GS-III · Science & Technology · Biology · Genetics
DNA — Deoxyribonucleic Acid 🧬
Complete UPSC Notes — What DNA is (double helix, 3 billion base pairs), structure (nucleotides, base pairs A=T/G≡C, sugar-phosphate backbone), types (A-DNA, B-DNA, Z-DNA), location (nucleus + mitochondria), replication (semi-conservative, helicase, DNA polymerase), functions (protein synthesis — transcription + translation, heredity), DNA fingerprinting, DNA vs RNA, nucleotides vs nucleosides, recombinant DNA, current affairs (Genome India, CRISPR FnCas9, Criminal Procedure Act 2022), and PYQs.
🧬 Double helix | 1953 Watson, Crick, Franklin, Wilkins | 3 billion base pairs in humans
🔗 Base pairing: A=T (2 H-bonds) | G≡C (3 H-bonds) | always anti-parallel strands
🔵 B-DNA = most common | Right-handed | Major + Minor grooves
🧪 Nucleotide = Sugar + Phosphate + Nitrogenous Base | Building block of DNA/RNA
🇮🇳 Genome India Project: 10,000 genomes sequenced | IBDC Faridabad (DBT) | 2025
📚 Legacy IAS — Civil Services Coaching, Bangalore · Updated: April 2026 · All Facts Verified
Section 01 — Foundation
🧬 What is DNA? — Made Simple
💡 The "Instruction Manual" Analogy
Think of DNA as the master instruction manual for building and running a living organism. If the human body is a complex machine with trillions of parts, DNA is the blueprint that describes every part, every assembly step, and every operating procedure. This manual is written in a 4-letter alphabet (A, T, G, C), organised into chapters called genes, bound together in volumes called chromosomes, and stored in the "filing cabinet" of the cell nucleus. The amazing thing: this 2-metre long instruction manual is coiled and compacted so efficiently that it fits inside a nucleus just 6 micrometres across — smaller than a red blood cell. Every single cell in your body (except mature red blood cells) contains the complete instruction manual — about 3 billion "letters" of code.
📌 Definition: DNA (Deoxyribonucleic Acid) is a double-stranded macromolecule that carries the genetic information for an organism's development, functioning, and reproduction. It is composed of nucleotides arranged in a specific sequence, and its structure is a right-handed double helix (B-DNA — most common form). DNA was discovered by Friedrich Miescher (1869) as "nuclein." Its double helix structure was established in 1953 by Watson, Crick, Franklin, and Wilkins.
📜 Discovery Timeline
1869 — Friedrich Miescher: Discovered "nuclein" (DNA) in white blood cells. First to isolate the substance from nuclei.
1944 — Avery, MacLeod, McCarty: Proved DNA (not protein) is the genetic material using Pneumococcus bacteria transformation experiments.
1950 — Erwin Chargaff: Established Chargaff's Rules: A=T and G=C in all DNA (ratios of purines = pyrimidines).
1952 — Rosalind Franklin + Raymond Gosling: X-ray crystallography (Photo 51) revealed the helical structure of DNA.
1953 — Watson + Crick: Proposed the double helix model using Franklin's X-ray data. Published in Nature. Nobel Prize 1962 (Watson, Crick, Wilkins — Franklin had died).
1958 — Meselson + Stahl: Proved semi-conservative replication of DNA.
🧬 Why DNA Matters — UPSC Significance
DNA is central to virtually every area of modern biology, medicine, and technology:
Heredity: Transmits traits from parents to offspring — explains inheritance of eye colour, blood group, disease susceptibility.
Protein synthesis: DNA codes for all proteins — enzymes, hormones, antibodies, structural proteins. Life's chemistry depends on DNA instructions.
Forensics: DNA fingerprinting identifies criminals, establishes paternity, identifies disaster victims. Accepted as "gold standard" of forensic evidence in Indian courts.
Medicine: Gene therapy, genetic diagnosis (cancer mutations, inherited diseases), personalised medicine based on individual genetic profiles.
Agriculture: GM crops (Bt cotton, Golden Rice) — desired genes inserted via recombinant DNA technology.
Evolution: Comparing DNA sequences across species reveals evolutionary relationships.
Section 02 — Building Blocks
🔬 Nucleotides & Nucleosides — The Building Blocks
🔬 Nucleoside vs Nucleotide
🔵 Nitrogen Base alone → Adenine (Purine) | Uracil (Pyrimidine)
🟢 Nucleoside = Base + Sugar → Adenosine (Adenine + Ribose) | Uridine (Uracil + Ribose)
🟠 Nucleotide = Base + Sugar + Phosphate → Adenylic acid (Adenine + Ribose + Phosphate)
🔑 DNA sugar = Deoxyribose | RNA sugar = Ribose | Joined by phosphodiester bonds
🔬 Nucleoside vs Nucleotide
Heterocyclic compound (Nitrogenous base): A ring structure with atoms of at least two different elements (N + C). Includes Adenine, Guanine, Cytosine, Thymine, Uracil.
Nucleoside = Nitrogenous base + Sugar (no phosphate).
Examples: Adenosine (A + Ribose), Deoxyadenosine (A + Deoxyribose), Thymidine (T + Deoxyribose).
Nucleotide = Nitrogenous base + Sugar + Phosphate group.
Examples: ATP (Adenosine Triphosphate), dAMP (Deoxyadenosine Monophosphate).
Nucleic acids (DNA and RNA) = Polymers of nucleotides — joined by phosphodiester bonds between the phosphate of one nucleotide and the sugar of the next → alternating sugar-phosphate backbone.
