Recombinant DNA Technology & Techniques

Recombinant DNA (rDNA) technology is the artificial joining of DNA from two sources to create novel sequences, then propagating and analysing them. For NEET PG this chapter is exploding in weightage — expect direct questions on enzyme functions, PCR logic, the three blots, Sanger sequencing, FISH, microarrays and CRISPR. Master the "which technique detects what" matrix and the cut-off facts.

Definition & core toolkit

Recombinant DNA = a DNA molecule formed by combining genetic material from multiple sources using molecular cloning. The workflow is: isolate gene → cut with restriction enzyme → insert into a vector → transform into host → select recombinant clones → express/analyse.

Key molecular "tools":

Tool	Function	Exam pearl
Restriction endonuclease	Cuts dsDNA at specific palindromic sites	Bacterial defence; named by organism (EcoRI from E. coli)
DNA ligase	Seals nicks (forms phosphodiester bond)	Needs ATP (or NAD⁺ in bacteria); "molecular glue"
DNA polymerase	Synthesises new strand 5′→3′	Needs primer + template
Reverse transcriptase	RNA → cDNA	From retroviruses; basis of RT-PCR & cDNA libraries
Alkaline phosphatase	Removes 5′ phosphate	Prevents vector self-ligation
Terminal transferase	Adds nucleotides without template	Used to make homopolymer tails

High-yield: A vector is a DNA vehicle that can self-replicate in a host. Plasmids (≤10 kb), bacteriophage λ (up to 20 kb), cosmids (~45 kb), BACs (~300 kb) and YACs (>1000 kb / 1 Mb) differ mainly in insert-carrying capacity — YAC carries the largest.

Restriction enzymes (molecular scissors)

Restriction endonucleases recognise specific palindromic sequences (read same 5′→3′ on both strands) and cleave DNA. The host protects its own DNA by methylation at the same sites (restriction–modification system).

• EcoRI recognises GAATTC and cuts between G and A leaving sticky (cohesive) ends with 5′ overhangs.
• Blunt-end cutters (e.g., SmaI — CCCGGG) cut straight across.
• Sticky ends are preferred for cloning because complementary overhangs base-pair, making ligation efficient.

High-yield: Sticky ends > blunt ends for cloning efficiency. The same enzyme used to cut the gene and the vector ensures compatible overhangs.

Mnemonic for naming: EcoRI → Escherichia (genus) + coli (species) + R (strain RY13) + I (order discovered).

Polymerase Chain Reaction (PCR)

PCR amplifies a target DNA region exponentially in vitro. Each cycle doubles the target → 2ⁿ amplification.

Essential components: template DNA, two primers (forward + reverse, ~18–25 nt), Taq polymerase (thermostable, from Thermus aquaticus), dNTPs, Mg²⁺ (cofactor), and buffer.

The three steps (one cycle):

1. Denaturation (~94–95 °C) — strands separate.
2. Annealing (~50–65 °C) — primers bind; temperature set ~5 °C below primer Tm.
3. Extension (~72 °C) — Taq synthesises new strands.

Denaturation → Annealing → Extension → (repeat ×25–35)

High-yield: Taq polymerase is favoured because it survives the 94 °C denaturation step. Mg²⁺ is the essential cofactor; too much Mg²⁺ causes non-specific amplification.

PCR variants (very commonly tested)

Variant	Special feature	Used for
RT-PCR	Reverse transcriptase makes cDNA first	Quantify/detect RNA (e.g., viral RNA, gene expression)
Real-time / qPCR	Fluorescent dye (SYBR Green) or TaqMan probe; measures product as it forms	Quantification; viral load (HIV, HCV, SARS-CoV-2)
Nested PCR	Two primer pairs in sequence	Increases specificity; low-copy targets (TB)
Multiplex PCR	Multiple primer pairs in one tube	Detect several targets simultaneously (DMD deletions)
Hot-start PCR	Polymerase activated only at high temp	Reduces non-specific products
ARMS / AS-PCR	Allele-specific primers	Point-mutation detection (sickle cell, CFTR)

High-yield: RT-PCR is the gold-standard molecular test for COVID-19 and other RNA viruses. qPCR/real-time PCR is used for viral load quantification and is the basis of the Ct value (cycle threshold — lower Ct = higher viral load).

Blotting techniques (the classic trio)

This is one of the highest-yield comparisons in molecular biology MCQs.

