Class 12 Biology Notes

Biotechnology: Principles and Processes Class 12 Notes

Complete, exam-ready notes on biotechnology principles and processes: the tools of recombinant DNA technology (restriction enzymes, vectors, host cells), PCR amplification, gel electrophoresis, gene cloning, selection of recombinants, bioreactors and downstream processing. Essential for CBSE Class 12 and NEET Biology.

Class12SubjectBiologyCoversCBSE · NEET

Written byDeep Narayan· Science & Mathematics EducatorReviewed byPushpanjali

What are the three basic steps of genetic engineering?

1) Identification and isolation of the desired gene, 2) Insertion of the gene into a vector (cloning vector) to form recombinant DNA, 3) Transfer of the recombinant DNA into a competent host cell for expression.

Principles of Biotechnology

Biotechnology

The use of living organisms or their products to make commercially useful substances for human welfare. Modern biotechnology rests on two core principles: (1) genetic engineering — the manipulation of genes to create recombinant DNA and alter organisms at the molecular level; (2) chemical engineering — designing bioreactors and optimising processes for large-scale production.

Genetic engineering involves gene isolation, manipulation and transfer to create organisms with new traits (e.g. Bt cotton, insulin-producing E. coli).
Process engineering ensures that recombinant organisms are grown under controlled conditions in bioreactors for maximum yield.
Cloning: producing identical copies of a gene or an organism. Molecular cloning copies a gene; reproductive cloning produces identical organisms (e.g. Dolly the sheep).

Tools of Recombinant DNA Technology

Restriction enzymes (molecular scissors)

Enzymes that cut DNA at specific nucleotide sequences called recognition sites. Over 900 restriction enzymes have been isolated from bacteria, where they serve as a defence against viral DNA. Each enzyme recognises a specific palindromic sequence — a sequence that reads the same on both strands in the 5' to 3' direction.

EcoRI: recognises the palindromic sequence 5'-GAATTC-3' and cuts between G and AATTC on each strand, producing sticky (cohesive) ends with single-stranded overhangs.
Sticky ends: short single-stranded overhangs at the cut ends that can base-pair with complementary sticky ends from another DNA fragment cut by the same enzyme, facilitating ligation.
Blunt ends: the enzyme cuts straight across both strands, producing no overhangs (e.g. SmaI cuts at CCCGGG). Blunt-end ligation is less efficient.

DNA ligase

The molecular glue that seals nicks in the phosphodiester backbone, joining DNA fragments by forming phosphodiester bonds. T4 DNA ligase is commonly used in recombinant DNA experiments. It joins sticky ends (hydrogen-bonded by complementarity) and blunt ends into a continuous strand.

Palindrome check

A restriction palindrome reads identically on both strands in the 5'-to-3' direction: EcoRI reads GAATTC on the top strand and GAATTC on the bottom strand (in the 5'-to-3' direction). It is NOT the same as a word palindrome.

Cloning Vectors and Competent Host Cells

Cloning vector

A DNA molecule that carries foreign DNA into a host cell and replicates independently. Common vectors include plasmids, bacteriophages (lambda), cosmids, BACs and YACs. A good cloning vector must have: (1) origin of replication (ori) — allows autonomous replication; (2) selectable marker — antibiotic resistance gene to identify transformed cells; (3) cloning site — unique restriction site(s) for inserting foreign DNA.

Plasmid pBR322

A widely used cloning vector derived from E. coli. It carries two selectable markers: ampicillin resistance (ampR) and tetracycline resistance (tetR). It has a single origin of replication and unique restriction sites within both antibiotic resistance genes, allowing insertional inactivation for recombinant selection.

Ti plasmid: from Agrobacterium tumefaciens, used to transfer genes into plant cells. The T-DNA region of the Ti plasmid integrates into the plant genome.
Competent cells: host cells that can take up foreign DNA from the environment. E. coli cells are made competent by CaCl2 treatment (calcium ions neutralise the negative charges on DNA and cell membrane, allowing DNA entry).
Other transformation methods: electroporation (brief electric pulse creates transient pores in the cell membrane), gene gun (biolistics — DNA-coated gold/tungsten particles are shot into plant cells), microinjection (direct injection of DNA into animal cells using a fine needle).