🔵 Nitrogenous Bases
Two categories:
Purines (double ring): Adenine (A) and Guanine (G). Found in both DNA and RNA. Larger molecules — 2 fused rings.
Pyrimidines (single ring): Cytosine (C), Thymine (T), Uracil (U). Smaller molecules — 1 ring.
In DNA: A, G, C, T (no Uracil)
In RNA: A, G, C, U (Uracil replaces Thymine — no Thymine)
Chargaff's Rules:
• A always pairs with T (or U in RNA) — 2 hydrogen bonds
• G always pairs with C — 3 hydrogen bonds
• [A] = [T] and [G] = [C] in any double-stranded DNA
• Purines = Pyrimidines always (ratio = 1)
UPSC TRAP: G≡C has 3 H-bonds (stronger); A=T has 2 H-bonds (weaker). Higher GC content = harder to denature (more stable).
A
Adenine
Purine (double ring)
= T (2 H-bonds)
T
Thymine
Pyrimidine | DNA only
= A (2 H-bonds)
G
Guanine
Purine (double ring)
≡ C (3 H-bonds)
C
Cytosine
Pyrimidine (single ring)
≡ G (3 H-bonds)
Section 03 — Structure
🧬 Structure of DNA — Double Helix
🧬 DNA Double Helix: Two anti-parallel strands wound around each other in a right-handed helix. The sugar-phosphate backbone (blue ribbon) forms the outside "handrails." The base pairs (coloured rungs) connect the two strands inside: Adenine-Thymine (yellow-green, 2 H-bonds) and Guanine-Cytosine (red-green, 3 H-bonds). The strands run anti-parallel: one 5'→3', the other 3'→5'.
🔬 DNA Structure (Detailed): Left: Double helix with Major groove (wider — accessible to proteins) and Minor groove (narrower). Centre: Base pair arrangement (5'→3' on one strand, 3'→5' on other — anti-parallel). Right: Single nucleotide showing Nitrogenous base (A or G, C or T), Sugar (deoxyribose — 5-carbon), and Phosphate group. Thymine (T) and Adenine (A) connected by 2 hydrogen bonds; Cytosine (C) and Guanine (G) by 3 hydrogen bonds.
🏗️ Key Structural Features
Double helix: Two polynucleotide strands wound around a common axis in a right-handed spiral (B-DNA, most common form).
Anti-parallel: One strand runs 5'→3', the other runs 3'→5'. Like a two-lane road with traffic in opposite directions.
Sugar-phosphate backbone: Alternating deoxyribose (5-carbon sugar) and phosphate groups form the structural "rails" — outside the helix. Connected by phosphodiester bonds.
Base pairs (rungs): Nitrogenous bases in the interior — A pairs with T (2 H-bonds), G pairs with C (3 H-bonds). Complementary and specific — no other pairing possible.
Major and Minor grooves: The helical twist creates two grooves of different widths. Major groove (wider) — accessible to DNA-binding proteins (transcription factors, enzymes). Minor groove (narrower).
Pitch: One complete turn = 10 base pairs = 3.4 nm. Diameter = 2 nm.
🔵 Types of DNA: A, B, Z
B-DNA (Most common):
• Right-handed double helix
• Most common form found in cells under normal conditions
• 10 base pairs per turn, 3.4 nm pitch, 2 nm diameter
• Has distinct major and minor grooves
• Watson-Crick model describes B-DNA
A-DNA:
• Right-handed double helix (like B-DNA)
• Wider and shorter than B-DNA
• Forms in dehydrated conditions or in RNA-DNA hybrids
• 11 base pairs per turn
• Anti-parallel strands, NOT symmetrical
Z-DNA:
• Left-handed double helix (unique among DNA forms)
• Zigzag backbone (hence "Z")
• Found in bacteria, eukaryotes, and viruses
• Narrower and more elongated than B-DNA
• Role in gene regulation being studied
UPSC Trap: B-DNA = right-handed (most common). Z-DNA = left-handed. A-DNA = right-handed.
Section 04 — Location
📍 Where is DNA Found? — Nucleus & Mitochondria
🗂️ DNA Packaging Hierarchy: DNA is 2 metres long but fits in a 6 μm nucleus through extraordinary compaction. Step 1: DNA wraps around 8 histone proteins → forming a Nucleosome ("bead on a string"). Step 2: Nucleosomes coil further → Chromatin fibre. Step 3: Chromatin loops and folds into a compact Chromosome (X-shape visible during cell division). Humans have 23 pairs (46 total) chromosomes per cell.
🔋 DNA Location in a Cell: Nuclear DNA (nDNA): Located in the cell nucleus. Contains ~99.9% of the cell's DNA — 3 billion base pairs, packed into 23 chromosome pairs. Mitochondrial DNA (mtDNA): A small circular DNA molecule inside each mitochondrion (the "powerhouse of the cell"). 16,569 base pairs. Codes for 13 proteins (for oxidative phosphorylation), 22 tRNAs, 2 rRNAs. Inherited ONLY from the mother (maternal inheritance). Used in forensics for identifying maternal lineages and ancient DNA analysis.
🔵 Nuclear DNA (nDNA)
Location: Cell nucleus — "control centre of the cell."
Amount: ~3 billion base pairs per haploid set (6 billion total in diploid cell).
Organisation: Packed into 23 pairs of chromosomes (46 total in humans). Each chromosome = one long DNA molecule + histone proteins.
Histones: Spool-like proteins (H2A, H2B, H3, H4 form octamer; H1 linker) around which DNA winds. 8 histones + ~147 bp of DNA = one nucleosome.
Chromatin: Complex of DNA + histones. Euchromatin (loosely packed, active) vs Heterochromatin (tightly packed, inactive).
