DNA Fingerprinting – Quick & Clear Notes 🧬
1. Why compare our DNA at all? 🤔
Humans share about 99.9 % of their DNA sequence. The remaining 0.1 %—roughly \(0.1\% \times 3 \times 10^{9} = 3 \times 10^{6}\) base-pairs—creates the unique blueprint that distinguishes one person from another. DNA fingerprinting lets us zoom straight into those variable bits instead of re-sequencing the whole genome every time. :contentReference[oaicite:0]{index=0}
2. Repetitive DNA & Satellite DNA 🛰️
- Repetitive DNA: Short sequences repeated many times in tandem.
- When separated by density-gradient centrifugation, most DNA forms a major peak (bulk DNA). Smaller side peaks—rich in repeats—are called satellite DNA. :contentReference[oaicite:1]{index=1}
- Based on length and A:T vs G:C content, satellites are grouped into microsatellites and minisatellites. These stretches rarely code for proteins but occupy a big slice of the genome. Their high variability (polymorphism) is the foundation of DNA fingerprinting. :contentReference[oaicite:2]{index=2}
3. Polymorphism — the heart of the technique 💓
A DNA change is called a polymorphism when an alternate allele appears in >1 % of the population. Such mutations pile up more freely in non-coding regions (they don’t affect survival or reproduction), creating abundant variation for us to detect.
4. Meet VNTR (minisatellite) loci 🚀
- Variable Number Tandem Repeats (VNTR) are minisatellite blocks arranged head-to-tail many times.
- The copy count differs from chromosome to chromosome and person to person, producing fragments from ≈ 0.1 kb to 20 kb. :contentReference[oaicite:3]{index=3}
- Because even maternal vs paternal copies vary, each individual shows a distinct band pattern—except identical twins! :contentReference[oaicite:4]{index=4}
5. Step-by-step DNA fingerprinting 🔬
- Isolate genomic DNA from blood, hair, bone, saliva, sperm—any tissue works because the polymorphism pattern stays the same throughout the body. :contentReference[oaicite:5]{index=5}
- Digest DNA with restriction endonucleases.
- Separate the resulting fragments by agarose gel electrophoresis.
- Blot (transfer) the separated fragments onto a nylon or nitrocellulose membrane.
- Hybridise with a radiolabelled VNTR probe (Southern blot).
- Detect the hybridised bands by autoradiography, revealing a barcode-like pattern unique to the individual. :contentReference[oaicite:6]{index=6}
6. Powerful upgrades 🛠️
- PCR now amplifies VNTRs, so a single cell provides enough DNA for analysis. :contentReference[oaicite:7]{index=7}
- Multiple probes can be used together to sharpen resolution or study biodiversity. :contentReference[oaicite:8]{index=8}
7. Applications that matter 🌟
- Forensic ID: Matching crime-scene material to a suspect with high confidence. :contentReference[oaicite:9]{index=9}
- Paternity & kinship: Inheritance of VNTR patterns settles family-relation disputes. :contentReference[oaicite:10]{index=10}
- Wild-life conservation & population genetics: Tracking genetic diversity within and between species. :contentReference[oaicite:11]{index=11}
- Evolutionary biology: Mapping polymorphism patterns to study speciation events. :contentReference[oaicite:12]{index=12}
8. Important Concepts for NEET 📝
- VNTR minisatellites—structure, size range, and why copy-number variation generates unique fingerprints. :contentReference[oaicite:13]{index=13}
- Satellite DNA classification—major vs satellite peaks; micro- vs mini-satellites. :contentReference[oaicite:14]{index=14}
- Definition of DNA polymorphism (>1 % allele frequency) and its link to non-coding regions. :contentReference[oaicite:15]{index=15}
- Six core steps of Southern-blot-based fingerprinting. :contentReference[oaicite:16]{index=16}
- Key applications: forensic identification & paternity testing. :contentReference[oaicite:17]{index=17}
💡 Tip for students: Focus on the logic—variation within repeats + inheritance = a powerful biological barcode!
