Biotechnology

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Biotechnology is the application of biological systems, living organisms, or their derivatives to develop products and technologies that improve human life. Modern biotechnology integrates molecular biology, genetics, microbiology, biochemistry, and chemical engineering to manipulate genetic material (DNA) for practical purposes.

Key Concepts:

Concept	Description
Genetic Engineering	Direct manipulation of an organism’s genome using recombinant DNA technology
rDNA	DNA formed by combining sequences from different sources
Vector	DNA molecule (plasmid, phage) that carries foreign DNA into a host cell
Restriction Enzyme	Bacterial enzyme that cuts DNA at specific sequences
Cloning	Creating genetically identical copies of a DNA fragment, cell, or organism
PCR	Polymerase Chain Reaction — amplifies specific DNA sequences exponentially

Tools of Genetic Engineering:

• Restriction endonucleases: Type II enzymes (EcoRI, HindIII, BamHI) recognise palindromic sequences and cut at specific sites
• DNA ligase: Joins DNA fragments together (forms phosphodiester bonds)
• Vectors: Plasmids (pBR322, pUC19), bacteriophages (lambda phage), cosmids, BACs, YACs
• Host organisms: E. coli (most common), Saccharomyces cerevisiae (yeast), cultured animal/plant cells

⚡ Exam Tips:

• EcoRI cuts between G and A in the sequence 5’-GAATTC-3’ (and complementary strand)
• Restriction sites are usually 4 or 6 base pairs long — 6 bp cutters give fewer fragments and larger fragments
• pBR322 has two antibiotic resistance genes (ampicillin and tetracycline) — useful for selection

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Restriction Enzymes — The Molecular Scissors:

Restriction enzymes are endonucleases encoded by bacteria that protect against invading bacteriophages by degrading foreign DNA. They recognise specific DNA sequences (4–8 bp) and cleave both DNA strands. The discovery of restriction enzymes by Smith, Arber, and Nathans (1970s) made genetic engineering possible — they shared the 1978 Nobel Prize.

Types of Restriction Enzymes:

• Type I: Cut at random sites >1 kb from recognition sequence; require Mg²⁺, ATP, SAM; multifunctional enzyme with restriction + modification activity
• Type II: Cut at specific sites within or near recognition sequence; require only Mg²⁺; most useful for genetic engineering (e.g., EcoRI, HindIII, HaeIII)
• Type III: Cut ~25 bp downstream of recognition sequence; require ATP

Type II enzymes are preferred because they are simple, catalytically self-sufficient, and produce predictable fragments.

Palindromic Sequences: Restriction sites are usually palindromic (reads the same 5’→3’ on both strands):

EcoRI: 5'-G ↓ A A T T C-3'
       3'-C T T A A ↑ G-5'

The enzyme cuts between G and A on both strands, producing sticky ends (overhanging single-stranded complementary ends). This is crucial for DNA ligation — complementary sticky ends anneal by base pairing, making ligation more efficient than with blunt ends.

Vectors — Carriers of Foreign DNA:

A good vector must have:

1. Origin of replication (ori) for replication in host
2. Selectable markers (antibiotic resistance genes)
3. Multiple cloning site (MCS) with unique restriction sites
4. Small size for easy manipulation

Plasmid vectors:

• pUC19: 2,686 bp; lacZ gene (for blue-white screening); ampicillin resistance; multiple cloning site
• pBR322: 4,361 bp; ampicillin + tetracycline resistance; two MCS regions

Bacteriophage lambda (λ):

• Can carry 12–25 kb foreign DNA (cos site allows in-vitro packaging into phage particles)
• Used for genomic library construction

Binary vectors for plants:

• Contain two components: T-DNA in plasmid (in Agrobacterium) + helper plasmid or chromosome
• Agrobacterium tumefaciens naturally transfers T-DNA (Ti plasmid) into plant cells causing crown gall disease

Gene Cloning — Step by Step:

1. Isolation of gene of interest: Use restriction enzymes or PCR to cut/amplify the gene
2. Insertion into vector: Ligate gene into plasmid using DNA ligase
3. Transformation: Introduce recombinant plasmid into E. coli (CaCl₂ method or electroporation)
4. Selection: Plate on antibiotic-containing agar; only transformed cells grow
5. Screening: Use blue-white screening (lacZ disruption) or colony hybridisation to identify colonies with correct insert
6. Expression: Culture positive colonies for protein production (if expression vector) or further analysis

PCR — Polymerase Chain Reaction:

PCR amplifies a specific DNA segment without requiring a vector or host organism. It uses:

• Template DNA: The DNA containing the region to amplify
• Primers: Short oligonucleotides (18–25 bases) complementary to flanking regions of the target
• Thermostable DNA polymerase: Taq polymerase (from Thermus aquaticus) — survives denaturation temperatures
• dNTPs: Deoxyribonucleotide triphosphates (dATP, dCTP, dGTP, dTTP)

PCR Cycles:

1. Denaturation (94–98°C): Double-stranded DNA separates
2. Annealing (50–65°C): Primers hybridise to complementary flanking sequences
3. Extension (72°C): Taq polymerase synthesises new DNA from primer (extends 5’→3’)

Each cycle doubles the amount of target DNA. After n cycles, the amount = 2ⁿ × (initial copies). Typical PCR: 25–35 cycles → billions of copies from a few molecules.