The Process — From DNA Isolation to Transformation

The overall workflow of recombinant DNA technology follows a logical sequence of steps:

Isolation of DNA: cells are lysed with detergents and enzymes (lysozyme for bacteria, cellulase for plants) to release DNA. Proteins are removed with proteases and RNA with RNases. The purified DNA is precipitated with chilled ethanol.
PCR (Polymerase Chain Reaction): amplifies a specific DNA segment in vitro through repeated cycles of three steps — (1) Denaturation at 94 deg C: the double-stranded DNA separates into single strands; (2) Annealing at 55-65 deg C: primers bind to complementary sequences flanking the target; (3) Extension at 72 deg C: Taq polymerase (thermostable DNA polymerase from Thermus aquaticus) synthesises new strands. After 30 cycles, the target sequence is amplified approximately 2^30 (about 1 billion) times.
Gel electrophoresis: DNA fragments are separated by size on an agarose gel using an electric field. DNA is negatively charged (phosphate backbone) and moves toward the positive electrode (anode). Smaller fragments move faster. The gel is stained with ethidium bromide and visualised under UV light. Specific bands are cut out for further use.
Ligation: the isolated gene of interest and the vector are cut with the same restriction enzyme to produce compatible sticky (or blunt) ends. DNA ligase joins them, creating a recombinant DNA molecule (recombinant vector).
Transformation: the recombinant vector is introduced into a competent host cell (usually E. coli) by CaCl2 treatment or electroporation. The host cell replicates the recombinant DNA along with its own genome.

Selection of Recombinantants

Insertional inactivation

When a foreign gene is inserted into a restriction site within a selectable marker gene (e.g. ampR or tetR on pBR322), that gene is disrupted and becomes non-functional. Cells carrying the recombinant vector lose resistance to that antibiotic while retaining resistance to the other. This allows screening by replica plating.

Example: if the gene is inserted into the tetR gene of pBR322, recombinants are AmpR but TetS. They grow on ampicillin plates but not on tetracycline plates.
Blue-white screening: uses the lacZ gene on the vector. The vector carries the lacZ gene encoding beta-galactosidase, and the host has a mutant lacZ. In the presence of X-gal, cells with non-recombinant vector produce blue colonies (functional lacZ). Recombinants (with foreign DNA inserted into lacZ) produce white colonies. This is a rapid visual screen.
Antibiotic resistance method: cells are plated on media containing the antibiotic encoded by the intact selectable marker. Only transformed cells (with the vector) grow; non-transformed cells die.

Bioreactors and Downstream Processing

Bioreactor (fermenter)

A large vessel in which raw materials are biologically converted into specific products by microorganisms, plant or animal cells under controlled conditions (pH, temperature, dissolved O2, agitation, nutrient supply). It allows large-scale production of enzymes, antibiotics, vaccines and other bio-products.

Simple stirred-tank bioreactor: has a motor-driven impeller for agitation, an air sparger for oxygen supply, pH and temperature control systems and ports for sampling.
Airlift bioreactor: mixing and aeration are achieved by pumping air; gentler than stirred tanks, suitable for shear-sensitive cells.
Batch vs continuous culture: in batch culture, all nutrients are added at the start and products are harvested at the end. In continuous culture, fresh medium is continuously added and product is withdrawn, maintaining cells in a log phase for maximum productivity.
Downstream processing: after the product is formed in the bioreactor, it must be separated, purified and packaged. Steps include cell separation (centrifugation, filtration), extraction, chromatographic purification (ion exchange, affinity chromatography), formulation and quality control.

NEET recall

Taq polymerase from Thermus aquaticus is thermostable and does not denature at 94 deg C — this is why PCR can use high temperatures for denaturation. This is a favourite NEET fact.

