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RGUHS Karnataka - MBBS 1st Year Biochemistry

Genetics & Molecular Biology: Deep PYQ Analysis (Last 5 Years)

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Official RGUHS Blueprint Allocation (Paper II)

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TIER 1 - Most Repeated Topics (Long Essay Level - 10 marks)

1. Recombinant DNA Technology ⭐⭐⭐

• Asked almost every year as a Long Essay (10 marks)
• What to cover:
  • Steps of recombinant DNA technology (cloning vectors, restriction enzymes, ligases, transformation)
  • Types of vectors: plasmids, bacteriophages, cosmids
  • Applications in medicine (insulin production, vaccines, gene therapy)
  • Chimeric DNA molecule definition
• Key past questions: "Discuss in detail recombinant DNA technology and its clinical applications" (seen across 2013, 2020, 2022, 2023, 2024)

2. DNA Replication ⭐⭐⭐

• Consistently asked as Long Essay or Short Essay
• What to cover:
  • Semi-conservative nature (Meselson-Stahl experiment)
  • Steps: initiation, elongation, termination
  • Enzymes: DNA polymerase I, II, III (prokaryotic), helicase, primase, ligase, topoisomerase
  • Differences: prokaryotic vs eukaryotic replication
  • Okazaki fragments, leading vs lagging strand
  • Replication fidelity and proofreading

3. Protein Synthesis (Translation) ⭐⭐⭐

• High-frequency Long Essay
• What to cover:
  • Ribosome structure (30S, 50S, 70S; 40S, 60S, 80S)
  • tRNA structure and functions of each arm (acceptor stem, anticodon loop, T-loop, D-loop)
  • Stages: initiation, elongation (transpeptidation), termination
  • Antibiotics that inhibit translation and their site of action:
    • Streptomycin - 30S, prevents initiation
    • Erythromycin - 50S, blocks translocation
    • Chloramphenicol - 50S, inhibits peptidyl transferase
    • Tetracycline - 30S, blocks aminoacyl-tRNA binding
  • Post-translational modifications

4. Transcription (RNA Synthesis) ⭐⭐⭐

• What to cover:
  • Template strand vs coding strand
  • RNA polymerase types (prokaryotic: sigma factor; eukaryotic: RNA pol I, II, III)
  • Steps in eukaryotes: initiation, elongation, termination
  • Post-transcriptional modifications:
    • 5' capping (7-methyl guanosine cap)
    • 3' polyadenylation (poly-A tail)
    • Splicing of introns (snRNPs, spliceosomes)
    • RNA editing
  • Differences from DNA replication

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TIER 2 - Frequently Asked (Short Essay Level - 5 marks)

5. Genetic Code ⭐⭐⭐

• One of the most-tested short essay topics
• What to cover:
  • Features: triplet, non-overlapping, comma-free, degenerate/redundant, universal, unambiguous
  • Wobble hypothesis (Crick) - third position flexibility of codon-anticodon pairing
  • Start codon (AUG), stop codons (UAA, UAG, UGA)
  • "Degeneracy" vs "ambiguity" distinction

6. Mutations ⭐⭐

• What to cover:
  • Types: point mutation (transition, transversion), frameshift mutation (insertion, deletion)
  • Silent, missense, nonsense mutations
  • Clinical examples: Sickle cell anemia (point mutation - GAG→GTG), Thalassemia (frameshift)
  • Mutagens: physical (UV), chemical (alkylating agents, base analogs)

7. DNA Repair Mechanisms ⭐⭐

• What to cover:
  • Mismatch repair
  • Base excision repair (BER)
  • Nucleotide excision repair (NER) - xeroderma pigmentosum
  • Direct repair (photolyase for UV-induced thymine dimers)
  • Double-strand break repair

