Chapter Overview & Weightage
Molecular Basis of Inheritance covers DNA structure, replication, transcription, translation, the genetic code, lac operon, DNA fingerprinting, and the Human Genome Project. This chapter bridges genetics and biochemistry — conceptual depth matters here.
This chapter carries 5-7% weightage in NEET with 4-5 questions. DNA replication, transcription-translation machinery, and the genetic code properties are the highest-yield topics.
Key Concepts You Must Know
Tier 1 (Core)
- DNA structure: double helix, antiparallel strands, complementary base pairing (A-T with 2 H-bonds, G-C with 3 H-bonds), sugar-phosphate backbone
- DNA replication: semi-conservative (Meselson-Stahl experiment), requires helicase, primase, DNA polymerase III, ligase
- Transcription: DNA → mRNA, needs RNA polymerase, promoter, terminator; occurs in nucleus
- Translation: mRNA → protein, requires ribosomes, tRNA, amino acids; occurs in cytoplasm
- Central dogma: DNA → RNA → Protein
Tier 2 (Frequently tested)
- Genetic code: triplet, degenerate (64 codons for 20 amino acids), universal, non-overlapping, comma-less
- Start codon: AUG (methionine); Stop codons: UAA, UAG, UGA
- Lac operon: structural genes (z, y, a), operator, promoter, regulatory gene (i), inducer (lactose/allolactose)
- DNA packaging: nucleosome (DNA + histone octamer), 30 nm fibre, chromatin loops, chromosome
Tier 3 (Occasionally tested)
- DNA fingerprinting: VNTR/STR analysis, Southern blotting
- Human Genome Project: 3.2 billion bp, ~20,000-25,000 genes
- RNA types: mRNA (messenger), tRNA (transfer, cloverleaf), rRNA (ribosomal)
- Reverse transcription: RNA → DNA (by reverse transcriptase in retroviruses)
Important Formulas
In double-stranded DNA:
- and (complementary base pairing)
- (purines = pyrimidines)
- ratio is species-specific
In single-stranded DNA or RNA, Chargaff’s rules do NOT apply.
| Feature | Replication | Transcription | Translation |
|---|---|---|---|
| Template | Both DNA strands | One DNA strand (template/antisense) | mRNA |
| Product | New DNA | mRNA (or tRNA, rRNA) | Protein |
| Enzyme | DNA polymerase III | RNA polymerase | Ribosome |
| Direction of synthesis | 5’ → 3’ | 5’ → 3’ | N-terminal → C-terminal |
| Location | Nucleus | Nucleus | Cytoplasm (ribosomes) |
| Condition | Repressor | Operator | Structural Genes |
|---|---|---|---|
| Lactose absent | Active (binds operator) | Blocked | OFF (not transcribed) |
| Lactose present | Inactive (lactose binds repressor) | Free | ON (transcribed) |
The lac operon is an example of negative regulation — the default state is OFF, and the inducer (lactose) turns it ON by inactivating the repressor.
For NEET, know the three stop codons by heart: UAA, UAG, UGA. Mnemonic: U Are Awesome, U Are Great, U Go Away (stop!). Also remember: AUG is both the start codon AND codes for methionine.
Solved Previous Year Questions
PYQ 1 — NEET 2024
Problem: Meselson and Stahl’s experiment proved that DNA replication is:
(A) Conservative (B) Semi-conservative (C) Dispersive (D) Non-conservative
Solution:
Using (heavy) and (light) isotopes of nitrogen, Meselson and Stahl showed that after one round of replication in medium, all DNA was of intermediate density (hybrid). After two rounds, half was intermediate and half was light. This pattern is consistent only with semi-conservative replication — each new DNA molecule has one old strand and one new strand.
Answer: (B) Semi-conservative
PYQ 2 — NEET 2023
Problem: How many hydrogen bonds are present between adenine and thymine?
(A) 1 (B) 2 (C) 3 (D) 4
Solution:
Adenine and thymine form 2 hydrogen bonds. Guanine and cytosine form 3 hydrogen bonds. This is why G-C rich DNA is more stable (higher melting temperature) than A-T rich DNA.
Answer: (B) 2
PYQ 3 — NEET 2022
Problem: In the lac operon, the
inducer molecule is:
(A) Glucose (B) Galactose (C) Lactose (allolactose) (D) Repressor protein
Solution:
The inducer in the lac operon is lactose (specifically, its isomer allolactose). When lactose is present, allolactose binds to the repressor protein, changing its shape so it can no longer bind to the operator. This frees the operator, allowing RNA polymerase to transcribe the structural genes.
Answer: (C) Lactose (allolactose)
Difficulty Distribution
| Difficulty | % of Questions | What to Expect |
|---|---|---|
| Easy | 35% | Base pairing rules, codon identification, H-bond count |
| Medium | 45% | Replication steps, transcription vs translation, lac operon mechanism |
| Hard | 20% | Meselson-Stahl analysis, DNA packaging levels, DNA fingerprinting |
Expert Strategy
Days 1-2: DNA structure and replication. Understand the semi-conservative model, know the enzymes (helicase unwinds, primase makes primer, DNA pol III synthesises, ligase joins Okazaki fragments). The leading vs lagging strand concept is frequently tested.
Days 3-4: Transcription and translation. Trace the flow: DNA → mRNA → protein. Know the roles of each RNA type. For translation, understand codon-anticodon pairing and the ribosomal A, P, E sites.
Day 5: Lac operon, genetic code properties, and DNA fingerprinting. The lac operon diagram from NCERT is directly used in NEET — learn it by heart.
A common NEET question type: “If the template strand sequence is 3’-ATGC-5’, what is the mRNA sequence?” Answer: 5’-UACG-3’ (complementary, with U replacing T). Always check strand polarity — template strand is read 3’ to 5’, mRNA is synthesised 5’ to 3’.
Common Traps
Trap 1 — Template strand vs coding strand. The template (antisense) strand is read 3’→5’ by RNA polymerase. The coding (sense) strand has the same sequence as mRNA (except T instead of U). NEET questions can give either strand — identify which one before answering.
Trap 2 — A-T has 2 H-bonds, G-C has 3. Students sometimes swap these. More G-C content = more H-bonds = higher melting temperature = more stable DNA. This is directly asked.
Trap 3 — Chargaff’s rules apply only to double-stranded DNA. In single-stranded DNA, RNA, or within one strand of dsDNA, A does NOT equal T. This exception is tested when NEET gives a single-strand composition and asks you to apply Chargaff’s rules.
Trap 4 — The genetic code is degenerate, NOT ambiguous. Degenerate = multiple codons for the same amino acid (e.g., leucine has 6 codons). Ambiguous would mean one codon codes for multiple amino acids — this is NOT the case. NEET tests this terminology distinction.