Arrange steps of DNA fingerprinting in correct sequence:

BiologyBiotechnology

Arrange steps of DNA fingerprinting in correct sequence:
A. Isolation of DNA + digestion by restriction endonucleases
B. Hybridisation using labelled VNTR probe
C. Transferring separated DNA fragments to synthetic membranes
D. Detection of hybridised DNA by autoradiography
E. Separation of DNA fragments by electrophoresis

Options

A, E, C, B, D

A, E, B, C, D

A, B, D, C, E

A, D, B, E, C

Correct Answer

Option 1 : A → E → C → B → D

Solution

A. Isolate DNA + digest with restriction enzymes → fragments of different sizes

E. Separate fragments by gel electrophoresis → bands based on size

C. Transfer (Southern blot) → DNA bands transferred to nylon/nitrocellulose membrane

B. Hybridise with labelled VNTR probe → probe binds to complementary sequences

D. Autoradiography → X-ray film detects radioactive bands = DNA fingerprint

A (Isolate+Digest) → E (Electrophoresis) → C (Blot to membrane) → B (Hybridise) → D (Autoradiography)

Theory: Biotechnology

1. DNA Fingerprinting — Discovery and Principle

DNA fingerprinting (DNA profiling, genetic fingerprinting) was developed by Alec Jeffreys at the University of Leicester, UK, in 1984. He discovered that certain regions of human DNA — called Variable Number Tandem Repeats (VNTRs) — varied enormously between individuals and that the pattern of these VNTRs was unique to each person (except identical twins). The technique works because: (1) Human genomes differ by only ~0.1% at single nucleotide polymorphism (SNP) level, but much more at tandem repeat level. (2) VNTRs are highly polymorphic — many different alleles at each locus. (3) The combination of VNTR alleles at multiple loci is statistically unique to each individual. First forensic use: 1986 Colin Pitchfork case (UK) — first criminal caught using DNA evidence. First in India: used in 1989 for immigration disputes.

2. VNTR (Variable Number Tandem Repeats)

VNTRs are tandemly repeated core sequences present in multiple copies throughout the genome. Key features: Core sequence: 10-15 base pairs repeated in tandem (one after another). Locus: VNTRs occur at specific chromosomal positions. Polymorphism: the NUMBER of repeats at each locus varies between individuals — some people have 3 copies, others 12 copies, etc. This creates alleles of different sizes. Between individuals: same position on chromosome has different numbers of repeats → produces DNA fragments of different sizes when cut with restriction enzymes. Since we are diploid, each locus gives 2 bands (one from each chromosome). At many VNTR loci across the genome: the combination of all band patterns = DNA fingerprint — statistically unique to each individual. VNTR probes: labelled DNA complementary to the core repeat sequence → hybridises to all VNTR-containing fragments simultaneously → reveals the pattern.

3. Steps of DNA Fingerprinting — Detailed

Step 1 — Isolation of DNA: extract DNA from sample (blood, saliva, semen, hair root, tissue). Typically requires 1-100 ng of DNA for RFLP-based methods; much less for PCR-based methods. Step 2 — Restriction digestion: DNA cut with restriction endonuclease (e.g., HinfI, AluI) at specific palindromic sequences → produces fragments of different sizes (depending on number of tandem repeats between restriction sites). Step 3 — Gel electrophoresis: DNA fragments separated by size using agarose gel electrophoresis. Smaller fragments → faster → further from well. Larger fragments → slower → closer to well. Creates pattern of bands. Step 4 — Southern blotting (Blotting): denatured DNA transferred from gel to nylon/nitrocellulose membrane (synthetic membrane). Gel is alkaline → DNA denatures to single-strands. Transferred by capillary action or vacuum blotting. Step 5 — Hybridisation: membrane incubated with labelled (radioactive or chemiluminescent) VNTR probe → probe hybridises to complementary sequences on membrane. Step 6 — Autoradiography: X-ray film placed over membrane → radioactive probe creates dark bands → pattern visualised = DNA fingerprint.

4. Southern Blotting — Named After E.M. Southern

Southern blotting (E.M. Southern, 1975): technique to transfer DNA from agarose gel to membrane for hybridisation. Process: Run DNA on agarose gel → denature DNA in gel (NaOH → single strands) → neutralise → transfer DNA to membrane (nylon/nitrocellulose) by capillary blotting (paper towels draw buffer through gel, carry DNA to membrane) → UV cross-link DNA to membrane → hybridise with labelled probe → wash away unbound probe → detect signal (autoradiography or chemiluminescence). Named variations: Southern blot: DNA on membrane (Edwin Southern). Northern blot: RNA on membrane (named analogously, no specific inventor named 'Northern'). Western blot: PROTEIN on membrane → probed with antibody (not nucleic acid hybridisation). Eastern blot: lipid or post-translational modification detection.

