Biotechnology: Principles and Processes — NEET notes

Principles of Biotechnology

Biotechnology deals with using living organisms or their products to modify processes, develop technologies, and create useful products. Modern biotechnology rests on two core techniques: genetic engineering (rDNA technology) and bioprocess engineering (sterile conditions for growing organisms on industrial scale).

Desired Gene + Vector Restriction Enzyme + Ligase Recombinant DNA Host Cell Product

Traditional vs Modern Biotechnology

Biotechnology is not new — humans have used microbes for thousands of years. Modern biotechnology adds precision by manipulating DNA directly.

Humans have been doing biotechnology since ancient times without knowing it. Making curd, bread, wine, cheese — all involve microbes. But traditional biotechnology was blind trial-and-error. Modern biotechnology (post-1970s) is precise: we can cut specific genes, paste them into new organisms, and produce exactly the protein we want.

Two Core Techniques

Modern biotechnology stands on two pillars: (1) genetic engineering to create rDNA and (2) bioprocess engineering to scale up production in bioreactors under sterile conditions.

Pillar 1: Genetic Engineering (rDNA Technology) — The technique of altering the DNA of an organism by inserting foreign genes. You take a useful gene from organism A, insert it into organism B's DNA, and organism B now produces the desired protein. The altered DNA is called recombinant DNA (rDNA).

Pillar 2: Bioprocess Engineering — Even after creating a genetically modified organism, you need to grow it on an industrial scale under sterile, optimized conditions. This involves bioreactors, maintaining optimal temperature, pH, oxygen levels, and then purifying the product (downstream processing).

• rDNA technology — combines DNA from two different sources into one molecule
• The organism carrying rDNA is called a genetically modified organism (GMO) or transgenic organism
• Bioprocess engineering — growing GMOs at industrial scale in bioreactors
• Both techniques together enable commercial production of therapeutic proteins, vaccines, enzymes
• First successful rDNA experiment: Stanley Cohen and Herbert Boyer (1973) — transferred antibiotic resistance gene between bacteria

Cohen & Boyer's 1973 experiment is considered the birth of genetic engineering. They used a plasmid vector and restriction enzymes — the exact tools you'll study in the next concepts.

Three Tools of rDNA Technology

To do genetic engineering, you need three basic tools: restriction enzymes (to cut DNA), ligase (to join DNA), and vectors (to carry DNA into host cells).

Think of genetic engineering as a cut-copy-paste operation on DNA. You need: 1. Restriction enzymes (molecular scissors) — cut DNA at specific sequences 2. DNA ligase (molecular glue) — joins DNA fragments together 3. Vectors (molecular vehicles) — carry the recombinant DNA into host cells

• Restriction enzymes — discovered in bacteria, cut foreign DNA at specific palindromic sites
• DNA ligase — seals the sugar-phosphate backbone, joining two DNA fragments
• Vectors — plasmids, bacteriophages, cosmids that carry foreign DNA into host
• Host organism — the cell that receives and expresses the recombinant DNA (usually E. coli, yeast, or plant cells)
• Additional tools: PCR (amplify DNA), gel electrophoresis (separate DNA), competent cells (take up DNA)

Restriction Enzymes

Restriction enzymes are molecular scissors that cut DNA at specific palindromic recognition sequences. Discovered in bacteria as a defense against bacteriophages, they are the most critical tool of rDNA technology. EcoRI, HindIII, BamHI — each cuts at a unique sequence, producing sticky or blunt ends.

EcoRI: 5'-G -3' / 3'-CTTAA -5'

Discovery & Nomenclature

Restriction enzymes were discovered when scientists noticed bacteria could destroy foreign (phage) DNA while protecting their own. The naming convention encodes the source organism.

In the 1960s, scientists observed that bacteriophage DNA was cut into pieces when it entered certain bacteria — but the bacterium's own DNA stayed intact. The bacterial enzymes responsible were called restriction endonucleases because they 'restrict' the growth of phages. Werner Arber, Daniel Nathans, and Hamilton Smith won the 1978 Nobel Prize for this discovery.

