CUET UG Biology Biotechnology Recombinant DNA Technology Restriction Enzymes Vectors PCR

Must-Know (15–20 detailed bullets)

• Restriction enzymes are produced by bacteria as a defense against bacteriophages; for example, EcoRI is isolated from Escherichia coli RY13.
• Restriction enzymes cut DNA at specific palindromic sequences; EcoRI recognizes 5'-GAATTC-3' and cuts between G and A on both strands.
• Sticky ends are overhanging ends produced by some restriction enzymes (e.g., EcoRI), which facilitate the action of DNA ligase.
• Blunt ends are formed when restriction enzymes cut both strands at the same position (e.g., AluI from Arthrobacter luteus).
• The first restriction enzyme discovered was HindII, which recognizes a six-base pair sequence and cuts at a specific site.
• The term "restriction" refers to the ability of bacterial enzymes to restrict viral DNA by cleaving it.
• The enzyme DNA ligase joins two DNA fragments by forming phosphodiester bonds; T4 DNA ligase is commonly used in rDNA technology.
• Plasmids are the most commonly used vectors in recombinant DNA technology; pBR322 is a well-known plasmid vector with 4361 base pairs.
• Vectors must have an origin of replication (ori), a selectable marker (e.g., ampicillin resistance gene), and a cloning site (MCS).
• In pBR322, the amp^R gene codes for ampicillin resistance and the tet^R gene for tetracycline resistance.
• Insertional inactivation is used to identify recombinant plasmids; for example, insertion into the tet^R gene makes bacteria sensitive to tetracycline.
• The polymerase chain reaction (PCR) was developed by Kary Mullis in 1985 and allows amplification of specific DNA sequences.
• PCR involves three steps: denaturation (94–96°C), annealing (55–60°C), and extension (72°C).
• Taq polymerase, isolated from Thermus aquaticus, is heat-stable and used in PCR to synthesize new DNA strands.
• Each PCR cycle doubles the DNA amount; after 30 cycles, DNA is amplified approximately 2^30 times (~1 billion-fold).
• Primers in PCR are short, single-stranded oligonucleotides (18–22 nucleotides) that are complementary to the ends of the target sequence.
• Gel electrophoresis separates DNA fragments based on size; DNA moves toward the anode due to its negative charge.
• Ethidium bromide is used to stain DNA in agarose gel, which fluoresces under UV light for visualization.
• The unit for measuring DNA fragment size is base pairs (bp) or kilobase pairs (kb); e.g., a 500 bp fragment is smaller than 1 kb.
• Verify from NCERT: The exact number of restriction enzymes isolated to date.

Difficulty Level

Intermediate — Requires understanding of molecular tools and their interplay, but no complex calculations or abstract theory.

Common CUET Traps (3 bullets)

Trap: Students assume all restriction enzymes produce sticky ends.
Avoid: Remember that some (like AluI) produce blunt ends; always check the enzyme type.

Trap: Confusing selectable markers with origin of replication in vectors.
Avoid: Selectable markers (e.g., antibiotic resistance genes) help identify transformed cells, while ori is essential for replication.

Trap: Believing PCR requires helicase to unwind DNA.
Avoid: PCR uses high temperature (94–96°C) for denaturation, not helicase.

Practice MCQs (5 questions)

Q1. Which of the following restriction enzymes produces blunt ends?
A. EcoRI
B. HindIII
C. AluI
D. BamHI

Answer: C
Explanation: AluI cuts DNA at the site AGCT and produces blunt ends.
Why others fail: EcoRI, HindIII, and BamHI all produce sticky ends, making them tempting but incorrect choices.

Q2. What is the function of Taq polymerase in PCR?
A. To cut DNA at specific sites
B. To join DNA fragments
C. To synthesize new DNA strands
D. To denature double-stranded DNA

Answer: C
Explanation: Taq polymerase synthesizes new DNA strands during the extension step of PCR.
Why others fail: Denaturation is done by heat, not Taq polymerase; restriction enzymes cut DNA, and ligase joins fragments.

Q3. In pBR322, insertion of foreign DNA into the tet^R gene results in:
A. Resistance to both ampicillin and tetracycline
B. Resistance to ampicillin but sensitivity to tetracycline
C. Sensitivity to both antibiotics
D. Resistance to tetracycline but sensitivity to ampicillin

Answer: B
Explanation: Insertional inactivation of tet^R makes the cell sensitive to tetracycline, while amp^R remains functional.
Why others fail: Students often reverse the markers or assume both genes are inactivated.

Q4. Which of the following is NOT a required feature of a cloning vector?
A. Origin of replication
B. Promoter for RNA polymerase
C. Selectable marker
D. Cloning site

Answer: B
Explanation: A promoter is needed for expression vectors but not essential for basic cloning vectors.
Why others fail: Promoters are crucial in expression systems, leading students to overgeneralize.

Q5. During PCR, the annealing step typically occurs at:
A. 72°C
B. 95°C
C. 55–60°C
D. 4°C

Answer: C
Explanation: Primers anneal to complementary sequences at 55–60°C, depending on primer melting temperature.
Why others fail: 72°C is for extension, 95°C for denaturation, and 4°C is for storage, so confusion arises from mixing up steps.

Last‑Minute Revision (15–20 one‑liners)

• ⚠️ EcoRI recognizes 5'-GAATTC-3' and produces sticky ends.
• ⚠️ HindII was the first restriction enzyme discovered.
• ⚠️ Palindromic sequence in DNA reads the same on both strands when oriented 5'→3'.
• ⚠️ Blunt ends: no overhangs (e.g., AluI); sticky ends: overhangs (e.g., EcoRI).
• ⚠️ DNA ligase forms phosphodiester bonds between nucleotides.
• ⚠️ pBR322 has 4361 bp and two antibiotic resistance genes.
• ⚠️ Insertional inactivation inactivates a gene by inserting foreign DNA into it.
• ⚠️ Recombinant plasmid detection: loss of tetracycline resistance indicates insert in tet^R.
• ⚠️ PCR: developed by Kary Mullis in 1985.
• ⚠️ PCR cycle: denaturation → annealing → extension.
• ⚠️ Denaturation in PCR occurs at 94–96°C.
• ⚠️ Annealing temperature: 55–60°C (depends on primer Tm).
• ⚠️ Extension by Taq polymerase at 72°C.
• ⚠️ Taq polymerase is thermostable, isolated from Thermus aquaticus.
• ⚠️ After 30 PCR cycles, DNA amplification ≈ 2^30-fold.
• ⚠️ Primers are 18–22 nucleotides long, single-stranded.
• ⚠️ Gel electrophoresis: DNA moves to anode due to negative charge.
• ⚠️ Ethidium bromide + UV light = orange fluorescence of DNA bands.
• ⚠️ DNA size measured in bp or kb; smaller fragments move faster in gel.
• ⚠️ Mnemonic: "PAL" — Palindromic, Annealing, Ligase — key terms in rDNA tech.