Genome: The complete set of an organism's DNA. Human genome = ~20,000–25,000 protein-coding genes (but only ~2% of DNA codes for proteins — the rest is non-coding DNA).
🔋 Mitochondrial DNA (mtDNA)
Location: Mitochondria (cytoplasm of cell).
Structure: Circular, double-stranded DNA (like bacterial DNA — evidence for endosymbiotic origin of mitochondria).
Size: ~16,569 base pairs (tiny compared to nuclear DNA).
Codes for: 13 proteins (for cellular respiration — oxidative phosphorylation), 22 transfer RNAs, 2 ribosomal RNAs.
Inheritance: Exclusively from mother (maternal inheritance). Sperm mitochondria are destroyed after fertilisation. This allows tracing maternal lineage through generations.
Forensic use: Identifying maternal lineage from hair roots, teeth, bones (ancient DNA). Used in Nirbhaya case, Tsunami identification, royal lineage tracing.
Mutations: mtDNA mutations cause mitochondrial diseases (Leigh syndrome, MELAS).
UPSC TRAP: mtDNA inherited ONLY from mother — not from father.
Section 05 — Replication
🔄 DNA Replication — Semi-Conservative
🔄 DNA Replication — Semi-Conservative: Stage 1 (top): Helicase enzyme unwinds the double helix at the replication fork. RNA primers initiate synthesis. Stage 2 (middle): DNA polymerase adds new nucleotides — Leading strand synthesised continuously (5'→3'); Lagging strand synthesised discontinuously as Okazaki fragments. Both strands grow in same direction from the fork. Stage 3 (bottom): Two identical DNA molecules formed — each has one original (teal) + one new (purple) strand = Semi-conservative replication.
📌 Semi-Conservative Replication (Meselson-Stahl, 1958): Each new DNA molecule retains one original parental strand and one newly synthesised complementary strand. Proved using heavy nitrogen (¹⁵N) vs light nitrogen (¹⁴N) density experiment with E. coli. Alternative models (conservative and dispersive) were ruled out.
1
Initiation — Origin of Replication: Replication begins at specific sites on the DNA called origins of replication. Bacteria have one origin (oriC); human cells have thousands. Initiator proteins recognise these sites and recruit the replication machinery.
2
Unwinding — Helicase: Helicase enzyme breaks the hydrogen bonds between base pairs and unwinds the double helix, creating a replication fork. Topoisomerase relieves the tension created by unwinding (prevents supercoiling). Single-strand binding (SSB) proteins stabilise the unwound strands.
3
Priming — RNA Primase: DNA polymerase cannot start a new chain from scratch — it can only extend existing strands. Primase synthesises short RNA primers (complementary to the template) to provide a starting point. These are later removed and replaced with DNA.
4
Elongation — DNA Polymerase: DNA polymerase III (in bacteria) adds new deoxyribonucleotides in the 5'→3' direction only. Leading strand: Synthesised continuously in the same direction as the fork. Lagging strand: Synthesised discontinuously as short Okazaki fragments (because it runs 3'→5', polymerase must work away from the fork in segments).
5
Primer Removal + Gap Filling: RNA primers are removed by RNase H / DNA Polymerase I (in bacteria). The gaps are filled with DNA nucleotides by DNA Pol I.
6
Ligation — DNA Ligase: DNA ligase seals the nicks between Okazaki fragments on the lagging strand, creating a continuous strand. Result: two identical double-stranded DNA molecules — each with one original + one new strand (semi-conservative).
Section 06 — Functions
⚙️ Functions of DNA — Protein Synthesis & Beyond
📌 The Central Dogma of Molecular Biology (Francis Crick, 1958): The flow of genetic information in a cell: DNA → RNA → Protein. DNA is transcribed to mRNA; mRNA is translated to protein. Retroviruses (HIV) violate this by using RNA → DNA (reverse transcriptase). This is the fundamental principle of all molecular biology.
📋 Step 1: Transcription (DNA → mRNA)
Location: Nucleus (in eukaryotes).
Enzyme: RNA polymerase.
Process: RNA polymerase binds to the promoter region of a gene on the DNA. It unwinds the double helix and reads the template strand (3'→5'). It synthesises a complementary messenger RNA (mRNA) molecule (5'→3') using the same base pairing rules — except U replaces T in RNA.
Product: Pre-mRNA (in eukaryotes) → processed by removing introns (non-coding) and retaining exons (coding) → mature mRNA.
Note: Coding strand = non-template strand (same sequence as mRNA except T→U). Template strand = the one read by RNA polymerase.
🔨 Step 2: Translation (mRNA → Protein)
Location: Cytoplasm (ribosomes).
Machinery: Ribosomes + tRNA (transfer RNA) + amino acids.
Process: mRNA carries the genetic code from nucleus to ribosomes. Each codon (3 consecutive bases on mRNA) codes for one amino acid. tRNA molecules carry the correct amino acids to the ribosome (tRNA has an anticodon complementary to the codon). Ribosome links amino acids into a polypeptide chain (protein).
Genetic Code: 64 codons total (4³) for 20 amino acids + 3 stop codons. Code is universal (same in bacteria and humans — basis for GM crops and gene therapy). Degenerate — multiple codons can code for same amino acid.
🗄️ Stores & Transmits Genetic Information
DNA is the chemical basis of heredity — the repository of genetic information transmitted from parents to offspring through reproduction.
Inheritance mechanism: During cell division (mitosis), DNA replication ensures each daughter cell receives an exact copy of the genome. During sexual reproduction (meiosis), each gamete receives half the parent's genome — offspring inherit a mix from both parents.