8. PCR (Polymerase Chain Reaction) ⭐⭐⭐

• Asked consistently as a short note / short essay
• What to cover:
  • Principle (exponential amplification of target DNA)
  • Steps: denaturation (94°C), annealing (55-65°C), extension (72°C, Taq polymerase)
  • Components: template DNA, primers, dNTPs, Taq polymerase, thermocycler
  • Medical applications: diagnosis of HIV, tuberculosis, genetic diseases, forensics, paternity testing
  • Variants: RT-PCR, real-time PCR

9. Regulation of Gene Expression ⭐⭐

• What to cover:
  • Lac operon (negative regulation) - inducible system
    • Structural genes: lacZ, lacY, lacA
    • Repressor protein, inducer (allolactose)
    • Positive regulation by catabolite activator protein (CAP)
  • Trp operon (negative regulation) - repressible system
  • Eukaryotic regulation: transcription factors, enhancers, silencers, histone modification

10. Restriction Endonucleases ⭐⭐

• What to cover:
  • Definition and types (Type I, II, III)
  • Type II most used in recombinant DNA technology
  • Examples: EcoRI, BamHI, HindIII with their recognition sequences
  • Palindromic sequences, sticky ends vs blunt ends
  • RFLP (Restriction Fragment Length Polymorphism) - forensic and diagnostic use

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TIER 3 - Short Notes / Short Answer (3-5 marks)

11. Gene Therapy ⭐⭐

• Definition, types (somatic vs germline)
• Ex vivo vs in vivo approaches
• Vectors: viral (retroviruses, adenoviruses) and non-viral
• Examples: ADA deficiency (SCID), cystic fibrosis, hemophilia
• Limitations and ethical issues

12. tRNA Structure and Function ⭐⭐

• Cloverleaf secondary structure with all 4 loops
• CCA-3' end (aminoacyl attachment)
• Anticodon loop function
• "Adaptor molecule" role

13. DNA Structure (A, B, Z forms) ⭐

• B-DNA (Watson-Crick) as physiological form
• Differences between A, B, Z-DNA
• Base pairing rules (Chargaff's rules): A=T, G≡C
• Major and minor grooves

14. Southern/Northern/Western Blotting ⭐⭐

• Southern blot - DNA detection
• Northern blot - RNA detection
• Western blot - protein detection (used in HIV confirmation)
• Procedure overview

15. DNA Fingerprinting / RFLP ⭐⭐

• Principle using restriction enzymes + gel electrophoresis + Southern blot
• Uses: forensic identification, paternity testing, disease diagnosis

16. Salient Features of Genetic Code ⭐⭐

• This exact phrase appears repeatedly as a short answer question
• 9 features: triplet, non-overlapping, comma-free, degenerate, universal, unambiguous, ordered, initiator codon, terminator codons

17. Post-Translational Modifications ⭐⭐

• Phosphorylation, glycosylation, hydroxylation, acetylation, ubiquitination
• Clinical relevance: collagen hydroxylation (scurvy), glycoprotein formation

18. Nucleotide Structure (Deoxyribonucleotides / Ribonucleotides) ⭐

• Structure of a deoxyribonucleotide with diagram
• Purine ring atom sources

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Topics with Recent RGUHS Trend (2023-2025)

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Priority Study Order for RGUHS Karnataka

MUST DO (Paper II Long Essay):
1. Recombinant DNA Technology + Applications
2. DNA Replication (all enzymes + diagram)
3. Protein Synthesis / Translation (full mechanism + antibiotics)
4. Transcription (eukaryotic + post-transcriptional modifications)

HIGH YIELD (Short Essays):
5. Genetic Code (all features + Wobble hypothesis)
6. PCR (principle + applications)
7. Lac Operon / Gene Regulation
8. DNA Repair Mechanisms
9. Mutations (types + clinical examples)
10. Restriction Endonucleases