Nomenclature system — The name tells you exactly where the enzyme came from: - First letter = Genus (italicized, capitalized) - Next two letters = Species (italicized, lowercase) - Fourth letter = Strain - Roman numeral = Order of discovery

• Restriction endonucleases cut DNA at specific internal sites (endo = within the strand)
• Exonucleases remove nucleotides from the ends — NOT used in rDNA technology
• First restriction enzyme isolated: HindII by Hamilton Smith (1970)
• Over 900 restriction enzymes isolated from 230+ bacterial strains
• Each enzyme recognizes a SPECIFIC DNA sequence (4-8 bp long)

Palindromic Sequences

Restriction enzymes recognize palindromic sequences — sequences where the 5' to 3' reading on one strand is identical to the 5' to 3' reading on the complementary strand.

In English, a palindrome reads the same forwards and backwards (MADAM, RACECAR). In DNA, a palindromic sequence reads the same on both strands when read in the 5' to 3' direction. This is NOT the same as reading a single strand backwards.

• Palindromic = same sequence on both strands when read 5'→3'
• Most restriction sites are 4-8 base pairs long
• 4-cutter (e.g., AluI: AGCT) cuts MORE frequently than 6-cutter (e.g., EcoRI: GAATTC)
• Why? A 4-bp sequence occurs once every 4^4 = 256 bp on average; a 6-bp sequence once every 4^6 = 4096 bp
• All restriction enzymes cut within or near their palindromic recognition sequence

Sticky Ends vs Blunt Ends

When restriction enzymes cut DNA, they produce either sticky ends (overhanging single-stranded tails) or blunt ends (flush cuts). Sticky ends are preferred in rDNA because they enable easy joining.

Restriction enzymes cut the two strands of DNA. If the enzyme cuts at different positions on the two strands (staggered cut), it creates sticky ends — short single-stranded overhangs that can base-pair with complementary overhangs. If it cuts both strands at the same position, it creates blunt ends — no overhangs.

• EcoRI cuts between G and A on both strands → creates 4-base 5' sticky overhangs (5'-AATT-3')
• Sticky ends from the SAME enzyme are always complementary → they anneal spontaneously
• DNA ligase then seals the phosphodiester bonds permanently
• Two different enzymes producing IDENTICAL sticky ends (compatible ends) can also be joined — called isocaudomers
• Blunt ends can be converted to sticky ends by adding synthetic linkers or adapters

Quick recall

Why must the same restriction enzyme be used to cut both vector and foreign DNA?

Because the same enzyme produces the same sticky ends on both pieces. Complementary sticky ends from vector and insert can base-pair and be sealed by DNA ligase, creating recombinant DNA.

Cloning Vectors

Cloning vectors are DNA molecules that carry foreign genes into host cells and replicate independently. Essential features: origin of replication (ori), selectable markers, restriction sites, and small size. pBR322 is the most NEET-tested vector — know its markers (ampR, tetR) and restriction sites cold.

pBR322: p = plasmid, B = Bolívar, R = Rodriguez, 322 = serial number

Essential Features of Vectors

Every cloning vector needs an origin of replication, selectable marker(s), restriction enzyme sites, and small size. Without any one of these, the vector fails.

A vector isn't just any DNA — it must have specific features to function as a gene-delivery vehicle. Think of it as a car that must have an engine (ori), a license plate (selectable marker), doors (restriction sites), and be compact enough to park (small size).

• Origin of replication (ori) — Sequence where replication begins. Controls copy number (how many copies per cell). High copy number = more protein produced
• Selectable marker — Gene that allows identification of transformed cells (e.g., antibiotic resistance genes). Without it, you can't tell which cells took up the vector
• Cloning site / Multiple Cloning Site (MCS) — Unique restriction enzyme sites where foreign DNA is inserted. Must be within or near a selectable marker for insertional inactivation screening
• Small size — Smaller vectors enter host cells more easily. Large DNA molecules are harder to transform
• Additional feature: Insertional inactivation — Foreign DNA insertion into a marker gene inactivates it, allowing screening

pBR322

pBR322 is an artificial plasmid vector with two selectable markers (ampR and tetR), multiple restriction sites, and a well-characterized ori. It uses insertional inactivation for screening recombinants.

pBR322 is the gold standard cloning vector and a NEET favorite. Created by Bolívar and Rodriguez in 1977. It's 4361 bp long and has everything a vector needs.

