Recombinant DNA Technology

The core technique that allows scientists to combine DNA from different sources.

Recombinant DNA Technology

Recombinant DNA Technology is a set of techniques used to join DNA molecules from two different sources and introduce the resulting recombinant DNA into a suitable host cell for replication or expression.

सरल शब्दों में, जब किसी organism के useful gene को दूसरे DNA molecule के साथ जोड़कर एक नया DNA molecule बनाया जाता है, तो उसे recombinant DNA (rDNA) कहते हैं और इस पूरी प्रक्रिया को Recombinant DNA Technology कहा जाता है।

Key Idea: Recombinant DNA Technology = Isolation of DNA + Cutting of DNA + Joining of DNA fragments + Introduction into host + Selection/Expression

1. What is Recombinant DNA?

Recombinant DNA is a DNA molecule formed by joining DNA fragments obtained from two different sources.

For example, if a human gene responsible for producing insulin is inserted into a bacterial plasmid, the resulting DNA molecule is recombinant DNA.

DNA from Source A + DNA from Source B → Recombinant DNA

2. Basic Requirements of Recombinant DNA Technology

Several important biological tools are required to perform recombinant DNA technology.

Component Function
Restriction enzymes Cut DNA at specific sites
DNA ligase Joins DNA fragments
DNA polymerase Synthesises new DNA strands
Cloning vector Carries the desired DNA into a host cell
Host cell Provides machinery for replication or expression
Selectable marker Helps identify transformed cells

3. Restriction Enzymes

Restriction enzymes are enzymes that recognise specific nucleotide sequences in DNA and cut the DNA at or near those specific sites.

These enzymes are also called molecular scissors because they cut DNA molecules at specific locations.

Restriction Endonucleases

Restriction enzymes belong to a group called restriction endonucleases. They cut DNA within the DNA molecule rather than removing nucleotides only from the ends.

The first restriction endonuclease was isolated in the early study of bacterial restriction systems. These enzymes provide the basic tool for cutting DNA in genetic engineering.

4. Recognition Sequence

A restriction enzyme recognises a specific DNA sequence called the recognition sequence or restriction site.

Many restriction sites are palindromic sequences.

Palindrome: A DNA sequence that reads the same in the 5′ → 3′ direction on both complementary strands.

For example, the restriction enzyme EcoRI recognises the following sequence:

5′ — GAATTC — 3′
3′ — CTTAAG — 5′

EcoRI cuts the DNA between G and A on both strands.

5′ — G     AATTC — 3′
3′ — CTTAA     G — 5′

5. Sticky Ends

When certain restriction enzymes cut DNA in a staggered manner, they produce short single-stranded overhanging ends called sticky ends.

Sticky ends are useful because complementary sticky ends can pair with each other through hydrogen bonding.

Important: Sticky ends make it easier to join DNA fragments having complementary sequences.

6. EcoRI

EcoRI is a commonly studied restriction endonuclease.

  • E → Escherichia
  • co → coli
  • R → strain RY13
  • I → first enzyme isolated from this strain

EcoRI recognises the sequence GAATTC and produces sticky ends.

7. Naming of Restriction Enzymes

The name of a restriction enzyme is generally derived from the organism from which it is isolated.

Part Meaning in EcoRI
E Escherichia
co coli
R Strain RY13
I First enzyme isolated

8. DNA Ligase

DNA ligase is an enzyme that joins DNA fragments by forming phosphodiester bonds between adjacent nucleotides.

After the desired DNA fragment and vector DNA have been cut, DNA ligase acts like molecular glue and joins the fragments.

Restriction enzyme → Cuts DNA

DNA ligase → Joins DNA fragments

9. DNA Polymerase

DNA polymerase is an enzyme involved in the synthesis of a new DNA strand using an existing strand as a template.

In recombinant DNA technology, DNA polymerases are also important in techniques such as PCR, where large numbers of copies of a specific DNA segment are produced.

10. Cloning Vector

A cloning vector is a DNA molecule used to carry a desired DNA fragment into a host cell where it can replicate.

The most commonly studied cloning vector in NCERT is the plasmid of bacteria.

11. Plasmid

A plasmid is a small, circular, extra-chromosomal DNA molecule found naturally in many bacteria.