Mutations: Changes in DNA sequence (substitution, insertion, deletion) can alter protein function — basis of evolution, genetic diseases, and cancer. DNA repair mechanisms (mismatch repair, base excision repair, nucleotide excision repair) correct errors.
🔗 DNA Replication in Cell Cycle
DNA replication occurs during the S-phase (Synthesis phase) of the cell cycle — between G1 (growth) and G2 (preparation for division).
Why replication matters: If DNA is not replicated, daughter cells would have half the genetic information — cells would die. Errors in replication can cause mutations → cancer if in tumour suppressor genes (p53, BRCA1/2) or proto-oncogenes.
In vitro replication: PCR (Polymerase Chain Reaction) — amplifies specific DNA sequences in test tubes. Uses heat to denature DNA, primers, and heat-stable Taq polymerase. Foundation of DNA fingerprinting, genetic testing, COVID-19 RT-PCR tests.
Section 07 — Applied DNA
🔍 DNA Fingerprinting & Recombinant DNA
🔍 DNA Fingerprinting
Developed by: Sir Alec Jeffreys (UK, 1984). In India: Dr. Lalji Singh (CCMB, Hyderabad) — "Father of DNA Fingerprinting in India."
Basis: 99.9% of human DNA is identical across all people. The 0.1% that varies contains unique repetitive sequences called VNTRs (Variable Number of Tandem Repeats) or STRs (Short Tandem Repeats). These repeat regions differ in number between individuals, creating a unique DNA "fingerprint."
Method: Extract DNA → cut with restriction enzymes → separate by gel electrophoresis → pattern of bands is unique per individual (except identical twins).
Applications:
1. Forensics — criminal identification (Nirbhaya case, 2012)
2. Paternity/maternity testing
3. Identifying disaster victims
4. Wildlife conservation — identifying poached animals
5. Tracing ancestry and racial/ethnic groups
India institutions: CDFD (Centre for DNA Fingerprinting & Diagnostics, Hyderabad); CFSL (Central Forensic Science Lab); CCMB, Hyderabad.
Legal status: Criminal Procedure (Identification) Act, 2022 — authorises police to collect biological samples including DNA from convicts and arrested persons. DNA evidence accepted as gold standard in Indian courts.
🔧 Recombinant DNA (rDNA) Technology
Definition: DNA molecules created in the laboratory by combining genetic material from multiple sources — creating sequences not found naturally. Basis of modern biotechnology and genetic engineering.
Key tools:
Restriction enzymes ("molecular scissors"): Cut DNA at specific sequences. Example: EcoRI cuts at GAATTC.
Ligase: "Molecular glue" — joins cut DNA fragments.
Vectors: Carriers (plasmids, bacteriophages, AAV) that transport foreign DNA into host cells.
PCR: Amplifies target DNA sequences.
Process: Cut desired gene from donor DNA → Insert into vector → Introduce into host cell → Host cell expresses the foreign gene → Harvests the protein product.
Applications:
• Insulin production: Human insulin gene in bacteria (E. coli) — no longer extracted from pigs/cows
• Bt crops: Bacillus thuringiensis gene in cotton/brinjal
• Gene therapy: Correcting genetic defects by inserting normal genes
• Vaccines: Hepatitis B vaccine (HBsAg produced in yeast via rDNA)
• Golden Rice: β-carotene gene inserted for Vitamin A production
Section 08 — Comparison
⚖️ DNA vs RNA — Complete Comparison
| Property | 🔵 DNA | 🟢 RNA |
|---|
| Full name | Deoxyribonucleic Acid | Ribonucleic Acid |
| Sugar | Deoxyribose (5-carbon, missing OH at 2' position) | Ribose (5-carbon, has OH at 2' position) |
| Strands | Double-stranded (dsDNA) — normally | Single-stranded — normally |
| Structure | Double helix (Watson-Crick) | Single strand, can fold on itself (secondary structure) |
| Bases | A, T, G, C (Thymine — contains methyl group) | A, U, G, C (Uracil replaces Thymine) |
| Base pairing | A=T (2 H-bonds), G≡C (3 H-bonds) | A=U, G≡C (within folds) |
| Location | Nucleus (primarily), mitochondria | Nucleus, cytoplasm, ribosomes |
| Amount in cell | Constant (except sex cells) | Variable — changes with metabolic activity |
| Stability | Very stable — preserved millions of years (ancient DNA) | Less stable — easily degraded by RNase enzymes |
| Function | Genetic information storage; heredity; template for RNA synthesis | mRNA (carries genetic code to ribosome); tRNA (carries amino acids); rRNA (ribosome structural component) |
| Replication | Self-replicating (semi-conservative) | Cannot replicate by itself (made from DNA template) |
| Role in protein synthesis | Template for transcription (contains the gene) | mRNA = message; tRNA = adapter; rRNA = ribosome machinery |
| UV sensitivity | Absorbs UV at 260 nm (used to measure DNA concentration) | Also absorbs at 260 nm |
| Examples | Nuclear DNA (nDNA), mitochondrial DNA (mtDNA) | mRNA, tRNA, rRNA, snRNA, siRNA, miRNA |
| Special forms | A-DNA (right-handed), B-DNA (most common, right-handed), Z-DNA (left-handed) | siRNA (gene silencing), miRNA (gene regulation), ribozymes (catalytic RNA) |
Section 09 — Current Affairs
📰 Current Affairs 2024–2026 (Fact-Verified)
2024–2025 — 🇮🇳 INDIA
Genome India Project (GIP) — 10,000 Genomes Sequenced & Made Public
🧬 What:The Genome India Project (GIP) — India's flagship genomics initiative — completed whole-genome sequencing of 10,074 samples covering 99 ethnic groups of India. The data has been made publicly accessible — a major milestone for Indian genetics research.
