent. If the child has alleles at any locus that are absent in both the alleged father and mother, the alleged father is excluded.

What is the biological reason why identical twins have the same DNA fingerprint?

Identical twins form from a single fertilized egg that splits, so they share the exact same DNA sequence, including the number of repeats at every VNTR/STR locus. Their fingerprints are indistinguishable (but other forensic methods like epigenetics can sometimes differentiate them).

Concept Map

Hinglish (regional understanding)

Intuition Hinglish mein samjho

DNA fingerprinting ek powerful technique hai jisse aap kisi bhi insaan ko uniquely identify kar sakte ho, bas thodi si biological sample se—chahe wo blood ho, saliva ho, ya sirf ek baal ka root. Sabka DNA similar hota hai genes mein (99.9% same), lekin kuch special regions hain jahan chhote sequences baar-baar repeat hote hain—inko VNTRs aur STRs kehte hain. Har insaan mein in repeats ki sankhya alag hoti hai (kisi mein 10 baar, kisi mein 25 baar), aur yahi uniqueness ko create karta hai, bilkul ek barcode ki tarah.

Process simple hai: pehle DNA nikalo, phir PCR se us specific repeat wale region ko lakhon baar copy karo (exponential growth: har cycle mein double!). Uske bad restriction enzymes se DNA ko cut karo—jo fragments bante hain unki length alag hogi kyunki repeat number alag hai. In fragments ko gel electrophoresis se separate karo: chhote pieces tez chalte hain (door jate hain), bade pieces dhere (pas mein ruk jate hain). Jab ap result dekhte ho, to bands ka pattern sabka alag hota hai—yahi hai DNA fingerprint.

Forensics mein isse crime solve hoti hai: agar crime scene pe mila blood ka pattern aur suspect ka pattern 13-20 loci pe match ho j

Test yourself — Genetic Engineering & CRISPR

Connections