Plasmids can replicate independently of the bacterial chromosome, making them useful as vectors in genetic engineering.

12. Important Features of a Good Cloning Vector

A useful cloning vector generally contains the following important features:

Feature Importance
Origin of replication (ori) Allows replication of the vector inside the host
Selectable marker Helps identify transformed cells
Cloning site Provides a site for insertion of foreign DNA

13. Origin of Replication (ori)

The origin of replication (ori) is a specific DNA sequence from where replication begins.

When a foreign DNA fragment is inserted into a vector, the vector must be able to replicate inside the host. Therefore, the presence of ori is essential for replication.

Remember: ori controls the replication of the vector and also influences the copy number of the linked DNA.

14. Selectable Marker

A selectable marker is a gene that helps identify cells which have successfully received the recombinant DNA or vector.

Common selectable markers in bacterial vectors include genes providing resistance to antibiotics such as:

  • Ampicillin
  • Tetracycline
  • Kanamycin
  • Chloramphenicol

For example, if a vector carries a gene for ampicillin resistance, cells containing that vector can survive on a medium containing ampicillin.

15. Cloning Sites

A cloning site is a specific restriction site in the vector where the foreign DNA fragment can be inserted.

A good vector should preferably contain unique restriction sites so that cutting at the cloning site does not disrupt essential functions of the vector.

16. Insertional Inactivation

Insertional inactivation is a method used to identify recombinant clones based on the disruption of a marker gene by insertion of foreign DNA.

For example, in some plasmid vectors, insertion of foreign DNA into a particular antibiotic-resistance gene can inactivate that gene.

Thus, recombinant and non-recombinant colonies can be distinguished based on their ability to grow on selective media.

17. Gene Cloning

Gene cloning is the production of many identical copies of a desired DNA fragment.

The basic sequence is:

Desired gene → Vector → Host cell → Replication → Many copies of gene

18. Steps in Recombinant DNA Technology

The major steps involved in recombinant DNA technology are:

  1. Isolation of DNA
  2. Cutting of DNA at specific sites
  3. Amplification of the desired DNA
  4. Ligation of DNA fragment with vector
  5. Introduction of recombinant DNA into host cell
  6. Selection of transformed cells
  7. Expression of the desired gene
  8. Downstream processing and purification

19. Step 1 – Isolation of DNA

The first step is to isolate DNA from the source organism.

DNA is present inside cells along with proteins, RNA and other cellular components. Therefore, these unwanted substances must be removed to obtain relatively pure DNA.

Different enzymes and chemicals are used during DNA isolation.

Enzyme Function
Lysozyme Breaks bacterial cell wall
Cellulase Breaks plant cell wall
Chitinase Breaks fungal cell wall
Protease Removes proteins
RNase Removes RNA

20. Step 2 – Cutting of DNA

The isolated DNA is cut using suitable restriction endonucleases.

The same restriction enzyme is generally used to cut both the desired DNA and the vector DNA. This produces compatible ends that can be joined together.

Desired DNA + Vector DNA → Compatible ends

21. Step 3 – Amplification of Desired DNA

Sometimes a specific DNA segment needs to be produced in a large number of copies before further manipulation. This can be achieved using Polymerase Chain Reaction (PCR).

PCR is an in vitro technique used to amplify a specific DNA sequence.

22. Polymerase Chain Reaction (PCR)

PCR can produce millions of copies of a desired DNA segment in a relatively short time.

The three basic steps of PCR are:

  1. Denaturation
  2. Annealing
  3. Extension

Denaturation

DNA is heated so that the two complementary strands separate.

Annealing

Specific primers bind to complementary sequences on the template DNA.

Extension

A thermostable DNA polymerase synthesises new DNA strands using the primers as starting points.

Taq Polymerase: PCR commonly uses a thermostable DNA polymerase called Taq polymerase, originally isolated from the bacterium Thermus aquaticus.

Denaturation → Annealing → Extension → Repeated cycles → Amplification

23. Step 4 – Ligation

The desired DNA fragment is joined to the vector using DNA ligase.

The resulting DNA molecule is called recombinant DNA.

Foreign DNA fragment
        +
Vector DNA
        ↓
DNA Ligase
        ↓
Recombinant DNA

24. Step 5 – Introduction into Host Cell

The recombinant DNA must be introduced into a suitable host cell for replication and/or expression.