Variants of PCR:

• RT-PCR: Reverse transcription PCR — converts RNA to cDNA first using reverse transcriptase; used to study gene expression
• qPCR (Real-time PCR): Quantifies initial DNA amount using fluorescent dyes (SYBR Green) or probes
• Multiplex PCR: Multiple primer pairs to amplify different targets simultaneously

⚡ Common Mistakes:

• Students confuse sticky ends and blunt ends: sticky ends have overhanging 5’ or 3’ overhangs; blunt ends are flush cuts
• Don’t confuse DNA ligase (joins fragments) with restriction enzymes (cut DNA)
• The ori site is crucial — without it, the plasmid won’t replicate in the host cell

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🔴 Extended — Deep Dive (exam-level mastery)

For students preparing for top-rank selection.

Recombinant DNA Technology — Detailed Cloning Strategies:

Blue-White Screening: The pUC series plasmids contain the lacZ gene encoding β-galactosidase. When grown on X-gal (chromogenic substrate), colonies expressing functional lacZ are blue. Foreign DNA inserted into the multiple cloning site (within lacZ) disrupts the gene → white colonies. This allows rapid visual identification of recombinant colonies.

Replica Plating: Used to transfer colonies from one plate to another without losing spatial information. A velvet (or nitrocellulose) replica is pressed onto the original plate, then pressed onto selective media. Colony positions are compared to identify colonies with the desired phenotype.

cDNA Library Construction: Messenger RNA (mRNA) is extracted from cells expressing the gene of interest. Reverse transcriptase synthesises complementary DNA (cDNA) using an oligo-dT primer that binds to the poly-A tail of mRNA. This cDNA lacks introns (spliced out during mRNA processing), making it suitable for expression in prokaryotes.

Genomic vs cDNA Library:

• Genomic library: Contains all DNA fragments (coding + non-coding); requires larger capacity vectors (YACs can carry up to 1 Mb)
• cDNA library: Contains only expressed genes; intron-free; ideal for studying gene expression

Gene Delivery Methods in Animals:

1. Microinjection: Direct injection of DNA into the nucleus (used for transgenic mice)
2. Retroviral vectors: Integrate into host genome; used in gene therapy clinical trials
3. Biolistics (gene gun): Fire DNA-coated gold particles into cells at high velocity
4. CRISPR-Cas9: RNA-guided endonuclease for precise genome editing (see below)

CRISPR-Cas9 System — The Gene Editor:

The CRISPR-Cas9 system (Clustered Regularly Interspaced Short Palindromic Repeats) was adapted from a bacterial adaptive immune system. It consists of:

• Guide RNA (gRNA): 20-nucleotide sequence complementary to target DNA; part of a larger scaffold
• Cas9 endonuclease: The “molecular scissors”

The gRNA guides Cas9 to the target site by complementary base pairing. Cas9 then creates a double-strand break (DSB) 3 bp upstream of the PAM sequence (NGG for Streptococcus pyogenes Cas9).

The cell repairs the DSB via:

1. Non-homologous end joining (NHEJ): Error-prone; causes insertions/deletions → gene knockout
2. Homology-directed repair (HDR): Uses a donor template → precise gene editing

CRISPR was developed by Jennifer Doudna and Emmanuelle Charpentier (Nobel Prize 2020). Its applications include gene therapy (sickle cell disease, beta-thalassaemia — approved therapies exist), agricultural improvements, and basic research.

DNA Fingerprinting (RFLP Analysis):

Forensic and paternity identification uses variable number tandem repeats (VNTRs) — short DNA sequences repeated in tandem at specific loci. The number of repeats varies between individuals (inherited in Mendelian fashion).

Steps:

1. Extract DNA from sample (blood, semen, hair root)
2. Digest with restriction enzyme → fragments
3. Separate by gel electrophoresis
4. Transfer to nylon membrane (Southern blotting)
5. Hybridise with a VNTR probe
6. Autoradiography → band pattern

The probability of two unrelated individuals having identical VNTR patterns is less than 1 in 10¹¹ (smaller than the Earth’s population), making it virtually conclusive for identification.

Applications of Biotechnology:

Application	Technique	Example
Medical	Recombinant insulin	Humulin (E. coli); first biotech drug (1982)
Medical	Gene therapy	Luxturna (RPE65 mutation); CAR-T cells
Vaccines	Subunit vaccine	Hepatitis B vaccine (HBsAg in yeast)
Diagnostics	ELISA	HIV antibody detection
Agriculture	Bt cotton	Cry toxin gene from Bacillus thuringiensis
Agriculture	Golden rice	β-carotene biosynthesis genes in rice endosperm
Industry	Biodegradable plastics	PHB from Ralstonia eutropha
Environment	Bioremediation	Oil-degrading bacteria; Pseudomonas

Transgenic Animals:

• Oncomouse: Mouse with activated oncogene; model for cancer research
• GloFish: Fluorescent zebrafish (Danio rerio) with GFP gene; pet trade
• Pharming: Recombinant proteins in milk (antithrombin III from transgenic goats — ATryn)

Ethical Concerns:

• GMO safety for human consumption (extensively studied; no credible evidence of harm)
• Gene editing in human embryos (He Jiankui case, 2018 — edited CCR5 gene in twins; universally condemned)
• Patenting of life forms (Diamond v. Chakrabarty, 1979 — US Supreme Court upheld patent on oil-eating bacterium)
• Biosafety and biocontainment in laboratories

NEET High-Yield Pattern:

• EcoRI makes sticky ends; cuts at G↓AATTC
• pBR322 has ampicillin and tetracycline resistance markers
• Taq polymerase is heat-stable (from Thermus aquaticus); used in PCR
• The selectable marker in pUC19 is ampicillin resistance + lacZ for screening
• CRISPR-Cas9 creates double-strand breaks at 3 bp upstream of PAM (NGG)
• DNA fingerprinting uses VNTR probes