📍 Where:Data archived at Indian Biological Data Centre (IBDC), Faridabad, Haryana — India's first national life science data repository. Supported by Department of Biotechnology (DBT). Over 19,000 blood samples collected (targeting 20,000). Phase 1: 5,750 samples analysed.
🔧 Framework:Framework for Exchange of Data (FeED) Protocols launched — under Biotech-PRIDE Guidelines — ensures responsible, high-quality, nation-specific data sharing. Genome India Data Conclave held to launch these protocols.
📚 UPSC angle:Genome India Project; DBT; IBDC Faridabad; whole genome sequencing; India's biodiversity in DNA; personalised medicine; ethical concerns (data privacy, scientific racism); Biotech-PRIDE Guidelines.
2024 — 🇮🇳 INDIA (CSIR)
FnCas9 — India's Breakthrough CRISPR Gene Editor (CSIR-IGIB)
🔬 What:Scientists at CSIR-IGIB (Institute of Genomics and Integrative Biology) and L.V. Prasad Eye Institute developed a precise genome-editing system using FnCas9 enzyme (from Francisella novicida). More precise and efficient than the widely used SpCas9 (from Streptococcus pyogenes) with fewer off-target DNA cuts. Patent filed.
🎯 Significance:CRISPR-Cas9 works by: guide RNA (gRNA) directs Cas9 enzyme to specific DNA sequence → Cas9 cuts the DNA → cell repair mechanisms modify the genome. FnCas9 offers higher fidelity (precision) without sacrificing efficiency — a key challenge in gene editing.
🇮🇳 Context:India's indigenous TnpB gene editing tool (developed 2025) offers a compact, IP-free alternative to CRISPR for plant genome editing. CRISPR tools are patented by foreign institutions — licensing fees are a barrier. India's ICAR negotiating with Broad Institute for fee waivers for small/marginal farmers.
📚 UPSC angle:CRISPR-Cas9; gene editing; FnCas9; CSIR-IGIB; intellectual property in biotechnology; GM crops and gene editing in India; ICMR guidelines (germline editing forbidden in India).
2022 — 🇮🇳 INDIA LAW
Criminal Procedure (Identification) Act, 2022 — DNA in Forensics
📜 Act:The Criminal Procedure (Identification) Act, 2022 replaced the Identification of Prisoners Act, 1920. It authorises police and prison authorities to collect biological samples (including DNA) from: (1) convicted persons, (2) arrested persons, (3) persons in custody under preventive detention.
🔬 DNA angle:DNA samples can now be legally collected and stored in a national database. DNA analysis accepted as "gold standard of forensic investigation" in Indian courts. Replaces the need for the earlier (proposed but withdrawn) DNA Technology Bill, 2019. DNA profiling: uses STR/VNTR analysis — unique to each individual (except identical twins).
⚖️ Controversy:Privacy concerns — mandatory DNA collection raised questions about bodily autonomy and data misuse. Critics argued it could violate Article 20(3) (right against self-incrimination). Debate ongoing on oversight mechanism.
📚 UPSC angle:DNA fingerprinting; forensic DNA databases; CDFD Hyderabad; CFSL; DNA Technology Bill 2019 (withdrawn); Criminal Procedure (Identification) Act 2022; privacy vs security; STR profiling.
Section 10 — PYQs & MCQs
📝 Previous Year Questions & Practice MCQs — Interactive
PYQ — Prelims 2015 Consider the following statements about DNA:
1. The strands of a DNA molecule are anti-parallel to each other.
2. In a DNA molecule, Adenine always pairs with Guanine.
3. The sugar in DNA is deoxyribose, while in RNA it is ribose.
4. Z-DNA has a left-handed helix structure.
a) 1, 2 and 3 only
b) 1, 3 and 4 only
c) 2, 3 and 4 only
d) 1, 2, 3 and 4
Statement 1 ✓ — DNA strands are anti-parallel: one runs 5'→3' and the other 3'→5'. This is essential for base pairing to work and for the complementary arrangement of the double helix. Statement 2 ✗ — Classic trap: Adenine does NOT pair with Guanine. Both are purines — purine-purine pairing would make the helix too wide. Correct pairing: A pairs with T (2 hydrogen bonds) and G pairs with C (3 hydrogen bonds). This is Chargaff's Rule — purines always pair with pyrimidines. Statement 3 ✓ — In DNA, the sugar is deoxyribose (missing the 2'-OH group — "deoxy" = without oxygen). In RNA, the sugar is ribose (has 2'-OH). This structural difference makes DNA more stable (OH group in RNA makes it susceptible to hydrolysis). Statement 4 ✓ — Z-DNA has a left-handed helical structure — its backbone forms a zigzag pattern (hence "Z"). A-DNA and B-DNA are both right-handed. B-DNA is the most common form found in cells. Answer: (b).