• Name: p = plasmid, B = Bolívar, R = Rodriguez, 322 = serial number
• Size: 4361 base pairs
• Selectable markers: ampR (ampicillin resistance) and tetR (tetracycline resistance)
• Important restriction sites: BamHI (in tetR gene), PstI (in ampR gene), EcoRI, HindIII, SalI, ClaI
• Origin of replication (ori): derived from ColE1 plasmid, gives ~15-20 copies per cell
• Insertional inactivation: insert foreign DNA into BamHI site → tetR gene disrupted → cell loses tetracycline resistance

Quick recall

If foreign DNA is inserted at PstI site of pBR322, which antibiotic resistance is lost?

Ampicillin resistance (ampR) is lost because PstI site is within the ampR gene. Screen: grow on tetracycline first (both survive), then replica plate on ampicillin (recombinants die).

Other Vectors

Beyond pBR322, several specialized vectors exist for different purposes: pUC (blue-white screening), Ti plasmid (plant transformation), bacteriophage lambda, and cosmids (large inserts).

Different cloning jobs need different vehicles. Small gene? Use a plasmid. Large gene? Use a cosmid or BAC. Want to transform plants? Use Ti plasmid from Agrobacterium. Each vector has its niche.

• pUC vectors — improved pBR322. Have lacZ gene for blue-white screening (X-gal + IPTG). Recombinants = white colonies, non-recombinants = blue. Much faster than replica plating
• Ti plasmid — from *Agrobacterium tumefaciens*. Contains T-DNA that naturally integrates into plant genome. Modified Ti plasmids = best vector for plant genetic engineering
• Bacteriophage lambda (λ) — virus that infects E. coli. Can carry up to ~23 kb of foreign DNA. Middle portion of genome is replaceable
• Cosmids — hybrid of plasmid + phage lambda cos sites. Can carry 33-45 kb inserts. Used for genomic libraries
• BACs (Bacterial Artificial Chromosomes) — carry up to 300 kb. Used in Human Genome Project
• YACs (Yeast Artificial Chromosomes) — carry up to 1000 kb (1 Mb). Largest capacity vectors

Competent Host & Transformation

Host cells must be made 'competent' (able to take up foreign DNA) before transformation. Methods include CaCl2 treatment + heat shock, electroporation, microinjection, biolistics (gene gun), and Agrobacterium-mediated transfer. The choice depends on cell type — bacteria, plant, or animal.

CaCl_2 (ice, 0°C) heat shock (42°C, 90s) DNA enters through transient pores

Making Cells Competent

Bacterial cell walls and membranes normally block DNA entry. Treatment with divalent cations (Ca2+) and brief heat shock creates transient pores that allow rDNA to enter.

DNA is a large, negatively charged molecule. Bacterial cell membranes are also negatively charged. Two negatives repel — so DNA can't just waltz into a cell. The cell must be made competent (receptive to DNA uptake).

• CaCl₂ method: Treat bacteria with ice-cold CaCl₂ (divalent Ca²⁺ ions neutralize the negative charges on DNA and membrane)
• Heat shock: Brief pulse at 42°C for 60-90 seconds, then immediately back on ice
• Heat shock creates transient pores in the membrane through which rDNA enters
• This is the most common lab method for E. coli transformation
• Efficiency: ~1 in 10⁷ cells takes up DNA — still enough when you plate millions of bacteria

Other Methods

CaCl₂/heat shock works for bacteria, but plant and animal cells need different methods. Electroporation uses electric pulses, microinjection uses needles, biolistics uses gold bullets, and Agrobacterium is nature's own gene delivery system for plants.