SHORT NOTES (3-5 marks each):
11. Gene Therapy
12. tRNA structure
13. Southern/Northern/Western Blotting
14. DNA Fingerprinting / RFLP
15. Post-translational modifications
16. Oncogenes and tumor suppressor genes
17. Reverse Transcriptase / Retrovirus genome

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Key Exam Tips for RGUHS Karnataka

1. Always draw diagrams - RGUHS papers consistently say "Draw neat labeled diagrams wherever necessary." For DNA replication, translation, tRNA structure, and lac operon - diagrams can fetch 3-4 bonus marks.
2. Link to clinical applications - RGUHS rewards answers that connect to disease (e.g., sickle cell anemia with point mutation, xeroderma pigmentosum with NER defect, ADA deficiency with gene therapy).
3. Antibiotics and molecular biology - The mechanism of action of antibiotics on ribosomal subunits is a near-certain question every year.
4. Recombinant DNA Technology is the single most-repeated long essay topic across all RGUHS Karnataka papers over the past 5+ years - treat it as a guaranteed 10-mark question.
5. Molecular Biology Techniques (PCR, blotting, RFLP, DNA fingerprinting) carry 8 marks officially in the new blueprint - do not skip these.

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TIER 3 SHORT NOTES - Complete 5-Mark Answers

Genetics & Molecular Biology | RGUHS Karnataka MBBS 1st Year

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1. Gene Therapy

• Viral vectors (most common): Retroviruses, Adenoviruses, Adeno-associated viruses (AAV), Lentiviruses
• Non-viral vectors: Liposomes, naked plasmid DNA, gene guns

1. Isolate defective cells (e.g., T-lymphocytes or HSCs) from patient
2. Insert normal gene into cells using a retroviral vector in vitro
3. Expand the corrected cells in culture
4. Reinfuse into the patient

• ADA-SCID (Severe Combined Immunodeficiency) - First successful gene therapy (1990); ADA gene inserted into T-lymphocytes via retroviral vector
• Hemophilia - clotting factor genes delivered via AAV
• Cystic fibrosis - CFTR gene via adenoviral vector
• Certain cancers and blindness disorders

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2. tRNA Structure and Functions

• All tRNAs end in the sequence ...CCA-3' (added post-transcriptionally)
• The 3'-OH of the terminal adenosine is the amino acid attachment site
• Aminoacyl-tRNA synthetase enzyme charges the tRNA with its specific amino acid (aminoacylation)

• Synthesized by RNA Polymerase III
• Contains many unusual/modified bases (dihydrouridine, pseudouridine, inosine)
• At least 20 types of tRNA exist (one per amino acid)
• The anticodon reads mRNA in the 3'→5' direction while mRNA runs 5'→3'

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3. DNA Structure (A, B, Z forms)

• Double helix, antiparallel strands (one runs 5'→3', the other 3'→5')
• Right-handed helix
• Base pairing: A=T (2 hydrogen bonds), G≡C (3 hydrogen bonds) - Chargaff's Rules
• Pitch = 3.4 nm, diameter = 2 nm, 10 base pairs per turn
• Has a major groove and a minor groove - transcription factors bind in major groove

1. [A] = [T] and [G] = [C]
2. Purines = Pyrimidines
3. GC content is constant for a species and varies between species
4. Higher GC content = higher melting temperature (Tm) because G≡C has 3 H-bonds

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4. Southern, Northern, and Western Blotting

• Southern = DNA
• Northern = RNA
• Western = Protein

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1. Digest genomic DNA with restriction enzymes
2. Separate fragments by gel electrophoresis (smaller fragments migrate further)
3. Denature DNA in gel (NaOH), transfer to nitrocellulose membrane (blotting)
4. Hybridize with a labeled probe (radioactive or fluorescent) complementary to target sequence
5. Expose to X-ray film (autoradiography) → bands visible

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• RNA is isolated (not DNA) and not digested with restriction enzymes
• Run on formaldehyde-agarose gel (denaturing conditions to prevent secondary structures)
• Transfer to membrane → hybridize with labeled probe