PYQ — Prelims 2019 Which of the following is the most appropriate description of DNA fingerprinting?
a) It involves analysis of the entire human genome sequence
b) It involves identification of blood group antigens
c) It involves analysis of specific repetitive DNA sequences (VNTRs/STRs) that vary between individuals
d) It involves analysis of mitochondrial DNA only, since it is maternally inherited
DNA fingerprinting is based on the fact that 99.9% of human DNA is identical — only the remaining 0.1% varies between individuals. This variable region contains repetitive sequences called VNTRs (Variable Number of Tandem Repeats) or STRs (Short Tandem Repeats) — the number of times a short sequence repeats differs from person to person, creating a unique "barcode." Option (a) WRONG — analysing the entire genome is called whole genome sequencing (3 billion base pairs) — DNA fingerprinting analyses only specific loci (10–15 STR locations). Option (b) WRONG — blood group antigens are proteins, not DNA. Option (d) WRONG — mtDNA IS used in some special cases (e.g., when only hair is available, or for maternal lineage tracing) but standard DNA fingerprinting uses nuclear STR analysis. mtDNA-only analysis is less discriminating (shared by all maternal relatives). Developed by Alec Jeffreys (UK, 1984). India: Dr. Lalji Singh (CCMB Hyderabad). Legal in India under Criminal Procedure (Identification) Act, 2022. Answer: (c).
Q1 Chargaff's Rules state that in DNA:
1. [A] = [T] and [G] = [C]
2. The total amount of purines equals the total amount of pyrimidines
3. A always pairs with G and T always pairs with C
4. If a DNA strand has 30% adenine, the complementary strand has 30% thymine
a) 1, 2 and 3 only
b) 1, 2 and 4 only
c) 2, 3 and 4 only
d) 1, 2, 3 and 4
Statement 1 ✓ — Chargaff's fundamental rule: [Adenine] = [Thymine] and [Guanine] = [Cytosine] in any double-stranded DNA. This is because they always pair with each other. Statement 2 ✓ — Since A pairs with T (both counted once each) and G pairs with C (both counted once each), total Purines (A + G) = total Pyrimidines (T + C). This ratio = 1 in any organism's dsDNA. Statement 3 ✗ — Trap: A does NOT pair with G. Both are purines — they don't pair together. The correct pairing: A=T and G=C (purine always pairs with pyrimidine). A pairing with G would be purine-purine — geometrically impossible in the double helix. Statement 4 ✓ — If one strand has 30% A, the complementary strand has 30% T (because every A on one strand pairs with a T on the other). Similarly: if strand 1 has 30% A, 20% T, 25% G, 25% C, then strand 2 has 30% T, 20% A, 25% C, 25% G. The TOTAL DNA still obeys: [A]total = [T]total and [G]total = [C]total. Answer: (b).
Q2 Which of the following statements about DNA replication is/are correct?
1. DNA replication is semi-conservative — each new molecule has one parental + one new strand.
2. Helicase breaks the covalent phosphodiester bonds between base pairs to unwind DNA.
3. DNA polymerase can only synthesise DNA in the 5'→3' direction.
4. The lagging strand is synthesised discontinuously as Okazaki fragments.
a) 1, 2 and 3 only
b) 1, 3 and 4 only
c) 2, 3 and 4 only
d) 1, 2, 3 and 4
Statement 1 ✓ — Semi-conservative replication confirmed by Meselson-Stahl experiment (1958) using ¹⁵N/¹⁴N density labelling in E. coli. Each daughter molecule contains one original template strand + one newly synthesised complementary strand. Statement 2 ✗ — Trap: Helicase breaks the hydrogen bonds between complementary base pairs (A=T and G≡C) — NOT the covalent phosphodiester bonds of the backbone. Covalent bonds are much stronger — breaking them would destroy the DNA. Hydrogen bonds are weak non-covalent bonds that can be broken and reformed. Topoisomerase (not helicase) relieves the torsional stress in the backbone. Statement 3 ✓ — DNA polymerase can only add nucleotides to the 3'-OH end of the growing strand, meaning synthesis proceeds 5'→3'. This constraint is why the lagging strand must be synthesised in fragments. Statement 4 ✓ — The lagging strand runs 3'→5' relative to the fork direction. Since polymerase can only work 5'→3', it must synthesise short segments (Okazaki fragments, 100–2000 nucleotides in eukaryotes) moving away from the fork, then jump back. DNA ligase joins these fragments. Answer: (b).
Q3 With reference to mitochondrial DNA (mtDNA), which of the following is correct?
a) mtDNA is inherited equally from both parents, like nuclear DNA
b) mtDNA is linear and double-stranded, like nuclear DNA
c) mtDNA is circular, maternally inherited, and is used to trace maternal lineages
d) mtDNA contains the same number of base pairs as nuclear DNA (~3 billion)
Option (c) is correct. Mitochondrial DNA (mtDNA) has three key distinguishing features: (1) Circular structure — unlike the linear chromosomal DNA in the nucleus, mtDNA is a circular molecule (16,569 base pairs). This resembles bacterial DNA — supporting the endosymbiotic theory (mitochondria evolved from ancient bacteria engulfed by eukaryotic cells). (2) Maternal inheritance — mtDNA is inherited ONLY from the mother. During fertilisation, sperm contributes its nuclear DNA but its mitochondria are destroyed by the egg cell. Therefore, all maternal relatives (mother, siblings, maternal grandparents) share identical (or nearly identical) mtDNA. (3) Tracing maternal lineage — used in forensics (identifying disaster victims from maternal relatives' DNA), archaeology (tracing ancient human migrations — "Mitochondrial Eve"), wildlife conservation. Option (a) WRONG — father contributes NO mtDNA to offspring. Option (b) WRONG — mtDNA is circular (not linear). Option (d) WRONG — mtDNA is tiny: ~16,569 bp, compared to ~3 billion bp for nuclear DNA. Answer: (c).
Section 11
🧠 Memory Aid — Lock These In
🔑 DNA — All Critical Facts for UPSC
STRUCTURE
Double helix (B-DNA most common, right-handed). Two anti-parallel strands. Sugar-phosphate backbone (outside). Base pairs inside: A=T (2 H-bonds), G≡C (3 H-bonds). 10 bp per turn, 3.4 nm pitch. Major groove (wider) + Minor groove. Nucleotide = Base + Deoxyribose + Phosphate.
