Overview: Why Molecular Biology Matters for NEET
Molecular biology contributes 5–7 questions in NEET. DNA structure, replication, transcription, translation, and the genetic code are high-priority topics. The lac operon is frequently tested as a 1-mark or short-answer question.
1. Nucleic Acid Structure
Nucleotides — The Building Blocks
Every nucleotide consists of three parts: 1. Phosphate group — negatively charged, forms the backbone 2. Pentose sugar — deoxyribose in DNA, ribose in RNA 3. Nitrogenous base — carries genetic information
Bases: Purines vs Pyrimidines
| Type | Bases | Memory Aid |
|---|---|---|
| Purines (double ring) | Adenine (A), Guanine (G) | PURe As Gold |
| Pyrimidines (single ring) | Cytosine (C), Thymine (T) in DNA; Uracil (U) in RNA | CUT the Pye |
DNA vs RNA — Key Differences
| Feature | DNA | RNA |
|---|---|---|
| Sugar | Deoxyribose | Ribose |
| Bases | A, T, G, C | A, U, G, C |
| Strands | Double-stranded | Usually single-stranded |
| Location | Nucleus (mainly) | Nucleus and cytoplasm |
| Stability | Very stable | Less stable |
| Function | Genetic information storage | Protein synthesis intermediary |
2. Watson-Crick Double Helix Model (1953)
James Watson and Francis Crick proposed the B-form double helix based on:
- Chargaff's rules (A=T, G=C)
- X-ray crystallography data from Rosalind Franklin and Maurice Wilkins
Key Features of the Double Helix
1. Two antiparallel strands — one runs 5'→3', the other 3'→5' 2. Right-handed helix — coils clockwise when viewed from top 3. Base pairing rules: A=T (2 hydrogen bonds), G≡C (3 hydrogen bonds) 4. Sugar-phosphate backbone on outside; bases stacked inside 5. Helix diameter: 2 nm; pitch (one full turn) = 3.4 nm; 10 base pairs per turn
Chargaff's Rules (Must Memorise)
For any double-stranded DNA:
- A = T (always)
- G = C (always)
- A + G = T + C (total purines = total pyrimidines)
- % A + % T + % G + % C = 100%
Exam application: If A = 30%, then T = 30%, leaving G + C = 40%, so G = C = 20%.
3. DNA Replication
Semiconservative Model
Each new double helix has one old strand (template) + one new strand.
Meselson-Stahl Experiment (1958)
- Grew E. coli in ¹⁵N (heavy nitrogen) medium — all DNA becomes heavy
- Transferred to ¹⁴N (light nitrogen) medium
- After 1 generation: all DNA = hybrid density (one heavy, one light strand)
- After 2 generations: 50% hybrid + 50% light DNA
Only semiconservative replication predicts this pattern — proved definitively.
Key Enzymes in DNA Replication
| Enzyme | Function |
|---|---|
| Helicase | Unwinds double helix at replication fork |
| SSB proteins | Keep strands separated |
| Primase | Synthesises short RNA primer (needed to start DNA synthesis) |
| DNA Polymerase III | Main synthesis enzyme; adds nucleotides 5'→3' only |
| DNA Polymerase I | Removes RNA primers and fills gaps |
| DNA Ligase | Joins Okazaki fragments and seals nicks |
| Topoisomerase | Relieves torsional stress ahead of replication fork |
Leading vs Lagging Strand
- Leading strand: Synthesised continuously toward the fork (5'→3')
- Lagging strand: Synthesised away from fork in short segments (Okazaki fragments, ~1000–2000 nucleotides in bacteria)
- DNA ligase joins Okazaki fragments after RNA primers are replaced
4. Transcription (DNA → RNA)
Key Terminology
- Template strand (antisense strand): DNA strand used as template; read 3'→5' by RNA polymerase
- Coding strand (sense strand): DNA strand with same sequence as the mRNA (but with T instead of U)
- mRNA: Messenger RNA — carries code from nucleus to ribosomes
Steps of Transcription
1. Initiation: RNA polymerase binds to promoter sequence (TATA box in eukaryotes) upstream of gene
2. Elongation: RNA polymerase reads template 3'→5', synthesises mRNA 5'→3' using complementary base pairing (A→U, T→A, G→C, C→G)
3. Termination: RNA polymerase reaches terminator sequence → transcription stops → mRNA released
Post-transcriptional Modifications in Eukaryotes
| Modification | What happens | Purpose |
|---|---|---|
| 5' capping | 7-methylguanosine added to 5' end | Protects mRNA from degradation; needed for ribosome binding |
| Poly-A tail | 150–200 adenine nucleotides added to 3' end | Stability and export from nucleus |
| RNA splicing | Introns removed; exons joined | Only exons code for protein |
Introns = intervening sequences (non-coding, removed) Exons = expressed sequences (coding, retained)
Splicing is carried out by spliceosome (RNA + protein complex).
