BiologyCBSE Class 12 · Biotechnology: Principles and Processes

Processes of Recombinant DNA Technology

Updated 15 October 2026 · Fact-checked

Recombinant DNA technology proceeds in steps: isolate DNA, cut it with restriction enzymes, separate the fragment by gel electrophoresis, amplify the gene by PCR, ligate it into a vector, insert the recombinant DNA into a competent host, select transformants, grow the host in bioreactors to express the product, and purify the product by downstream processing.

Understand Processes of Recombinant DNA Technology

The sequence of steps. (i) Isolation of DNA → (ii) cutting it with restriction endonucleases → (iii) isolation of the desired fragment → (iv) amplification (PCR) → (v) ligation into a vector → (vi) transfer of recombinant DNA into a host → (vii) culturing host cells on a large scale → (viii) extraction and purification of the product.

1. Isolating DNA. DNA is enclosed in membranes, so the cell is broken open with enzymes: lysozyme (bacteria), cellulase (plant cells), chitinase (fungi). This releases DNA with RNA, proteins, polysaccharides and lipids. DNA is intertwined with proteins such as histones. RNA is removed with ribonuclease, proteins with protease, and other molecules by suitable treatments. Adding chilled ethanol precipitates purified DNA, seen as fine threads that can be spooled.

2. Cutting DNA. Purified DNA is incubated with a restriction enzyme under its optimal conditions. Agarose gel electrophoresis checks the progress of digestion. The vector is cut with the same enzyme.

3. Gel electrophoresis: separating fragments.

DNA fragments are negatively charged, so in an electric field they move towards the anode through a matrix, most commonly agarose (a natural polymer from seaweeds).
Agarose acts as a sieve: smaller fragments move farther; larger ones stay nearer the wells.
DNA is invisible in visible light, so the gel is stained with ethidium bromide and exposed to UV light: DNA shows as bright orange bands.
The desired band is cut out and the DNA extracted from the gel piece: elution. The purified fragment is joined to a vector.

4. PCR: polymerase chain reaction. Makes many copies of a gene in vitro using two sets of primers (short, chemically synthesised oligonucleotides complementary to the ends of the target region) and DNA polymerase, with nucleotides and the genomic DNA as template. Each cycle has three steps:
1. Denaturation: heating separates the two strands.
2. Annealing: primers bind to complementary sites on the single strands.
3. Extension: DNA polymerase extends the primers to make new strands.
Repeating the cycle doubles the DNA each time, so a segment can be amplified about a billion times. This needs a thermostable DNA polymerase from the bacterium *Thermus aquaticus* (Taq polymerase), which stays active during the high temperature of denaturation. The amplified fragment can be ligated into a vector for cloning.

5. Ligation and insertion into a host. The gene and vector, cut with the same enzyme, are joined by DNA ligase. The host (e.g. *E. coli*) is made competent and takes up the recombinant DNA. If the vector carries an ampicillin-resistance gene, cells spread on ampicillin agar reveal transformants (they grow) while untransformed cells die; the resistance gene is the selectable marker.

6. Obtaining the foreign gene product. The aim is usually a protein. A protein from a gene expressed in a different (heterologous) host is a recombinant protein. Small lab cultures give little product, so cells are grown in a continuous culture (used medium drained from one side, fresh medium added from the other) to keep them in the log (exponential) phase, giving more biomass and higher yield. Large volumes (100-1000 litres) are processed in bioreactors: vessels where raw materials are biologically converted into specific products using microbial, plant, animal or human cells. Bioreactors provide optimum temperature, pH, substrate, salts, vitamins and oxygen.

The most common type is the stirred-tank bioreactor: cylindrical or with a curved base to help mixing; a stirrer gives even mixing and oxygen availability; alternatively, air can be bubbled through (sparged tank).
Parts: agitator system, oxygen delivery system, foam control system, temperature control system, pH control system and sampling ports to withdraw small volumes periodically.

7. Downstream processing. After the biosynthetic stage, the product goes through separation and purification (together called downstream processing), is formulated with preservatives, undergoes clinical trials (for drugs) and strict quality control. These vary from product to product.

Flow chart: cell lysis (lysozyme/cellulase/chitinase) → remove RNA (ribonuclease) and protein (protease) → chilled ethanol (DNA precipitates) → restriction digestion → gel electrophoresis + elution → PCR amplification → ligation into vector → competent host (Ca²⁺, heat shock) → selection on antibiotic → bioreactor (log phase) → downstream processing → product.

Draw and label 1: agarose gel after electrophoresis. Checklist: a rectangle (gel) with wells along the top edge near the cathode (−); lanes running down to the anode (+); bands in each lane, small fragments far from the wells. Labels: wells, cathode (−), anode (+), direction of movement, larger fragments, smaller fragments, DNA bands (stained with ethidium bromide, seen under UV).