BASES
Purines (double ring): A and G. Pyrimidines (single ring): C, T (DNA), U (RNA only — no Thymine in RNA). Chargaff's Rules: A=T, G≡C → [A]=[T], [G]=[C] → Purines = Pyrimidines = 1. G≡C has 3 H-bonds (stronger, harder to denature). A=T has 2 H-bonds (weaker).
DNA TYPES
B-DNA: right-handed, most common, 10 bp/turn. A-DNA: right-handed, wider, dehydrated conditions. Z-DNA: LEFT-handed (unique), zigzag backbone, found in bacteria/eukaryotes/viruses. TRAP: Only Z-DNA is left-handed — A and B are both right-handed.
LOCATION
Nuclear DNA (nDNA): nucleus, 23 pairs of chromosomes, 3 billion bp. Mitochondrial DNA (mtDNA): circular, 16,569 bp, maternally inherited ONLY, codes for 13 proteins + 22 tRNAs + 2 rRNAs. TRAP: Father contributes ZERO mtDNA. mtDNA = circular (like bacteria — endosymbiotic theory).
REPLICATION
Semi-conservative (Meselson-Stahl 1958). Helicase unwinds (breaks H-bonds, NOT phosphodiester bonds). Primase adds RNA primer. DNA polymerase extends 5'→3' ONLY. Leading strand: continuous. Lagging strand: Okazaki fragments (discontinuous). Ligase joins fragments. In vitro: PCR (Taq polymerase).
CENTRAL DOGMA
DNA → (Transcription) → mRNA → (Translation) → Protein. RNA polymerase transcribes. Ribosomes + tRNA translate. Codon = 3 bases = 1 amino acid. 64 codons, 20 amino acids (degenerate code). Universal code (same in bacteria and humans). HIV breaks dogma: RNA → DNA (reverse transcriptase).
DNA FINGERPRINT
Alec Jeffreys (UK, 1984). India: Dr. Lalji Singh (CCMB Hyderabad). Based on VNTR/STR repeats (0.1% variable DNA). NOT whole genome analysis. mtDNA used for maternal lineage only. Indian institutions: CDFD Hyderabad, CFSL. Legal: Criminal Procedure (Identification) Act 2022. Used in: Nirbhaya, Tsunamis, paternity, wildlife poaching.
NUCLEOTIDE
Nucleoside = Base + Sugar (NO phosphate). Nucleotide = Base + Sugar + Phosphate. DNA sugar = Deoxyribose. RNA sugar = Ribose. Nucleic acids (DNA/RNA) = polymers of nucleotides joined by phosphodiester bonds. Heterocyclic compound = ring with 2+ different element types (N + C).
CURRENT AFFS
Genome India Project: 10,074 genomes (99 ethnic groups), IBDC Faridabad, DBT, data public 2025. FnCas9 CRISPR: CSIR-IGIB + LV Prasad Eye Institute, 2024, more precise than SpCas9. TnpB gene editing tool (India 2025): IP-free CRISPR alternative for crops. Criminal Procedure (Identification) Act 2022: DNA collection from arrestees authorised.
TRAPS 🪤
• Z-DNA = LEFT-handed (not A or B). • A pairs with T (NOT G). • mtDNA from mother ONLY (not both parents). • mtDNA = circular (not linear). • mtDNA ≠ 3 billion bp (only 16,569 bp). • DNA fingerprinting ≠ whole genome sequencing. • Helicase breaks H-bonds (NOT covalent phosphodiester bonds). • Thymine in DNA; Uracil in RNA (no thymine in RNA). • G≡C = 3 H-bonds (not 2).
Section 12
❓ FAQs — Concept Clarity
What is the difference between a gene, a chromosome, and the genome?
These three terms represent different levels of organisation of genetic information: Gene: A specific sequence of DNA that encodes the instructions to make a functional RNA molecule (often, but not always, a protein). The average human gene is about 27,000 base pairs long. Humans have approximately 20,000–25,000 protein-coding genes — but these represent only about 2% of the total genome. The rest is non-coding DNA (formerly called "junk DNA" — now known to have regulatory, structural, and other functions). Chromosome: A long, coiled DNA molecule associated with protein (histones). Each chromosome contains many genes arranged linearly. Humans have 23 pairs of chromosomes (46 total) — 22 autosomes + 1 pair of sex chromosomes (XX in females, XY in males). During cell division, chromosomes condense and become visible under a microscope. Each chromosome's DNA, if stretched out, would be several centimetres long — packed into a 6 μm nucleus through histone wrapping → nucleosome → chromatin → chromosome. Genome: The complete set of genetic material (DNA) in an organism. The human genome = all 46 chromosomes + mtDNA = approximately 3 billion base pairs of nuclear DNA. The Human Genome Project (completed 2003) sequenced the entire human genome. India's Genome India Project is building an Indian-specific reference genome database covering India's unique genetic diversity (99 ethnic groups sequenced).
What is CRISPR-Cas9 and how does it edit DNA?
