Biotechnology

Recombinant DNA — the toolkit

Modern biotechnology turns on three technical advances. First, restriction endonucleases: Werner Arber, Daniel Nathans, and Hamilton Smith (Nobel 1978) showed bacterial enzymes that cleave foreign DNA at palindromic sites — EcoRI cuts G↓AATTC, BamHI G↓GATCC, HindIII A↓AGCTT. Sticky ends allow ligation by T4 DNA ligase. Second, vectors: bacterial plasmids (pUC19, pBR322), bacteriophage λ, cosmids, and BACs each accept different insert sizes. Third, transformation: heat-shock or electroporation introduces recombinant DNA into E. coli. Punjab Textbook Board Biology XII Chapter 22 and Campbell 12e Chapter 20 develop these tools step-by-step.

Polymerase chain reaction

Kary Mullis devised PCR in 1983 (Nobel 1993). A reaction mix contains template DNA, two primers (~20 nt) flanking the target, dNTPs, Mg²⁺, and a thermostable DNA polymerase — originally Taq from Thermus aquaticus isolated from Yellowstone hot springs (Brock, 1969). Each cycle: denaturation at 95 °C (~30 s), annealing at 50–65 °C (depending on primer Tm), extension at 72 °C. Thirty cycles theoretically amplify 2³⁰ ≈ 10⁹-fold. Variants: RT-PCR for RNA quantification (used in COVID-19 diagnostics), qPCR with fluorescent probes (TaqMan, SYBR Green) for real-time quantification, and PCR-based forensic STR profiling.

Gel electrophoresis and DNA sequencing

Agarose gels separate DNA fragments by size (1 % gel resolves 0.5–10 kb). DNA is loaded near the cathode, migrates toward the anode under an electric field (typically 5 V/cm), and is visualised with ethidium bromide (intercalator) or safer dyes like SYBR Safe under UV. SDS-PAGE with polyacrylamide separates denatured proteins. Sanger's 1977 dideoxy chain-termination method used radiolabelled or fluorescent ddNTPs to sequence DNA; modern next-generation sequencing (Illumina, PacBio, Oxford Nanopore) reads gigabases per run, enabling whole-genome sequencing for under USD 1000 (per the NHGRI Genome Sequencing Cost dataset).

Recombinant therapeutics and GMOs

The first recombinant drug was human insulin (Humulin) — Genentech and Eli Lilly licensed it in 1982. The 51-amino-acid hormone (MW 5808 Da) was produced by inserting the synthetic A and B chain genes into E. coli plasmids; the chains were combined chemically. Today recombinant systems also produce growth hormone, erythropoietin, factor VIII, monoclonal antibodies, and vaccines (hepatitis B surface antigen). Transgenic crops include Bt cotton (carrying Bacillus thuringiensis cry genes for insect resistance — adopted on millions of hectares in Pakistan's Punjab) and Roundup Ready soybean. Golden Rice carries psy and crtI genes producing β-carotene to address vitamin A deficiency.

CRISPR–Cas9 and the genome-editing revolution

Jennifer Doudna and Emmanuelle Charpentier (Nobel Chemistry 2020) showed in 2012 that the bacterial CRISPR–Cas9 system could be reprogrammed for site-specific genome editing using a 20-nt guide RNA. Cas9 makes a double-strand break that cells repair by error-prone NHEJ (knockouts) or template-driven HDR (knock-ins). Clinical applications include Casgevy (exa-cel), the first approved CRISPR therapy for sickle-cell disease (FDA, December 2023). Gene therapy for severe combined immunodeficiency, Leber congenital amaurosis (Luxturna), and spinal muscular atrophy (Zolgensma, an AAV9 vector delivering SMN1) illustrate the trajectory. Bioethics around germline editing remains active after the 2018 He Jiankui controversy.