Different cell types have different barriers. Bacteria have cell walls. Plant cells have thick cell walls + cell membrane. Animal cells have only a membrane (easier to penetrate). Each needs a tailored approach.

• Electroporation — Brief high-voltage electric pulses (kV range) create temporary pores in cell membrane. Works for bacteria, plant, AND animal cells. Very versatile
• Microinjection — Direct injection of DNA into the nucleus using a fine glass micropipette under a microscope. Used for animal cells (especially for creating transgenic animals)
• Biolistics / Gene gun — DNA-coated gold or tungsten microparticles are shot at high velocity into plant cells. Used when Agrobacterium doesn't work (e.g., monocots like rice, wheat)
• Agrobacterium-mediated transfer — Natural plant pathogen delivers T-DNA into plant genome. Best method for dicots (and now also some monocots)
• Lipofection — DNA wrapped in lipid vesicles (liposomes) that fuse with cell membrane. Used for animal cell cultures

Selecting Recombinants

After transformation, most cells don't take up DNA. Selectable markers, insertional inactivation, and blue-white screening help identify the rare cells that carry recombinant DNA.

Transformation is inefficient — only a tiny fraction of cells take up the vector, and an even tinier fraction get the recombinant vector (with insert). You need a screening strategy to find them.

• Step 1: Antibiotic selection — plate on antibiotic media. Only cells with the vector (carrying resistance gene) survive
• Step 2: Distinguish recombinants from non-recombinants — use insertional inactivation or chromogenic screening
• Insertional inactivation — insert disrupts one marker gene; recombinants lose one resistance (pBR322 method)
• Blue-white screening — insert disrupts lacZ gene; recombinants form WHITE colonies on X-gal plates, non-recombinants form BLUE colonies
• Colony hybridization — use a labeled DNA probe to directly detect colonies carrying the target gene

PCR (Polymerase Chain Reaction)

PCR amplifies a specific DNA segment in vitro — from a single molecule to billions of copies in just a few hours. Invented by Kary Mullis (1983 Nobel Prize), it uses Taq polymerase (from Thermus aquaticus, a thermophilic bacterium) and repeats three steps: denaturation, annealing, and extension.

After n cycles: 2^n copies (e.g., 20 cycles = 2^ 20 10^6 copies)

PCR Principle

PCR mimics DNA replication in a test tube. By repeatedly heating and cooling the reaction mix with a heat-stable DNA polymerase, a single DNA target is exponentially amplified.

PCR is the single most important technique in molecular biology. Imagine you have ONE copy of a gene in a blood sample and need millions of copies for testing. PCR does this in ~2 hours. Kary Mullis invented it in 1983 and won the Nobel Prize in Chemistry (1993).

• PCR = in vitro DNA amplification (happens in a machine called a thermocycler, not inside a cell)
• Amplifies a specific target sequence, not the entire genome
• Requires: template DNA, two primers, Taq DNA polymerase, dNTPs (deoxynucleotide triphosphates), Mg²⁺ buffer
• Taq polymerase — from *Thermus aquaticus*, a bacterium from hot springs. Survives 95°C denaturation without denaturing itself
• Before Taq, scientists had to add fresh enzyme after EVERY denaturation cycle — Taq automated the whole process

Three Steps of PCR

Each PCR cycle has three temperature-controlled steps: denaturation (95°C), primer annealing (50-65°C), and extension (72°C). Each cycle doubles the target DNA, giving exponential amplification.

A single PCR cycle takes ~3-5 minutes and doubles the number of target DNA copies. Repeat 25-30 times and you go from 1 copy to over a billion.

• Step 1 — Denaturation (94-95°C, 30 sec): Double-stranded DNA melts into two single strands by breaking hydrogen bonds
• Step 2 — Annealing (50-65°C, 30 sec): Two short primers (18-25 nucleotides) bind to complementary sequences flanking the target region on each strand
• Step 3 — Extension (72°C, 1-2 min): Taq polymerase extends the primers by adding dNTPs in the 5'→3' direction, synthesizing new complementary strands
• One cycle = denature + anneal + extend → DNA doubled
• After n cycles: 2^n copies of the target sequence

Quick recall

After 10 cycles of PCR starting from 1 DNA molecule, how many copies do you have?