Blot	Detects	Probe	Mnemonic
Southern	DNA	Labelled DNA/RNA	Southern = DNA (S–D)
Northern	RNA	Labelled DNA/RNA	Northern = RNA
Western	Protein	Antibody	W = protein (West)
Southwestern	DNA-binding proteins	Labelled DNA probe	Protein detected by DNA
Eastern	Post-translational modification	—	Rarely asked

Mnemonic: "SNoW DRoP" → Southern–DNA, Northern–RNA, Western–Protein.

High-yield: Southern blot (named after Edwin Southern, the only one named after a person) was the original. The others are wordplay. Western blot is the confirmatory test for HIV (after ELISA screening) and for Lyme disease.

General blotting flow: electrophoresis (size separation) → transfer to membrane → probe hybridisation/antibody → detection.

DNA sequencing — Sanger method

Sanger (dideoxy chain-termination) sequencing is the classic technique and remains the gold standard for single-gene/confirmatory sequencing.

Principle: DNA synthesis is terminated whenever a dideoxynucleotide (ddNTP) is incorporated, because it lacks the 3′-OH needed to form the next phosphodiester bond. Four reactions (or one with fluorescent ddNTPs) generate fragments of every possible length, separated by size.

Template + primer + DNA polymerase + dNTPs + ddNTPs → chain termination at each ddNTP → capillary electrophoresis → read sequence

High-yield: The key reagent in Sanger sequencing is the ddNTP, which lacks the 3′-hydroxyl group, terminating elongation. Fred Sanger won two Nobel Prizes (protein + DNA sequencing).

Next-generation sequencing (NGS) = massively parallel sequencing; basis of whole-exome/whole-genome sequencing, used increasingly for cancer panels and inherited disease. Higher throughput, lower cost per base than Sanger, but Sanger still validates NGS findings.

FISH (Fluorescence In Situ Hybridisation)

FISH uses a fluorescent-labelled DNA probe that hybridises to a complementary chromosomal sequence, visualised under a fluorescence microscope. It bridges cytogenetics and molecular biology.

Uses:

• Microdeletion syndromes — DiGeorge (22q11.2), Prader-Willi/Angelman (15q11), Williams (7q11), Cri-du-chat (5p).
• Aneuploidy in interphase nuclei (rapid prenatal — trisomy 21, 13, 18, X, Y).
• Gene amplification/translocation in cancer — HER2/neu (breast), BCR-ABL (CML), N-myc (neuroblastoma).

High-yield: FISH is the investigation of choice for submicroscopic microdeletions (too small for routine karyotype) and for HER2 amplification in breast cancer. Interphase FISH gives rapid aneuploidy results without culture.

DNA microarray (gene chip)

A microarray carries thousands of immobilised oligonucleotide probes on a chip; labelled sample nucleic acid hybridises, and fluorescence intensity reflects expression level or copy number.

Applications: gene-expression profiling, comparative genomic hybridisation (array-CGH — detects copy-number variations across the whole genome at high resolution, now first-line for unexplained developmental delay/intellectual disability), and SNP genotyping.

High-yield: Array-CGH has replaced karyotype as the first-tier test for unexplained intellectual disability, autism and multiple congenital anomalies because it detects sub-microscopic copy-number variants genome-wide.

CRISPR-Cas9 (genome editing)

CRISPR (Clustered Regularly Interspaced Short Palindromic Repeats) evolved as a bacterial adaptive immune system against bacteriophages. Engineered into a gene-editing tool:

• A guide RNA (gRNA) directs the Cas9 nuclease to a complementary genomic target.
• Cas9 creates a double-strand break, repaired by NHEJ (error-prone — gene knockout) or HDR (precise — gene correction with a donor template).
• A PAM sequence (e.g., NGG) adjacent to the target is required for Cas9 cutting.

High-yield: Cas9 needs gRNA + a PAM sequence to cut. Casgevy (exa-cel) — the first approved CRISPR therapy — treats sickle cell disease & β-thalassaemia. Doudna and Charpentier won the 2020 Nobel Prize (Chemistry).