Types of RNA
| RNA Type | Function | Structure |
|---|---|---|
| mRNA | Carries genetic code from DNA to ribosome | Linear |
| tRNA | Carries amino acids to ribosome | Cloverleaf (L-shape in 3D) |
| rRNA | Structural and catalytic component of ribosome | Forms ribozyme |
tRNA structure: Cloverleaf with 4 arms:
- Anticodon loop: recognises mRNA codon (complementary, antiparallel)
- Acceptor stem (3'-CCA end): amino acid attaches here
5. Translation (mRNA → Protein)
Ribosome Structure
| Component | Prokaryotes | Eukaryotes |
|---|---|---|
| Small subunit | 30S | 40S |
| Large subunit | 50S | 60S |
| Complete ribosome | 70S | 80S |
The large subunit has three sites: A (aminoacyl), P (peptidyl), E (exit)
Genetic Code
- Triplet: Three nucleotides = one codon
- 64 codons total: 61 sense codons (specify amino acids) + 3 stop codons
- Start codon: AUG → methionine (first amino acid in all proteins)
- Stop codons: UAA, UAG, UGA ("UAA = U Are All done")
- Degenerate: Multiple codons → same amino acid (e.g., GGU, GGC, GGA, GGG all = glycine)
- Unambiguous: Each codon → only one amino acid
- Universal: Same code in nearly all organisms (minor exceptions in mitochondria)
- Non-overlapping: Each nucleotide is part of only one codon
Steps of Translation
6. Gene Regulation — Lac Operon
Background
Bacteria control gene expression based on environmental conditions. The lac operon is the model for negative inducible regulation in E. coli.
Components of the Lac Operon
When Lactose is ABSENT (genes OFF)
1. Regulator gene constitutively expressed → repressor protein synthesised 2. Repressor binds to operator → blocks RNA polymerase from reaching structural genes 3. lacZ, lacY, lacA are NOT transcribed → no enzyme made
When Lactose is PRESENT (genes ON)
1. Lactose enters cell → converted to allolactose (the inducer) 2. Allolactose binds to repressor → changes repressor shape → repressor can no longer bind operator 3. RNA polymerase reads through operator → lacZ, lacY, lacA transcribed 4. Enzymes made → lactose digested 5. When lactose is used up → allolactose disappears → repressor active again → genes turn off
Why this is "negative inducible":
- Negative: repressor normally turns genes OFF
- Inducible: inducer (allolactose) turns genes ON
7. Human Genome Project — Key Facts
| Feature | Data |
|---|---|
| Total base pairs | ~3 billion |
| Protein-coding genes | ~20,000–25,000 |
| Non-coding DNA | ~98% of genome |
| Completed | 2003 (draft 2001) |
| Technology used | Automated DNA sequencing, bioinformatics |
Significance:
- Identifies genes linked to diseases (e.g., BRCA1/BRCA2 for breast cancer)
- Enables pharmacogenomics — personalised medicine
- Forensic identification using DNA fingerprinting
- Evolutionary studies across species
Board Examination Questions
1-Mark Questions
Q1: Name the enzyme that seals nicks in DNA and joins Okazaki fragments. DNA Ligase
Q2: What is the anticodon of tRNA that recognises the codon AUG? The anticodon is UAC (antiparallel and complementary: A pairs with U, U pairs with A, G pairs with C).
Q3: Which strand of DNA serves as the template for transcription? The template strand (also called antisense strand or non-coding strand), read in the 3'→5' direction.
Q4: What are introns? Introns are intervening, non-coding sequences within eukaryotic genes that are transcribed into pre-mRNA but are removed by RNA splicing before translation. Only exons are retained in the mature mRNA.
Q5: Name two stop codons. UAA and UAG (also UGA — any two are accepted).
2-Mark Questions
5-Mark Questions
NEET Focus Points
1. Chargaff's rules: If %A is given, immediately find %T (equal), then split remainder equally between G and C 2. Replication enzymes: Helicase unwinds, primase makes primer, DNA Pol III synthesises, DNA Pol I removes primer, Ligase joins fragments 3. Codons: AUG = start/methionine; UAA, UAG, UGA = stop. Know these by heart 4. Lac operon: Allolactose is the inducer (not lactose directly); repressor binds operator; negative regulation 5. tRNA anticodon: Complement of codon and antiparallel — practice working these out