2^ 10 = 1024 copies. After 20 cycles: ~1 million. After 30 cycles: ~1 billion. This exponential power is why PCR can detect even a single molecule of pathogen DNA.

Applications of PCR

PCR has revolutionized diagnostics, forensics, genetic testing, and research. It can detect pathogens, identify criminals from trace DNA, diagnose genetic diseases, and detect mutations.

PCR is arguably the most impactful biological technique ever invented. Any field that involves DNA uses PCR.

• Medical diagnostics — Detect HIV, Hepatitis, TB, COVID-19 (RT-PCR) from tiny blood/swab samples
• Forensic science — DNA fingerprinting from crime scene evidence (hair, blood, skin cells). Even a single cell's DNA can be amplified
• Genetic disorders — Prenatal diagnosis of sickle cell anemia, thalassemia, cystic fibrosis by amplifying and analyzing specific genes
• Paternity testing — Compare amplified DNA markers between parent and child
• Ancient DNA analysis — Amplify degraded DNA from fossils, mummies, preserved specimens
• Gene cloning — Amplify a gene before inserting into a vector

Kary Mullis reportedly conceived PCR while driving on a California highway in 1983. He won the Nobel Prize in 1993 — one of the fastest lab-to-Nobel timelines in history. The technique single-handedly launched the genomics revolution.

Gel Electrophoresis

Gel electrophoresis separates DNA, RNA, or proteins by size using an electric field. DNA is negatively charged (phosphate backbone) and migrates toward the anode (+). Smaller fragments move faster through the agarose gel matrix. Bands are visualized using ethidium bromide (EtBr) under UV light.

DNA^ - electric field migrates to anode (+) | smaller fragments travel farther

Principle

DNA carries a net negative charge due to its phosphate groups. In an electric field, DNA fragments migrate through an agarose gel matrix toward the positive electrode (anode). Smaller fragments travel faster and farther.

Every nucleotide in DNA has a negatively charged phosphate group. So DNA is naturally negative at physiological pH. Place it in an electric field and it moves toward the positive pole (anode). The agarose gel acts as a molecular sieve — small fragments slip through easily, large fragments get tangled and move slowly.

• DNA migrates from cathode (−) to anode (+) because DNA is negatively charged
• Agarose (a polysaccharide from seaweed) forms the gel matrix
• Higher agarose concentration → smaller pore size → better separation of small fragments
• Lower agarose concentration → larger pores → better for separating large fragments
• Rate of migration depends on: size (primary factor), charge (roughly equal per bp for DNA), and gel concentration
• DNA fragments of known sizes (molecular weight markers / DNA ladder) run alongside samples for size estimation

Visualization & Elution

DNA is colorless and invisible. Ethidium bromide (EtBr) intercalates between base pairs and fluoresces bright orange under UV light. Desired bands can be cut out and eluted for further use.

After electrophoresis, you have separated DNA fragments — but you can't see them. The gel looks blank. You need a staining step.

• Ethidium bromide (EtBr) — the classic DNA stain. It intercalates (inserts between) DNA base pairs
• When exposed to UV light, EtBr-DNA complex fluoresces bright orange
• Each band on the gel = DNA fragments of the same size
• Elution — cut the desired band from the gel and extract the DNA using chemical or physical methods
• The extracted DNA can be used for cloning, sequencing, or further analysis
• Modern alternatives to EtBr: SYBR Green (less toxic, more sensitive)

Southern, Northern & Western Blotting

Blotting techniques transfer separated molecules from gel to a membrane for specific detection using probes. Southern = DNA, Northern = RNA, Western = protein. Named after Edwin Southern (the others are directional jokes).

Gel electrophoresis separates molecules by size, but doesn't tell you WHICH specific gene or protein is in a band. Blotting transfers the bands to a membrane and uses specific probes to identify the target.