5. Modern DNA Profiling — STR Analysis

Modern forensic DNA profiling uses Short Tandem Repeats (STRs) instead of VNTRs. STRs: shorter repeat unit (2-7 base pairs), analysed by PCR (not restriction digest). PCR amplifies specific STR loci using fluorescently labelled primers → PCR products of different sizes (depending on number of repeats) → separated by capillary electrophoresis → automated readout of allele sizes. Advantages over RFLP: needs much less DNA (picogram quantities), works with degraded DNA, fully automated, faster. CODIS (Combined DNA Index System, USA): uses 20 STR loci. NDNAD (UK): uses 17 STR loci. The probability of two unrelated individuals having identical profiles at 20+ loci is extremely small (1 in 10¹⁸ or less). India: DNA Technology (Use and Application) Regulation Bill — proposes national DNA database.

6. Applications of DNA Fingerprinting

Forensic science: criminal investigation (matching suspect DNA to crime scene evidence), identifying victims of disasters/accidents (tsunami, plane crashes), identifying human remains. Paternity/maternity testing: child inherits half DNA from each parent → child's DNA fingerprint contains half the bands of each parent. Immigration disputes: proving family relationships. Used by immigration authorities to verify biological relationships. Medical: diagnosis of genetic disorders, cancer diagnosis (somatic mutations in tumour cells), bone marrow transplant engraftment monitoring (is the transplant taking?). Conservation biology: determining parentage in endangered species breeding programmes, detecting poaching (matching confiscated wildlife products to poaching sites). Agriculture: varietal identification of crop plants (e.g., Basmati rice — protects Indian origin). Archaeology: ancient DNA analysis, identifying royal remains.

7. Autoradiography — Visualising the Fingerprint

Autoradiography is the technique used to detect radioactive signals from labelled probes. Process: After hybridisation with radioactive probe (³²P-labelled): membrane is placed in contact with X-ray film (in dark, usually at −70°C to improve sensitivity). Radioactive decay from ³²P probe → exposes silver grains in film → dark bands where probe hybridised. Film developed → dark bands visible against clear background = DNA fingerprint pattern. Bands are located at positions corresponding to DNA fragments of specific sizes (can be determined using DNA ladder/marker). Modern alternatives: fluorescence-based detection (non-radioactive, faster, safer), chemiluminescence (ECL), digital imaging. Radioactive methods are most sensitive but require radiation safety precautions and licenses.

8. Limitations and Ethical Considerations

Limitations: Contamination: DNA from multiple individuals at crime scene → mixed profiles → complex interpretation. Degradation: old, heat-exposed, or bleach-treated samples → fragmented DNA → poor results. Identical twins: have identical DNA → cannot be distinguished by DNA fingerprinting. Chain of custody: evidence handling must be documented to prevent tampering claims. Technical errors: lab errors → false results → wrongful convictions. Ethical issues: DNA databases: privacy concerns — who has access? Can DNA reveal more than just identity (disease risk, ancestry). DNA phenotyping: predicting physical appearance from DNA — ethical limits? Familial searching: if suspect's DNA not in database, search for relatives → identifies family members who never consented. False exoneration: mixed samples may fail to match innocent person → potential misuse. India: DNA Technology Regulation Bill pending since 2018 — balances utility vs privacy rights.

Frequently Asked Questions

1. Why must DNA be transferred to a membrane before hybridisation? ⌄

After electrophoresis in agarose gel: DNA is still in gel matrix. Problems with hybridising directly to gel: (1) Gel is difficult to handle and prone to tearing. (2) Diffusion of small DNA fragments from gel. (3) Large-molecule probe cannot penetrate gel efficiently. (4) Cannot dry and store gel easily. Solution: Southern blotting transfers DNA to a solid, stable nylon membrane: (1) DNA binds strongly to nylon (UV cross-linked covalently). (2) Membrane is thin → probe penetrates easily. (3) Membrane is durable → can be hybridised, washed, stripped, and re-probed. (4) Easy to handle, store, and expose to X-ray film. The spatial pattern of bands is preserved exactly during transfer → same band positions on membrane as in gel.

2. What are VNTRs and why are they useful for identification? ⌄

VNTRs (Variable Number Tandem Repeats): specific genomic regions where a short core sequence (10-15 bp) is repeated many times in tandem (head-to-tail). Useful because: (1) Highly polymorphic: number of repeats at each VNTR locus varies enormously between individuals. At one locus, different people may have 3, 8, 14, 23 repeats (creating alleles of different sizes). (2) When cut with restriction enzymes (which cut outside the repeat region), each VNTR allele produces a fragment of a different size. (3) Multiple VNTR loci combined: the probability of two unrelated individuals having identical patterns at multiple loci is astronomically small (~1 in 30 billion for 4-6 loci). (4) Child inherits one allele of each VNTR from each parent → can be used for paternity testing.

