Is it correct to say: Confusing DNA (double-stranded) with RNA (single-stranded)?

DNA usually ds with 2-deoxyribose and T. RNA usually ss with ribose and U.

Is it correct to say: Writing DNA replication as conservative?

Semi-conservative — each daughter has 1 parental + 1 new strand. Meselson–Stahl proved it.

Is it correct to say: Forgetting stop codons in translation questions?

UAA, UAG, UGA terminate translation. They do NOT code for any amino acid.

Is it correct to say: Assuming genetic code is universal without exceptions?

Mostly universal but mitochondrial code shows minor deviations. State 'nearly universal'.

Is it correct to say: Confusing codon (mRNA) with anticodon (tRNA)?

Codon is on mRNA; anticodon is on tRNA, base-complementary to codon.

Is it correct to say: Writing DNA polymerase can synthesize 3' to 5'?

DNA polymerase reads 3' to 5' but SYNTHESIZES 5' to 3'. Only one direction.

Molecular Basis of Inheritance

DNA structure, replication, transcription, translation, gene expression — Maharashtra HSC Biology Ch 5

📐 11 formulas✏️ 3 examples🎯 5 practice⚖️ 5 marks🏫 Maharashtra State Board📚 Class 12✓ 2025–26 syllabus

💡

Board Exam Tips

→Watson–Crick model of DNA with labelled diagram (2 nm width, 3.4 nm/turn, 10 bp/turn) is a Maharashtra HSC 3-marker every year.
→Semi-conservative replication (Meselson–Stahl experiment) — diagram and interpretation for 3 marks.
→Transcription in prokaryotes vs eukaryotes — Balbharati asks the comparison in tabular form.
→Translation: initiation, elongation, termination stages with diagram of ribosome and tRNA.
→Lac operon with regulatory genes — Maharashtra HSC 4-marker classic.

📐 Formulas(11)

Chargaff's Rule★ Board fav

% A = % T, % G = % C, A + G = T + C

Watson–Crick Structure Parameters★ Board fav

Diameter = 2 nm; Rise/bp = 0.34 nm; Pitch = 3.4 nm; 10 bp/turn

Base Pairing★ Board fav

A = T (2 H-bonds), G \equiv C (3 H-bonds)

DNA Length Calculation

Length = No. of bp \times 0.34 nm

Semi-conservative Replication★ Board fav

Each daughter DNA = 1 parental strand + 1 new strand

Central Dogma★ Board fav

DNA replication DNA transcription RNA translation Protein

Codon and Anticodon

Codon: mRNA triplet; Anticodon: tRNA triplet complementary to codon

Genetic Code Properties★ Board fav

Triplet, degenerate, universal, non-overlapping, comma-less

Lac Operon Components★ Board fav

i \to repressor, P, O \to control, z, y, a \to structural genes

Start and Stop Codons

Start: AUG (Met); Stop: UAA, UAG, UGA

Enzymes in Replication

Helicase, Primase, DNA polymerase, Ligase, Topoisomerase

✏️ Solved Examples

1Solved Exampleeasy3 steps

If a DNA molecule contains 30% adenine, what is the percentage of each other base?

Chargaff's rule: %A = %T.

% T = 30%

2Solved Exampleboard4 steps

Explain the Meselson–Stahl experiment that proved semi-conservative replication of DNA.

E. coli cells grown for many generations in medium containing ¹⁵NH₄Cl → DNA fully heavy.

Parent: all^{15} N (heavy)

3Solved ExampleHOTS5 steps

Explain the working of the lac operon in the presence and absence of lactose.

In absence of lactose: the i gene produces active repressor which binds to operator O.

Repressor bound: transcription blocked

⚠️ Traps & Common Mistakes

⚠️Common Mistakes6

1
Confusing DNA (double-stranded) with RNA (single-stranded)
✓DNA usually ds with 2-deoxyribose and T. RNA usually ss with ribose and U.
2
Writing DNA replication as conservative
✓Semi-conservative — each daughter has 1 parental + 1 new strand. Meselson–Stahl proved it.
3
Forgetting stop codons in translation questions
✓UAA, UAG, UGA terminate translation. They do NOT code for any amino acid.
4
Assuming genetic code is universal without exceptions
✓Mostly universal but mitochondrial code shows minor deviations. State 'nearly universal'.
5
Confusing codon (mRNA) with anticodon (tRNA)
✓Codon is on mRNA; anticodon is on tRNA, base-complementary to codon.
6
Writing DNA polymerase can synthesize 3' to 5'
✓DNA polymerase reads 3' to 5' but SYNTHESIZES 5' to 3'. Only one direction.

🎯 Practice Yourself

🎯Practice Yourself5 questions

Q1
State the base sequence complementary to 5'-ATTGCAT-3'.
Q2
Name the enzyme that seals nicks in the DNA strand during replication.
Q3
How many nucleotides are needed to code for a protein of 100 amino acids?
Q4
Differentiate between exons and introns.
Q5
Name the scientists who proposed the double-helix model of DNA.

📝 Notes

Molecular Basis of Inheritance — Maharashtra HSC Overview

Chapter 5 of the Balbharati Class 12 Biology textbook covers DNA structure (Watson–Crick), replication, transcription, translation, genetic code, and gene regulation (lac operon).

Maharashtra syllabus specifics

Watson–Crick structural parameters (2 nm diameter, 0.34 nm rise per bp, 10 bp per turn) — Balbharati asks the exact numbers.
Meselson–Stahl experiment — Maharashtra-specific 3-mark diagram-based question. Learn density gradient centrifugation interpretation.
Lac operon — Jacob and Monod's model with all four genes (i, z, y, a), promoter, operator — is a 4-marker.
Human Genome Project — Balbharati has a short section on features and applications; sometimes asked as 2-marker.
DNA fingerprinting — Balbharati briefly discusses VNTRs and RFLP; 2-marker.

Study tips

Learn diagrams: DNA double helix, replication fork, transcription bubble, ribosome with tRNA.
Memorise names: helicase, primase, DNA pol I, II, III, ligase, topoisomerase.
Distinguish transcription (RNA from DNA) vs translation (protein from mRNA).
Central dogma: DNA → RNA → Protein (with reverse transcription in retroviruses).
Post-transcriptional processing: 5' cap + poly-A tail + splicing (introns removed).