Draw and label 2: simple stirred-tank bioreactor. Checklist: tall cylindrical vessel with a curved base; a central shaft with flat paddles; an air inlet at the bottom; outlets at the top; side ports. Labels: motor, stirrer (impeller), sterile air inlet (sparger), foam breaker, acid/base for pH control, culture broth, sampling port, steam/temperature jacket, exhaust gas outlet.

Processes of Recombinant DNA Technology: key facts

PCR amplification: $copies after n cycles = 2^{n} (from one double-stranded molecule)$; 30 cycles ≈ 1.07 × 10⁹, about a billion.
PCR cycle: $denaturation \to annealing (primers) \to extension (Taq polymerase)$; Thermostable polymerase from Thermus aquaticus.
Gel electrophoresis: $D N A^{-} \to anode; distance moved \propto \frac{1}{size} (qualitatively)$; Agarose sieve; ethidium bromide + UV shows orange bands.
Enzymes in DNA isolation: $lysozyme (bacteria), cellulase (plant), chitinase (fungus); RNase, protease$; Chilled ethanol precipitates DNA.
Bioreactor scale: $100 - 1000 litres; optimum: temperature, pH, substrate, salts, vitamins, O_{2}$; Stirred-tank is the most common type.

How to answer Processes of Recombinant DNA Technology questions

Process questions expect the right step in the right order with its tool. Use the flow chart as your answer skeleton.

1Identify which step is asked (isolation, cutting, separation, amplification, insertion, expression, purification).
2Name the tool for that step: specific enzymes (lysozyme, cellulase, chitinase, RNase, protease, restriction enzyme, ligase, Taq polymerase), reagents (chilled ethanol, ethidium bromide), or equipment (gel, thermal cycler, bioreactor).
3For gel questions, apply: negative DNA → anode; small fragments farther; stain + UV to see; elute the band.
4For PCR, write the three steps and why Taq polymerase is used; for numbers use $2^{n}$ .
5For bioreactors, list the control systems and say why a stirred tank (mixing and oxygen).
6End with downstream processing for any 'product' question: separation, purification, formulation, trials, quality control.

Processes of Recombinant DNA Technology in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

PCR amplification and gel reading

Suppose a diagnostic lab receives a sample with just 10 copies of a viral DNA segment and needs at least 10 million copies to see a clear band on a gel.

What is the minimum number of PCR cycles needed (assuming perfect doubling), and where will the 500 bp amplified band lie on the gel compared with a 2000 bp marker band?

Try it first, then show the solutionSolution

Copies after n cycles = 10 × 2ⁿ; we need 10 × 2ⁿ ≥ 10,000,000, so 2ⁿ ≥ 1,000,000.
2¹⁹ = 524,288 (too few); 2²⁰ = 1,048,576 (enough), so n = 20 cycles, giving about 10.5 million copies.
On the gel, DNA moves towards the anode and smaller fragments move farther, so the 500 bp band lies farther from the well (nearer the anode) than the 2000 bp marker.

Answer: 20 cycles (about 1.05 × 10⁷ copies); the 500 bp band runs farther from the well than the 2000 bp band.

Memory hook: Every cycle doubles; every small fragment runs farther.

Tricks to remember Processes of Recombinant DNA Technology

Short memory aids. Say each one aloud once, then use it on a question the same day.

PCR = D-A-E
Denature, Anneal, Extend: 'Dancing Around Everywhere'.
Use it when: Order of PCR steps.
Run to red
DNA is negative and runs to the positive (red) anode.
Use it when: Direction in electrophoresis.
L-C-C break walls
Lysozyme for bacteria, Cellulase for plants, Chitinase for fungi.
Use it when: DNA isolation enzymes.

Processes of Recombinant DNA Technology: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Primers in PCR · 2 marks · Very short answerBoard level

What are primers in PCR? Why does each PCR reaction need two of them?

Try it first, then show the solutionSolution

Primers are small, chemically synthesised oligonucleotides complementary to the regions of DNA at the two ends of the target segment; DNA polymerase extends them (1 mark).
One primer pairs with each of the two separated strands, so both strands are copied and only the region between the two primers is amplified (1 mark).

Answer: Short synthetic oligonucleotides complementary to the ends of the target; two are needed so both strands are copied and the region between them is amplified.

Example 2: Isolating DNA · 3 marks · Short answerBoard level

Describe how pure DNA is isolated from (a) a bacterial cell and (b) a plant cell. How is the DNA finally obtained?