Solved Examples

Example: A researcher wants to clone a gene into pBR322. The gene has EcoRI sites at both ends. After cutting both the gene and pBR322 with EcoRI, the fragments are ligated with DNA ligase. The recombinant plasmid is introduced into E. coli by CaCl2 treatment. How would you select for cells containing the recombinant plasmid?

Solution: EcoRI sites on pBR322 are located within the tetR gene. Insertion of the foreign gene disrupts tetR (insertional inactivation) but leaves ampR intact. Plate the bacteria on ampicillin-containing medium — only cells that took up the plasmid (recombinant or non-recombinant) will grow. Replica-plate onto tetracycline medium. Cells that grow on ampicillin but NOT on tetracycline are the recombinants (AmpR, TetS). Verify by further screening.

Example: Explain why the polymerase used in PCR must be thermostable, and name the organism from which it is isolated.

Solution: PCR involves repeated cycles of denaturation at 94 deg C, which would denature (unfold) a normal DNA polymerase. The polymerase must withstand this high temperature without losing activity across 30 or more cycles. Taq polymerase, isolated from the thermophilic bacterium Thermus aquaticus (found in hot springs), is thermostable and functions optimally at 72 deg C, making it ideal for PCR.

Revision

Key formulas at a glance

Memorise these before attempting numericals — most exam questions hinge on one of them.

PCR amplification

2^n \text{ copies after } n \text{ cycles}

EcoRI recognition

5'-GAATTC-3'

Denaturation temp

94^{\circ}\text{C}

Annealing temp

55\text{--}65^{\circ}\text{C}

Extension temp

72^{\circ}\text{C}

Plasmid pBR322 markers

amp^R + tet^R

Competent cell treatment

CaCl_2 \to \text{competent } E.\,coli}

Exam tips

How this chapter is asked

Where this topic appears in CBSE, JEE Main and NEET papers.

EcoRI cuts GAATTC between G and AATTC — producing sticky ends. SmaI cuts CCCGGG to produce blunt ends.
pBR322 has two selectable markers: ampR and tetR. Insertional inactivation is used for recombinant screening.
Taq polymerase from Thermus aquaticus is thermostable — essential for the denaturation step of PCR.
PCR cycles: denaturation (94 deg C) -> annealing (55-65 deg C) -> extension (72 deg C) -> repeat.
CaCl2 makes E. coli cells competent; electroporation uses electric pulses; gene gun is for plant cells.
Blue-white screening uses lacZ gene and X-gal: white colonies = recombinants.
Ti plasmid from Agrobacterium tumefaciens is used for plant genetic engineering via T-DNA transfer.
Downstream processing includes separation, purification and formulation after bioreactor production.

FAQ

Common questions

What is the role of restriction enzymes in genetic engineering?

Restriction enzymes cut DNA at specific palindromic recognition sequences, generating sticky or blunt ends. They allow precise cutting of both the gene of interest and the vector so that compatible ends can be joined by DNA ligase to create recombinant DNA.

How does insertional inactivation help select recombinants?

When a foreign gene is inserted into a selectable marker gene (e.g. tetR on pBR322), that gene is disrupted. Recombinants lose resistance to that antibiotic but retain resistance to the other. By replica-plating on media with different antibiotics, recombinants are identified as colonies that grow on one antibiotic but not the other.

Why is CaCl2 used in transformation?

CaCl2 treatment makes E. coli cells 'competent' — the Ca2+ ions neutralise the negative charges on both the DNA and the cell membrane, reducing electrostatic repulsion and allowing the foreign DNA to pass through the cell membrane into the cell.

What is the advantage of using a bioreactor over batch culture in flasks?

Bioreactors allow large-scale, controlled production with precise regulation of pH, temperature, dissolved oxygen and nutrient supply. They support continuous culture, maintaining cells in the log phase for maximum productivity, and are essential for commercial-scale manufacturing of enzymes, antibiotics and vaccines.

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