Applications: diagnosis & forensics

• Genetic disease diagnosis: PCR + sequencing for point mutations (sickle cell, CF), multiplex PCR for DMD deletions, FISH for microdeletions, MLPA for copy-number changes.
• Infectious disease: RT-PCR (HIV, HCV, COVID-19), GeneXpert/CBNAAT (cartridge-based nested real-time PCR) for TB + rifampicin resistance.
• Forensics & paternity: STR (Short Tandem Repeat) analysis is the current standard for DNA fingerprinting (replaced older RFLP/VNTR). Mitochondrial DNA used for degraded/maternal-lineage samples.
• Prenatal: cell-free fetal DNA (NIPT) for trisomies via NGS.

Clinical need	Best technique
Detect/quantify RNA virus	RT-PCR / qPCR
Confirm HIV after ELISA	Western blot
22q11 microdeletion	FISH
Unexplained ID / CNV screen	Array-CGH
Single-gene point mutation	Sanger sequencing
Whole-exome screen	NGS
Paternity / crime scene	STR DNA fingerprinting
TB + rifampicin resistance	CBNAAT (GeneXpert)

Gene therapy & expression vectors (brief)

Recombinant DNA enables therapeutic protein production — recombinant human insulin (first rDNA drug, 1982), growth hormone, erythropoietin, clotting factor VIII, and hepatitis B vaccine (recombinant surface antigen in yeast). Viral vectors (adeno-associated virus, lentivirus) deliver corrective genes in gene therapy (e.g., Luxturna for retinal dystrophy, Zolgensma for SMA).

High-yield: The first recombinant DNA product approved for human use was insulin ("Humulin"). The hepatitis B vaccine is the prototype recombinant subunit vaccine.

Complications / limitations & pitfalls

• PCR contamination → false positives (amplicon carry-over); needs strict workspace separation.
• Off-target effects in CRISPR editing → unintended mutations; a major safety concern.
• Ethical/biosafety issues: germline editing (e.g., the He Jiankui controversy), gene drives, biosecurity.
• Sanger sequencing cannot economically scale to whole genomes (hence NGS).
• FISH only detects the specific locus probed — it is not a genome-wide screen.

Key differentials / "don't confuse these"

• Restriction enzyme vs DNA ligase: scissors (cut) vs glue (join).
• Southern vs Northern: DNA vs RNA — remember DNA contains a "D" matching SoutherD... use SNoW DRoP.
• PCR vs cloning: PCR amplifies in vitro (no living cell); cloning amplifies inside a host cell.
• FISH vs karyotype: FISH detects sub-microscopic deletions a karyotype misses, but karyotype sees the whole genome at low resolution.
• qPCR vs conventional PCR: real-time quantification vs end-point detection.

Recently asked / exam angle

• Direct match-the-following: technique ↔ molecule detected (blots), enzyme ↔ function.
• "Which enzyme lacks 3′-OH requirement?" → ddNTP in Sanger.
• "Investigation of choice for DiGeorge / 22q11 deletion" → FISH.
• "Confirmatory test for HIV" → Western blot.
• "Thermostable polymerase in PCR" → Taq.
• "First recombinant DNA drug" → insulin.
• "CRISPR component that directs Cas9" → guide RNA; "sequence needed for cutting" → PAM.
• "First-line test for unexplained intellectual disability" → chromosomal microarray (array-CGH).
• "Standard method of DNA fingerprinting today" → STR analysis.
• "Ct value belongs to which technique?" → real-time / qPCR.

Rapid revision

1. Restriction enzymes cut at palindromic sequences; EcoRI → GAATTC, sticky ends.
2. DNA ligase forms phosphodiester bonds (molecular glue).
3. PCR steps: denaturation → annealing → extension; Taq polymerase, Mg²⁺ cofactor, 2ⁿ amplification.
4. RT-PCR detects RNA; qPCR quantifies (Ct value, viral load).
5. SNoW DRoP: Southern–DNA, Northern–RNA, Western–Protein.
6. Western blot = confirmatory test for HIV and Lyme disease.
7. Sanger uses ddNTP lacking the 3′-OH → chain termination.
8. FISH = investigation of choice for microdeletions (22q11, 15q11) and HER2.
9. Array-CGH = first-tier test for unexplained intellectual disability/CNVs.
10. CRISPR-Cas9 needs gRNA + PAM; Casgevy treats sickle cell & β-thalassaemia.
11. Insulin = first recombinant DNA therapeutic; HBV vaccine = recombinant subunit vaccine.
12. Forensics today uses STR analysis; CBNAAT/GeneXpert detects TB + rifampicin resistance.