The introduction of recombinant DNA into a bacterial cell is called transformation.

In bacterial transformation, cells may be treated with calcium ions and exposed to a suitable temperature treatment to facilitate uptake of DNA.

25. Competent Cells

Cells capable of taking up foreign DNA are called competent cells.

In bacteria, treatment with calcium ions can increase the efficiency of DNA uptake.

26. Step 6 – Selection of Transformants

Not every host cell successfully receives recombinant DNA. Therefore, cells must be selected using appropriate selectable markers.

Only cells containing the desired vector or recombinant construct will show the expected selectable characteristic.

27. Step 7 – Expression of the Desired Gene

After successful introduction into the host, the desired gene can be expressed to produce the required protein or other product.

For example, a gene coding for human insulin can be introduced into a suitable host so that the host produces insulin-related polypeptides.

28. Step 8 – Downstream Processing

After the desired product is produced, it must be separated, purified and processed to obtain the final usable product.

This stage is called downstream processing.

It may involve:

  • Separation of the product
  • Purification
  • Quality testing
  • Formulation
  • Packaging

29. Complete Flow of Recombinant DNA Technology

Isolation of DNA
       ↓
Cutting by Restriction Enzyme
       ↓
Isolation/Amplification of Desired Gene
       ↓
Cutting of Vector
       ↓
Joining by DNA Ligase
       ↓
Recombinant DNA
       ↓
Introduction into Host Cell
       ↓
Selection of Transformants
       ↓
Expression of Desired Gene
       ↓
Product Formation
       ↓
Downstream Processing
       ↓
Purified Product

30. Gel Electrophoresis

Gel electrophoresis is a technique used to separate DNA fragments according to their size.

DNA molecules are negatively charged because of their phosphate groups. Therefore, when an electric field is applied, DNA moves towards the positive electrode (anode).

Smaller DNA fragments move faster and farther through the gel than larger fragments.

Exam Point: DNA is negatively charged and therefore migrates towards the positive electrode during gel electrophoresis.

31. Agarose Gel Electrophoresis

Agarose gel is commonly used for separation of DNA fragments.

After electrophoresis, DNA fragments can be visualised after staining with a suitable dye and exposure to ultraviolet radiation.

32. Elution

The desired DNA fragment can be cut from the agarose gel and purified. The process of recovering the desired DNA fragment from the gel is called elution.

33. Important Tools of Recombinant DNA Technology

Tool Main Function
Restriction endonuclease Cuts DNA at specific sites
DNA ligase Joins DNA fragments
DNA polymerase Synthesises DNA
Taq polymerase Amplifies DNA during PCR
Vector Carries foreign DNA into host
Host cell Replicates/expresses recombinant DNA
Selectable marker Identifies selected cells
Agarose gel Separates DNA fragments

34. Vector vs Host Cell

Vector Host Cell
Carries foreign DNA Receives recombinant DNA
Usually a plasmid or other suitable DNA molecule May be bacterial, yeast, plant or animal cell
Contains features such as ori and selectable marker Provides cellular machinery for replication/expression

35. Recombinant DNA Technology and Insulin

Recombinant DNA technology has been used to produce important therapeutic proteins such as human insulin.

Human insulin consists of two polypeptide chains, A-chain and B-chain, linked by disulphide bonds in the functional insulin molecule.

Genetic engineering made it possible to produce insulin using microorganisms, reducing dependence on insulin obtained from animal sources.

36. Recombinant DNA Technology in Medicine

Recombinant DNA technology has enabled the production of several medically important substances.

  • Human insulin
  • Human growth hormone
  • Vaccines
  • Interferons
  • Other therapeutic proteins

37. Recombinant Vaccines

Genes encoding specific antigenic proteins can be introduced into suitable host organisms to produce recombinant vaccine components.

These technologies help in developing vaccines that can stimulate an immune response against particular pathogens.