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1. Separate proteins by SDS-PAGE (sodium dodecyl sulfate polyacrylamide gel electrophoresis)
2. Transfer to PVDF or nitrocellulose membrane
3. Block non-specific sites (skim milk or BSA)
4. Incubate with primary antibody (specific to target protein)
5. Add secondary antibody (conjugated to enzyme or fluorescent dye)
6. Detect by chemiluminescence or colorimetry

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5. DNA Fingerprinting / RFLP

• When restriction enzymes cut genomic DNA, the sizes of resulting fragments differ between individuals due to polymorphisms in restriction sites
• These different-sized fragments are the basis of DNA fingerprinting

1. Obtain DNA sample (blood, saliva, hair follicle, semen)
2. Cut with restriction enzymes → produces fragments of varying sizes
3. Separate by gel electrophoresis
4. Southern blot - transfer to membrane
5. Hybridize with labeled probes for VNTR regions
6. Autoradiography → produces a unique banding pattern ("DNA fingerprint")

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6. Post-Translational Modifications (PTMs)

• Endoplasmic Reticulum: Glycosylation, disulfide bond formation, signal peptide cleavage
• Golgi apparatus: Further glycosylation, sulfation, phosphorylation
• Cytoplasm: Phosphorylation, ubiquitination, acetylation

• Defective glycosylation → I-cell disease (mucolipidosis)
• Defective hydroxylation (Vit C deficiency) → Scurvy (weakened collagen)
• Abnormal phosphorylation → cancer, diabetes

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7. Oncogenes and Tumor Suppressor Genes

• Proto-oncogenes: Normal genes that promote cell growth and division (e.g., Ras, Myc, Src)
• Tumor suppressor genes: Normal genes that inhibit cell growth and promote apoptosis (e.g., p53, Rb, BRCA1)

• Proto-oncogenes that become mutated or overexpressed → convert to oncogenes (gain-of-function mutation)
• Oncogenes drive uncontrolled cell proliferation
• Act in a dominant manner (one mutated allele is sufficient)

• Normally restrain cell growth; both alleles must be inactivated for tumor formation (Knudson's "Two-Hit" hypothesis)
• Act in a recessive manner

• Normal Ras: GDP-bound (inactive) → GTP-bound (active) → GTPase activity → GDP (returns to inactive)
• Mutant Ras: GTPase activity lost → permanently GTP-bound → constitutively active → uncontrolled proliferation

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8. Reverse Transcriptase / Retrovirus Genome

• Also called RNA-dependent DNA polymerase
• Discovered by Temin and Baltimore (1970) - Nobel Prize 1975
• Found in all retroviruses

1. RNA-dependent DNA polymerase - synthesizes complementary DNA (cDNA) from RNA template
2. RNase H activity - degrades the original RNA strand of the RNA-DNA hybrid
3. DNA-dependent DNA polymerase - synthesizes the second DNA strand to form double-stranded DNA

1. Virus attaches and injects ssRNA into host cell
2. Reverse transcriptase uses viral RNA as template → synthesizes cDNA (RNA-DNA hybrid)
3. RNase H degrades original RNA strand
4. Second DNA strand synthesized → double-stranded DNA (dsDNA)
5. Viral dsDNA integrates into host chromosome as a provirus (by integrase enzyme)
6. Provirus transcribed by host RNA polymerase → new viral RNA and mRNA
7. Viral proteins assembled → budding of new virions

• gag - codes for core proteins (p24, p17, p9, p7)
• pol - codes for reverse transcriptase, integrase, protease
• env - codes for envelope proteins (gp120, gp41)

• RT is the target of NRTIs (Nucleoside Reverse Transcriptase Inhibitors) like Zidovudine (AZT), Lamivudine - first-line HIV drugs
• Reverse transcriptase is used in labs to make cDNA from mRNA for cloning expressed genes
• RT-PCR uses reverse transcriptase to detect RNA viruses (COVID-19 diagnosis uses RT-PCR)

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Quick Revision Table - All 8 Topics at a Glance

Exam Tip: For any topic involving a structure (tRNA, DNA forms), always draw a labeled diagram. RGUHS examiners give 1-2 marks specifically for diagrams even in short notes.