• All three follow the same logic: separate → transfer to membrane → detect with specific probe
• Southern blotting uses labeled complementary DNA probes (radioactive or fluorescent)
• Northern blotting measures gene expression (which genes are being transcribed)
• Western blotting uses antibodies (not nucleic acid probes) to detect specific proteins
• A fourth type: Eastern blotting — detects post-translational modifications of proteins (rarely asked in NEET)

Bioreactors & Downstream Processing

Bioreactors are vessels for large-scale culture of organisms under controlled conditions (temperature, pH, oxygen, nutrients). Stirred-tank bioreactors are most common. After growth, downstream processing involves separation, purification, and quality control to obtain the final product.

Upstream (fermentation) arrow Downstream (separation + purification + QC) arrow Final Product

Types of Bioreactors

Bioreactors provide optimal growth conditions for microbes at industrial scale. The stirred-tank bioreactor (with mechanical agitator) is most widely used. Systems can operate in batch mode (closed) or continuous mode (constant input/output).

Growing cells in a small lab flask is easy. Growing them at 10,000-liter scale while maintaining perfect conditions? That needs a bioreactor. A bioreactor is essentially a controlled environment that provides everything cells need: nutrients, oxygen, correct temperature, correct pH.

• Simple stirred-tank bioreactor — most commonly used. Has a mechanical stirrer (agitator) that mixes contents and ensures uniform distribution of oxygen and nutrients
• Sparged bioreactor — air is bubbled through the medium (no mechanical stirrer). Simpler but less effective mixing
• Batch culture — all nutrients added at start; nothing added or removed during growth. Product harvested at the end
• Continuous culture (chemostat) — fresh medium continuously added, spent medium continuously removed. Cells maintained in log phase indefinitely
• Bioreactor components: agitator, oxygen delivery system, foam control, temperature control, pH control, sampling ports

Bioreactor Design

A stirred-tank bioreactor has specific components: agitator for mixing, sparger for air supply, baffles to break vortex, jacket for temperature control, probes for pH and dissolved O2, and ports for sampling and harvesting.

The design of a bioreactor determines the quality and quantity of the product. Every component serves a specific purpose.

• Agitator (impeller) — mechanical stirrer that ensures uniform mixing of cells, nutrients, and oxygen
• Sparger — device that introduces sterile air/oxygen into the medium (usually at the bottom)
• Baffles — internal plates that prevent vortex formation during stirring. Without baffles, liquid just spins in a circle without mixing
• Jacket / coil — surrounds the vessel for heating or cooling to maintain optimal temperature
• pH probe + acid/base addition — monitors and adjusts pH automatically
• Dissolved oxygen probe — ensures adequate oxygen for aerobic organisms
• Foam breaker / antifoam — vigorous aeration creates foam that can overflow; antifoam agents or mechanical breakers control it

The NCERT diagram of a simple stirred-tank bioreactor (Fig. 11.7) is frequently shown in NEET and you're asked to label parts. Know the position of: agitator, sparger, baffles, jacket, pH/DO probes.

Downstream Processing

After the bioreactor run, the crude product is mixed with cells, media components, and waste. Downstream processing involves separation, purification, and quality control to get a pure, safe, market-ready product.

The bioreactor produces the desired protein — but it's mixed with millions of cells, leftover nutrients, waste products, and other proteins. Downstream processing is the art of extracting your needle from this haystack.

• Step 1 — Separation: Remove cells from the broth by filtration or centrifugation. If the product is intracellular, cells must be lysed (broken open) first
• Step 2 — Purification: Isolate the target protein using chromatography (ion exchange, gel filtration, affinity chromatography), precipitation, or ultrafiltration
• Step 3 — Formulation: Convert the purified product into a stable, usable form (lyophilization/freeze-drying, adding stabilizers, packaging)
• Step 4 — Quality control: Test for purity, potency, safety, sterility. Must meet regulatory standards (GMP — Good Manufacturing Practice)
• Downstream processing can account for 60-80% of total production cost — it's the most expensive part!