Try it first, then show the solutionSolution

Cells are broken open with lysozyme (bacteria) or cellulase (plant cells) to release DNA with RNA, proteins, polysaccharides and lipids (1 mark).
RNA is removed with ribonuclease and proteins (including histones) with protease; other molecules by suitable treatments (1 mark).
Chilled ethanol precipitates the purified DNA, seen as fine threads that can be spooled (1 mark).

Answer: Lysozyme/cellulase → RNase, protease → chilled ethanol → spooled DNA threads.

Example 3: Gel electrophoresis · 3 marks · Short answerHarder board level

Explain how DNA fragments are separated and visualised by agarose gel electrophoresis. What is elution?

Try it first, then show the solutionSolution

DNA fragments are negatively charged and move towards the anode through agarose gel in an electric field; the gel acts as a sieve, so smaller fragments move farther (1 mark).
The gel is stained with ethidium bromide and exposed to UV light; DNA appears as bright orange bands (1 mark).
Elution: cutting out the desired band and extracting the DNA from the gel piece, for use in making recombinant DNA (1 mark).

Answer: Negative DNA to anode, size sieving; EtBr + UV orange bands; elution recovers DNA from a band.

Example 4: PCR steps · 3 marks · Short answerHarder board level

Describe the three steps of a PCR cycle and calculate how many copies of a DNA segment are present after 25 cycles, starting from one molecule.

Try it first, then show the solutionSolution

Denaturation: high temperature separates the DNA strands; annealing: two primers bind to complementary regions flanking the target (1 mark).
Extension: Taq polymerase adds nucleotides to the primers, copying each strand; the DNA doubles (1 mark).
After 25 cycles: 2²⁵ = 33,554,432 copies (about 3.4 × 10⁷) (1 mark).

Answer: Denaturation, annealing, extension; 2²⁵ ≈ 3.4 × 10⁷ copies.

Example 5: Stirred-tank bioreactor · 5 marks · Long answerHarder board level

Why are bioreactors needed, and why is the stirred-tank type most commonly used? List the components that maintain optimal conditions.

Try it first, then show the solutionSolution

Small cultures cannot give appreciable quantities; bioreactors process 100-1000 litres to produce large amounts of product (1 mark).
They provide optimum temperature, pH, substrate, salts, vitamins and oxygen (1 mark).
Stirred-tank: cylindrical or curved base to aid mixing; the stirrer gives even mixing and oxygen availability throughout (1 mark).
Components: agitator system, oxygen delivery system, foam control system (1 mark).
Temperature control system, pH control system, sampling ports for periodic withdrawal (1 mark).

Answer: Large-scale optimal production; stirring ensures mixing and oxygen; agitator, O₂ delivery, foam, temperature, pH controls and sampling ports.

Example 6: Gel reading puzzle · 3 marks · Short answerHOTS / competency

Suppose a linear 6 kb DNA is cut with an enzyme that has two sites, at 1.5 kb and 4.0 kb from one end. (a) How many fragments form and of what sizes? (b) Arrange them from the band nearest the wells to the farthest. (c) If a single mutation destroyed the site at 4.0 kb, how would the pattern change?

Try it first, then show the solutionSolution

(a) 3 fragments: 1.5 kb, 2.5 kb (4.0 − 1.5) and 2.0 kb (6.0 − 4.0) (1 mark).
(b) Largest stays nearest the wells: 2.5 kb, then 2.0 kb, then 1.5 kb farthest (1 mark).
(c) Only one site at 1.5 kb: 2 fragments, 1.5 kb and 4.5 kb; the 2.5 and 2.0 kb bands disappear and a 4.5 kb band appears near the wells (1 mark).

Answer: (a) 1.5, 2.5, 2.0 kb (b) 2.5 > 2.0 > 1.5 kb from the wells (c) 1.5 and 4.5 kb only.

Where students lose marks in Processes of Recombinant DNA Technology

The slips examiners see most often, and what to write instead.

Mistake: DNA moves to the cathode
Charge of DNA forgotten.
Wrong working
DNA fragments move towards the cathode.
Right working
DNA is negatively charged and moves towards the anode.
Do this: Phosphate groups make DNA negative.
Usually costs: 1 mark.
Mistake: Larger fragments travel farther
Sieving is misunderstood.
Wrong working
Bigger fragments move faster and farther.
Right working
Smaller fragments move farther through the agarose sieve.
Do this: Small slips through the mesh.
Usually costs: 1 mark.
Mistake: Wrong PCR order or missing primers
Steps memorised loosely.
Wrong working
Extension, denaturation, annealing.
Right working
Denaturation → annealing (primers) → extension (Taq polymerase).
Do this: D-A-E, and name primers and Taq.
Usually costs: 1 mark.
Mistake: Seeing DNA without staining
Students assume bands are visible.
Wrong working
DNA bands are seen directly in the gel.
Right working
Stain with ethidium bromide and expose to UV: orange bands.
Do this: Always mention EtBr + UV.
Usually costs: ½-1 mark.