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TIER 2 SHORT ESSAYS - Complete 5-Mark Answers

Genetics & Molecular Biology | RGUHS Karnataka MBBS 1st Year

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1. Genetic Code and Wobble Hypothesis

Definition

Features of Genetic Code (9 Features - Must Know All)

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Wobble Hypothesis (Crick, 1966)

• Strict base pairing (A-U, G-C) applies only at the 1st and 2nd positions of the codon
• At the 3rd position (called the "wobble position"), the base pairing rules are relaxed
• A single tRNA anticodon can base-pair with more than one codon by wobbling at the 3rd codon position

• UGA codes for Tryptophan (not stop)
• AUA codes for Methionine (not Isoleucine)

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2. PCR - Polymerase Chain Reaction

Definition

Components Required

1. Template DNA - DNA containing the target sequence
2. Two primers - short oligonucleotides (~18-25 bp), one complementary to each strand, flanking the target
3. Taq DNA polymerase - heat-stable DNA polymerase from Thermus aquaticus (withstands 94°C)
4. dNTPs (dATP, dTTP, dCTP, dGTP) - building blocks
5. MgCl₂ - cofactor for Taq polymerase
6. Buffer - maintains optimal pH
7. Thermocycler - machine that automatically cycles temperatures

Three Steps of Each PCR Cycle

Exponential Amplification

• After 1 cycle: 2 copies
• After 20 cycles: 2²⁰ = ~10⁶ copies
• After 30 cycles: 2³⁰ = ~10⁹ copies
• Product doubles with each cycle

Medical Applications of PCR

Important Variant - RT-PCR

• First, viral RNA is converted to cDNA using Reverse Transcriptase
• Then cDNA is amplified by PCR
• Used for: COVID-19 diagnosis, HIV quantification (viral load), gene expression studies

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3. Regulation of Gene Expression - Lac Operon

What is an Operon?

• Structural genes - encode the actual enzymes/proteins
• Operator - binding site for repressor protein
• Promoter - binding site for RNA polymerase
• Regulator gene - encodes the repressor protein (located separately)

The Lac Operon (Jacob and Monod, 1961)

Regulation of the Lac Operon

• LacI repressor protein is active
• Repressor binds to operator region
• RNA polymerase CANNOT bind to promoter
• No transcription → no enzymes made

• Lactose → converted to allolactose (the true inducer) by small amounts of existing β-galactosidase
• Allolactose binds to the LacI repressor → repressor changes shape → CANNOT bind to operator
• RNA polymerase binds to promoter and transcribes lacZ, lacY, lacA as a polycistronic mRNA
• Enzymes produced → lactose metabolized

• No glucose → high cAMP levels → cAMP binds to CAP (Catabolite Activator Protein / CRP)
• cAMP-CAP complex binds to the promoter → stimulates RNA polymerase binding → maximum transcription
• Glucose present → low cAMP → CAP inactive → reduced transcription (even if lactose is present)

Trp Operon (Repressible Operon - Briefly)

• Normally ON (tryptophan synthesis occurs constitutively)
• When tryptophan is available, it acts as a corepressor - binds inactive repressor → active repressor → binds operator → transcription stops
• This saves energy when tryptophan is already available

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4. DNA Repair Mechanisms

Why DNA Repair is Needed

Four Major DNA Repair Mechanisms

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1. MutS protein recognizes the mismatched base pair
2. MutL is recruited by MutS; complex activates MutH
3. MutH cleaves the unmethylated daughter strand (discrimination from methylated parental strand via GATC methylation)
4. Exonuclease removes the mismatched stretch
5. DNA pol III fills the gap; DNA ligase seals