CRISPR (Clustered Regularly Interspaced Short Palindromic Repeats) was originally discovered as the immune system of bacteria — bacteria store snippets of viral DNA between CRISPR repeats to recognise and destroy future viral attacks. Scientists (Jennifer Doudna + Emmanuelle Charpentier, Nobel Prize 2020) adapted this into a powerful gene editing tool. How it works: (1) Design a guide RNA (gRNA) — a short RNA sequence complementary to the target DNA location. (2) gRNA binds to the Cas9 enzyme (molecular scissors). (3) The gRNA-Cas9 complex scans the DNA until it finds the matching sequence. (4) Cas9 makes a double-strand break in the DNA at that location. (5) The cell's repair machinery fixes the break — either inaccurately (deleting or inserting bases = gene knockout) or using a provided template (gene correction). Applications: Treating genetic diseases (sickle cell disease — first CRISPR therapy Casgevy approved 2023); cancer immunotherapy; GM crops; diagnostic tools. India context: ICMR guidelines — germline editing (changing sperm/egg/embryo DNA) is FORBIDDEN in India. CSIR-IGIB developed FnCas9 (2024) — more precise variant. India's indigenous TnpB tool (2025) — IP-free CRISPR alternative for plant genome editing, avoiding foreign patent licensing fees. UPSC angle: CRISPR is central to Gene Technology, biotechnology, and bioethics — all in GS-III syllabus.
What is recombinant DNA and how has it transformed India's agriculture and medicine?
Recombinant DNA (rDNA) is DNA created by combining genetic material from different organisms — creating gene sequences that would not occur naturally. It is the foundation of modern genetic engineering. Key tools: restriction enzymes (cut DNA at specific sequences), ligase (join fragments), vectors (carriers like plasmids), and PCR. In Indian agriculture: Bt cotton — the Cry gene from Bacillus thuringiensis soil bacteria, which produces insecticidal proteins, was inserted into cotton plants' DNA using rDNA technology. India approved Bt cotton in 2002 — it now accounts for over 90% of India's cotton area. Bt brinjal was approved for field trials but remains commercially blocked due to a moratorium (2010). India's ICAR developing multiple GM crops using rDNA technology — drought-resistant wheat, vitamin-A-enriched rice (Golden Rice), pest-resistant tomatoes. In Indian medicine: Insulin produced by inserting the human insulin gene into bacteria (E. coli) — manufactured commercially, replacing pig/cow insulin extracts. Hepatitis B vaccine produced by inserting the HBsAg gene into yeast cells — mass-produced and part of India's Universal Immunisation Programme (UIP). Erythropoietin (EPO) — produced via rDNA for treating anaemia. Human growth hormone — recombinant version for treating growth disorders. India's DBT (Department of Biotechnology) and institutions like CSIR, DBT-BIRAC support rDNA research. India is the world's largest producer of generic biologics (protein drugs made via rDNA technology) — exported to 200+ countries.
Section 13
🏁 Conclusion — UPSC Synthesis
🧬 From a Twisted Ladder to the Blueprint of Life
When Watson and Crick described the double helix in a single page in Nature in 1953 — using Rosalind Franklin's X-ray data without full acknowledgement — they handed humanity the key to understanding life itself. Every organism on Earth — from a bacterium to a blue whale, from rice in India's paddy fields to the Asiatic lion in Gir — runs on the same four-letter code (A, T, G, C). That code, written in 3 billion letters and coiled into 46 chromosomes inside a nucleus smaller than a grain of sand, determines eye colour, disease risk, and the shape of every protein our bodies make.
India is now writing its own chapter in this story. The Genome India Project is decoding the genetic diversity of India's 1.4 billion people across 99 ethnic groups — building a reference database at IBDC Faridabad that will power personalised medicine for Indians, who are severely underrepresented in global genetic databases. CSIR-IGIB's FnCas9 CRISPR tool brings India closer to designing its own gene therapies. And the Criminal Procedure (Identification) Act, 2022 has put DNA fingerprinting at the centre of India's forensic justice system.
📋 For UPSC Prelims — Key Facts
🧬 Structure:Double helix (1953) — Watson, Crick, Franklin, Wilkins
🍬 Sugar:Deoxyribose (DNA) vs Ribose (RNA)
🔗 Base pairs:A=T (2 H-bonds) | G≡C (3 H-bonds) — GC stronger
🔵 DNA types:B-DNA = right-handed (most common) | Z-DNA = LEFT-handed only
🔋 mtDNA:Circular | 16,569 bp | Maternally inherited ONLY
🔄 Replication:Semi-conservative (Meselson-Stahl 1958)
🪤 Traps:Helicase breaks H-bonds (NOT phosphodiester) | Okazaki fragments = lagging strand
🧬 Building blocks:Nucleoside = base+sugar | Nucleotide = base+sugar+phosphate
🔍 Fingerprinting:VNTR/STR analysis — NOT whole genome sequencing
📖 Central Dogma:DNA → RNA → Protein
🇮🇳 Current affs:Genome India: IBDC Faridabad, DBT, 10,074 genomes, 99 ethnic groups | Criminal Procedure (Identification) Act 2022
📝 For UPSC Mains GS-III — Topics
🧬 Structure:DNA structure & significance for inheritance; chromosome packaging (histones, nucleosomes)
⚙️ Protein synthesis:Transcription + Translation; genetic code (universal, degenerate); Central Dogma
🔄 Replication:Semi-conservative; role in cell cycle; DNA repair mechanisms; cancer mutations
🔧 rDNA technology:Bt cotton, insulin, Hep-B vaccine, gene therapy — India's biotech applications
🔍 Forensic DNA:DNA fingerprinting in Indian law; CDFD Hyderabad; Criminal Procedure Act 2022; privacy concerns
✂️ Gene editing:CRISPR-Cas9; FnCas9 (CSIR-IGIB); TnpB (India, IP-free); ICMR germline editing ban; patent barriers for farmers
🇮🇳 Genome India:Personalised medicine; genetic privacy; IBDC Faridabad; FeED protocols; Biotech-PRIDE
🔋 mtDNA:Maternal inheritance; forensics; endosymbiotic theory (circular like bacteria)