Exam tips

Write the steps of rDNA technology in order as a numbered list; examiners award marks per step.
For each step, name the specific enzyme or reagent.
Draw the gel with wells at the cathode end and label anode and cathode.
Give the bioreactor volume range (100-1000 L) and list its control systems.
Use 'downstream processing' and 'elution' as technical terms.

Practice questions on Processes of Recombinant DNA Technology

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
In agarose gel electrophoresis, DNA fragments move towards the:
(a) cathode, larger ones farther (b) anode, smaller ones farther (c) anode, larger ones farther (d) cathode, smaller ones farther
Hint
DNA is negatively charged; gel is a sieve.
Answer
(b) anode, smaller ones farther.
Q2 · 1 mark · Very short answerBoard level
Which enzyme would you use to release DNA from a fungal cell, and which to remove RNA from the extract?
Hint
Fungal walls contain chitin.
Answer
Chitinase to break the fungal cell wall; ribonuclease to remove RNA.
Q3 · 1 mark · Assertion-reasonHarder board level
Assertion (A): In PCR, a thermostable DNA polymerase is used.
Reason (R): The double-stranded DNA is denatured at high temperature in every cycle.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Why must the enzyme survive heat?
Answer
(a).
Q4 · 2 marks · Very short answerHarder board level
What is downstream processing? List its main steps and explain why it differs between products.
Hint
After the bioreactor.
Answer
Downstream processing is the series of processes after biosynthesis that make the product marketable: separation and purification, formulation with preservatives, clinical trials (for drugs) and strict quality control. It varies because each product has different properties, purity needs and safety tests.
Q5 · 4 marks · Case-basedHarder board level
Read and answer.
Suppose a biotech start-up grows recombinant E. coli in a 500-litre vessel where used medium is drained from one side and fresh medium added from the other, with a stirrer, an air sparger and probes for pH and temperature.
(a) Name this type of culture and the growth phase it maintains. (1)
(b) Why does this give a higher yield than a batch culture? (1)
(c) Give two functions of the stirrer and sparger and one reason a foam control system is needed. (2)
Hint
Continuous culture; log phase.
Answer
(a) Continuous culture, keeping cells in the log (exponential) phase. (b) Cells stay in their most active growth phase, producing more biomass and therefore more of the desired protein. (c) The stirrer mixes the contents evenly and spreads oxygen; the sparger bubbles sterile air to supply oxygen. Vigorous aeration of protein-rich medium produces foam, which must be controlled to avoid overflow and contamination.
Q6 · 2 marks · Very short answerHOTS / competency
Suppose a student forgets to add protease while isolating DNA from liver tissue and then tries to cut the DNA with a restriction enzyme. Predict the problem and explain.
Hint
DNA in eukaryotes is wrapped around histones.
Answer
The DNA would remain bound to proteins such as histones, so the restriction enzyme cannot reach many recognition sites; digestion would be incomplete or fail. Protease removes these proteins to give purified DNA.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Which reagent precipitates DNA?Show answer
Chilled ethanol.
2.What is agarose obtained from?Show answer
Seaweeds.
3.Which stain makes DNA visible under UV?Show answer
Ethidium bromide.
4.Name the three steps of PCR.Show answer
Denaturation, annealing, extension.
5.Copies of DNA after 10 PCR cycles from one molecule?Show answer
2¹⁰ = 1024.
6.Most common type of bioreactor?Show answer
Stirred-tank bioreactor.

Processes of Recombinant DNA Technology: frequently asked questions

What are the steps of recombinant DNA technology?

Isolating DNA, cutting it with restriction enzymes, separating the desired fragment, amplifying it by PCR, ligating it into a vector, inserting it into a host, culturing the host to express the product, and downstream processing.

How does gel electrophoresis separate DNA?

Negatively charged DNA moves towards the anode through agarose gel, which acts as a sieve, so smaller fragments move farther.

What is PCR?

Polymerase chain reaction: repeated cycles of denaturation, primer annealing and extension by Taq polymerase that amplify a DNA segment about a billion times.

What is a bioreactor?

A vessel (100-1000 litres) in which cells convert raw materials into a product under optimum temperature, pH, nutrients and oxygen; the stirred-tank type is most common.

What is downstream processing?

Separation and purification of the product after biosynthesis, followed by formulation, clinical trials (for drugs) and quality control.