38. Important Terminology

Term Meaning
Recombinant DNA DNA formed by joining DNA fragments from different sources
Restriction enzyme Enzyme that cuts DNA at specific recognition sites
Sticky end Single-stranded overhang produced by staggered DNA cutting
DNA ligase Enzyme that joins DNA fragments
Vector DNA carrier used to introduce foreign DNA into a host
Transformation Introduction of foreign DNA into a bacterial cell
Clone Genetically identical copy or population derived from a common source
ori Origin of replication
PCR Technique for amplification of a specific DNA sequence
Elution Recovery of desired DNA fragment from gel
Downstream processing Separation and purification of the desired product

39. Restriction Enzyme vs DNA Ligase

Restriction Enzyme DNA Ligase
Cuts DNA Joins DNA fragments
Recognises specific DNA sequences Forms phosphodiester bonds between DNA fragments
Acts like molecular scissors Acts like molecular glue

40. PCR vs Gel Electrophoresis

PCR Gel Electrophoresis
Amplifies DNA Separates DNA fragments
Produces many copies of a specific sequence Separates fragments mainly according to size
Uses primers and DNA polymerase Uses an electric field and gel

41. Common Mistakes to Avoid

  • Do not confuse restriction enzyme with DNA ligase.
  • Restriction enzymes cut DNA; they do not join DNA.
  • DNA ligase joins DNA fragments; it does not amplify DNA.
  • ori is the origin of replication, not the cloning site.
  • DNA moves towards the positive electrode during gel electrophoresis.
  • Smaller DNA fragments generally move faster through agarose gel.
  • PCR is an amplification technique, not a DNA-separation technique.
  • Transformation refers to uptake of foreign DNA by a bacterial cell.

42. Board Important Questions

Very Short Answer Questions

  1. What is recombinant DNA?
  2. What are restriction enzymes?
  3. What is a sticky end?
  4. What is the function of DNA ligase?
  5. What is a cloning vector?
  6. What is the full form of PCR?
  7. What is the function of ori?
  8. What is a selectable marker?
  9. What is transformation?
  10. What is elution?

Short Answer Questions

  1. Explain the role of restriction endonucleases in recombinant DNA technology.
  2. What are sticky ends? Why are they useful?
  3. Explain the important features of a cloning vector.
  4. What is the role of DNA ligase?
  5. Explain the three basic steps of PCR.
  6. Why are competent cells required in genetic engineering?
  7. Explain the role of selectable markers.
  8. What is gel electrophoresis? How does it separate DNA fragments?

Long Answer Questions

  1. Explain the major steps involved in recombinant DNA technology.
  2. Describe the structure and important features of a plasmid cloning vector.
  3. Explain PCR with its major steps and the role of Taq polymerase.
  4. Describe how recombinant DNA is formed using restriction enzymes and DNA ligase.
  5. Explain the role of recombinant DNA technology in the production of useful biological products.

43. Multiple Choice Questions

  1. Which enzyme cuts DNA at specific recognition sequences?

    (A) DNA ligase   (B) Restriction endonuclease   (C) Amylase   (D) Protease

    Answer: (B) Restriction endonuclease

  2. Which enzyme joins DNA fragments?

    (A) DNA ligase   (B) Helicase   (C) RNase   (D) Lipase

    Answer: (A) DNA ligase

  3. EcoRI recognises which sequence?

    (A) AAGCTT   (B) GAATTC   (C) GGATCC   (D) GGTACC

    Answer: (B) GAATTC

  4. Which structure is commonly used as a cloning vector?

    (A) Ribosome   (B) Plasmid   (C) Lysosome   (D) Golgi body

    Answer: (B) Plasmid

  5. What does ori represent?

    (A) Origin of replication   (B) Origin of transcription   (C) Operator region   (D) RNA origin

    Answer: (A) Origin of replication

  6. PCR is mainly used for:

    (A) DNA amplification   (B) Protein digestion   (C) Cell division   (D) Lipid synthesis