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1. UvrABC excinuclease recognizes the bulky dimer
2. Cuts the damaged strand on both sides (5' and 3') of the lesion
3. A short oligonucleotide (~12-13 bp in prokaryotes, ~25-30 bp in eukaryotes) is excised
4. DNA pol I fills the gap
5. DNA ligase seals the nick

• Cannot repair UV-induced pyrimidine dimers
• Extreme sensitivity to sunlight
• Multiple skin cancers in early childhood

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1. DNA glycosylase removes the abnormal base by cleaving the N-glycosidic bond → creates an AP site (apurinic/apyrimidinic site)
2. AP endonuclease cleaves the phosphodiester backbone at the AP site
3. Deoxyribophosphodiesterase removes the deoxyribose remnant
4. DNA pol β (in eukaryotes) inserts the correct nucleotide
5. DNA ligase seals the nick

• Deamination: C → U, A → Hypoxanthine
• Oxidation: Guanine → 8-oxoguanine (can cause G-C to T-A transversion)
• Alkylation: N7-methylguanine

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• Directly reverses UV-induced thymine dimers in the presence of visible light (photoactivation)
• Not present in placental mammals (present in bacteria, plants, lower organisms)

• Directly removes methyl groups from O⁶-methylguanine
• Suicide enzyme - permanently inactivated after one repair event

Summary Table

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5. Mutations - Types and Clinical Examples

Definition

Classification of Mutations

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A. Based on Size / Extent

• Insertion or deletion of 1-2 nucleotides (not multiples of 3)
• Shifts the reading frame of all codons downstream
• Results in completely different amino acid sequence from point of mutation onwards
• Often creates a premature stop codon

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B. Based on Cause

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C. Based on Location

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Important Clinical Examples

Carcinogenesis and Mutations

• Most cancers result from acquired somatic mutations in proto-oncogenes (gain-of-function) and tumor suppressor genes (loss-of-function)
• Mutagens are therefore also called carcinogens
• Ames test: identifies potential carcinogens by their mutagenicity in bacteria

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6. Restriction Endonucleases

Definition

Discovery

• Arber, Smith, and Nathans (1978 Nobel Prize) - discovered restriction endonucleases

Types of Restriction Enzymes

Recognition Sequences - Palindromic Nature

Types of Cut Ends

• Staggered cuts leave short single-stranded overhangs
• Can anneal with complementary sticky ends from any other DNA cut with the same enzyme
• More efficient for ligation (DNA joining)

5'...G     AATTC...3'    →    5'...G           3' + 5' AATTC...3'
3'...CTTAA     G...5'         3'...CTTAA  5'         3'     G...5'

• Cut straight through both strands at the same position
• No single-stranded overhang
• Can be ligated but less efficiently

Role in Recombinant DNA Technology

1. Both the vector (plasmid) and foreign DNA are cut with the same restriction enzyme → compatible sticky ends
2. Foreign DNA fragment anneals to the cut vector by hydrogen bonding
3. DNA ligase covalently seals the nicks → recombinant DNA (chimeric DNA)
4. Inserted into host bacterium (transformation) for cloning and expression

Other Applications

Bacterial Protection Against Own Restriction Enzymes

• Bacteria methylate their own DNA at the same recognition sequences (using methyltransferases)
• Methylated DNA is NOT cut by the restriction enzyme
• Foreign DNA (unmethylated) IS cut

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Quick Revision - All 6 Tier 2 Topics at a Glance

Exam strategy: For Lac Operon - always draw the ON/OFF diagram showing RNA polymerase, repressor, operator, and structural genes. For Mutations - always give the sickle cell anemia example (GAG→GTG). For PCR - write the 3 steps with temperatures. These details are what separate a 4-mark answer from a 5-mark answer in RGUHS exams.