3. Why is gel electrophoresis done BEFORE Southern blotting? ⌄

The purpose of gel electrophoresis is to SEPARATE the DNA fragments by size BEFORE they are transferred to the membrane. If you blotted all the unseparated DNA directly to a membrane and probed it, you would just get a large smear — you would not be able to distinguish between fragments of different sizes. The separation step (electrophoresis) creates a spatial pattern where small fragments are at one end and large fragments at the other. This separation is preserved when the DNA is transferred to the membrane (Southern blotting). The hybridisation then reveals which specific-sized fragments hybridise to the probe. The positions of the bands tell you the sizes of the VNTR-containing fragments → different individuals show different band positions → different DNA fingerprints.

4. What is the role of the VNTR probe? ⌄

The VNTR probe is a short, single-stranded DNA fragment that is complementary to the VNTR core repeat sequence. It is labelled (radioactively with ³²P, or with a fluorescent/chemiluminescent tag). Function: when applied to the membrane (under conditions that allow hybridisation: appropriate salt concentration and temperature), the probe base-pairs (hybridises) with complementary sequences on the membrane. It will find and bind to ALL fragments on the membrane that contain the VNTR core sequence. This means it will highlight all the VNTR-containing fragments at once. Different individuals will have VNTR fragments of different sizes (due to different numbers of repeats) → probe hybridises to differently sized fragments → different band patterns → unique 'fingerprint.' A single probe reveals multiple bands because VNTRs are found at multiple chromosomal locations.

5. What is autoradiography and how does it work? ⌄

Autoradiography: technique to detect and visualise radioactive molecules by their own radiation. Process: After hybridisation: membrane has probe bound to specific bands. ³²P (radioactive phosphorus) in probe emits beta particles (high-energy electrons). X-ray film placed tightly against membrane in darkness (usually at −70°C with an intensifying screen). Beta particles from ³²P hit silver halide crystals in film → reduce them to metallic silver → latent image. Film developed (like photo film) → metallic silver appears black → dark bands on film where probe hybridised. Film shows the DNA fingerprint pattern. Exposure time: 1 hour to several days depending on radioactivity and amount of DNA. Modern alternatives: (1) Phosphorimager: direct electronic detection of radioactivity. (2) Fluorescent probes: non-radioactive, visualised with UV light or laser scanner. (3) Chemiluminescence (ECL): enzyme-antibody systems that emit light.

6. Compare RFLP and STR methods of DNA profiling. ⌄

RFLP (Restriction Fragment Length Polymorphism) — the original Jeffreys method: Uses restriction enzymes, gel electrophoresis, Southern blotting, probe hybridisation, autoradiography. Requires large amounts of DNA (1-10 μg). Takes 1-2 weeks. Cannot work with degraded DNA. Analyses VNTRs (10-15 bp repeats). STR (Short Tandem Repeat) — modern method: Uses PCR amplification of specific STR loci with fluorescently labelled primers. Capillary electrophoresis separates PCR products by size. Automated readout. Requires very small amounts of DNA (1-10 ng or less). Takes hours. Works with partially degraded DNA. Analyses STRs (2-7 bp repeats). STR is the current standard in forensic labs worldwide due to higher sensitivity, speed, automation, and ability to work with challenging samples.

7. What was the first case where DNA fingerprinting solved a crime? ⌄

The first forensic use of DNA fingerprinting was the Colin Pitchfork case (Narborough, Leicestershire, UK) in 1986. Background: Two teenage girls were murdered near the village of Narborough in 1983 and 1986. Initially, a young man named Richard Buckland confessed to the second murder (under pressure during questioning). Alec Jeffreys' DNA test: DNA from crime scenes tested. Buckland's DNA did NOT match — he was exonerated (first person proven innocent by DNA evidence). Police then collected blood samples from thousands of local men. Colin Pitchfork's sample (initially provided fraudulently by someone else) was eventually matched to crime scene DNA when the deception was discovered. Pitchfork was convicted in 1988 based on DNA evidence — first criminal convicted using DNA profiling. This case established DNA fingerprinting as a revolutionary forensic tool.

8. What is Southern blotting used for besides DNA fingerprinting? ⌄

Southern blotting has many applications in molecular biology beyond DNA fingerprinting: (1) Restriction mapping: determining positions of restriction enzyme sites in a gene. (2) Gene detection: identifying whether a specific gene is present in a genome. (3) RFLP analysis: for genome mapping, disease gene linkage analysis. (4) Transgenic verification: confirming integration of transgene in GMO organisms. (5) Diagnosis of genetic diseases: detecting specific mutations or deletions (e.g., diagnosing thalassaemia, DMD gene deletions). (6) Measuring gene copy number: how many copies of a gene are present. (7) Zygosity determination: is a transgene insertion homozygous or heterozygous? (8) Strain typing of microorganisms. Modern replacements: PCR-based methods (faster, less DNA needed) have replaced Southern blotting for many routine applications, but Southern blot remains important for accurate sizing of large fragments and detecting gross rearrangements.

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