    Answer: (A) DNA amplification

  7. Which enzyme is commonly used in PCR?

    (A) Taq polymerase   (B) DNA ligase   (C) Pepsin   (D) Cellulase

    Answer: (A) Taq polymerase

  8. DNA is negatively charged because of its:

    (A) Nitrogen bases   (B) Phosphate groups   (C) Sugars   (D) Hydrogen bonds

    Answer: (B) Phosphate groups

  9. During gel electrophoresis, DNA moves towards:

    (A) Cathode   (B) Anode   (C) Both equally   (D) It does not move

    Answer: (B) Anode

  10. Smaller DNA fragments in agarose gel generally:

    (A) Move slower   (B) Move faster   (C) Do not move   (D) Remain at the well

    Answer: (B) Move faster

  11. The process of introducing recombinant DNA into a bacterial cell is called:

    (A) Translation   (B) Transformation   (C) Transcription   (D) Transduction

    Answer: (B) Transformation

  12. Which of the following is a selectable marker?

    (A) Antibiotic-resistance gene   (B) Ribosomal RNA   (C) ATP   (D) Water

    Answer: (A) Antibiotic-resistance gene

  13. Which enzyme removes RNA during DNA isolation?

    (A) RNase   (B) Ligase   (C) Polymerase   (D) Amylase

    Answer: (A) RNase

  14. Which enzyme helps remove proteins during DNA isolation?

    (A) Protease   (B) Ligase   (C) Taq polymerase   (D) Restriction enzyme

    Answer: (A) Protease

  15. Recovery of a desired DNA fragment from agarose gel is called:

    (A) Transformation   (B) Elution   (C) Translation   (D) Denaturation

    Answer: (B) Elution

44. Fill in the Blanks

  1. Restriction enzymes are also called molecular ________.
  2. DNA fragments are joined by the enzyme ________.
  3. The origin of replication is abbreviated as ________.
  4. PCR stands for ________.
  5. DNA is negatively charged because of its ________ groups.
  6. DNA moves towards the ________ electrode during electrophoresis.
  7. The introduction of foreign DNA into bacteria is called ________.
  8. EcoRI recognises the sequence ________.
  9. The thermostable DNA polymerase used in PCR is called ________ polymerase.
  10. Recovery of DNA from an agarose gel is called ________.

45. Match the Following

Column A Column B
1. Restriction endonuclease a. DNA amplification
2. DNA ligase b. Origin of replication
3. PCR c. Cuts DNA
4. ori d. Joins DNA fragments
5. Taq polymerase e. Thermostable DNA polymerase

Answers: 1-c, 2-d, 3-a, 4-b, 5-e

46. Assertion and Reason

  1. Assertion: Restriction enzymes are useful in recombinant DNA technology.

    Reason: They can cut DNA at specific recognition sequences.

    Answer: Both Assertion and Reason are true, and Reason correctly explains Assertion.

  2. Assertion: DNA moves towards the positive electrode during gel electrophoresis.

    Reason: DNA has negatively charged phosphate groups.

    Answer: Both Assertion and Reason are true, and Reason correctly explains Assertion.

  3. Assertion: DNA ligase is used to amplify DNA.

    Reason: DNA ligase joins DNA fragments.

    Answer: Assertion is false, but Reason is true.

47. Quick Revision

  • Restriction enzyme → Cuts DNA at specific sites.
  • Sticky ends → Single-stranded overhangs produced by staggered cuts.
  • DNA ligase → Joins DNA fragments.
  • Plasmid → Common cloning vector.
  • ori → Origin of replication.
  • Selectable marker → Helps identify selected cells.
  • PCR → Amplifies specific DNA.
  • Taq polymerase → Thermostable enzyme used in PCR.
  • Transformation → Introduction of foreign DNA into a bacterial cell.
  • Gel electrophoresis → Separates DNA fragments according to size.
  • Elution → Recovery of desired DNA fragment from gel.
  • Downstream processing → Purification and processing of the final product.

48. One-Line Exam Facts

  • EcoRI recognises the palindromic sequence GAATTC.
  • Restriction enzymes are molecular scissors.
  • DNA ligase is molecular glue.
  • Plasmids are commonly used as cloning vectors.
  • ori is required for replication of the vector.
  • Antibiotic-resistance genes can act as selectable markers.
  • PCR involves denaturation, annealing and extension.
  • Taq polymerase is obtained from Thermus aquaticus.
  • DNA is negatively charged.
  • DNA moves towards the positive electrode in gel electrophoresis.
  • Smaller DNA fragments move faster through agarose gel.
  • DNA ligase joins compatible DNA fragments.
Must Remember: The complete logic of recombinant DNA technology is Cut → Amplify → Join → Transfer → Select → Express → Purify.
Lesson 16 of 24
On This Page