Is it correct to say: Writing uracil (U) as a base of DNA?

DNA has A, T, G, C. RNA replaces T with U. Never mix the two lists.

Is it correct to say: Claiming replication is conservative or dispersive?

Meselson-Stahl proved DNA replication is semiconservative — each daughter duplex has one old + one new strand.

Is it correct to say: Confusing template strand with coding (sense) strand?

Template strand is read 3'→5' by RNA polymerase. The coding (sense) strand has the same sequence as mRNA (with T replacing U).

Is it correct to say: Believing all human DNA codes for protein?

Less than 2% of the human genome codes for protein. The rest is regulatory, repetitive or non-coding RNA.

Is it correct to say: Mixing up the numbers of hydrogen bonds in AT and GC pairs?

A=T uses 2 H-bonds; G≡C uses 3 H-bonds. That's why GC-rich DNA has a higher melting temperature.

Is it correct to say: Saying the genetic code has 20 codons because there are 20 amino acids?

The code has 64 codons (4³): 61 sense + 3 stop. Because 61 codons specify only 20 amino acids the code is degenerate.

Molecular Basis of Inheritance

DNA structure, replication, transcription, genetic code, translation, regulation of gene expression, Human Genome Project — NCERT Class 12 Biology Ch 5

📐 10 formulas✏️ 3 examples🎯 6 practice⚖️ 7-8 marks🏫 CBSE📚 Class 12✓ 2025–26 syllabus

💡

Board Exam Tips

→Watson-Crick model with Chargaff's rule is a 3-mark favourite. Learn (A = T) and (G ≡ C) base-pair details.
→Semiconservative replication (Meselson & Stahl) — 5-mark question with density-gradient diagram.
→Central dogma, transcription unit and post-transcriptional processing appear almost every year.
→Never write DNA base uracil (U). RNA has U, DNA has T. Marks are cut for this every year.
→Learn the Lac operon components (i, p, o, z, y, a) and the role of the inducer lactose.

📐 Formulas(10)

Chargaff's Rule★ Board fav

[A] = [T], [G] = [C]; \frac{A + G}{T + C} = 1

DNA Double Helix Geometry★ Board fav

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

Length of Human DNA

L = (6.6 \times 1 0^{9} bp) \times (0.34 nm) \approx 2.2 m per diploid cell

Semiconservative Replication★ Board fav

Each daughter duplex = 1 parental strand + 1 newly synthesised strand

Nucleotide = Sugar + Base + Phosphate

Nucleotide = deoxyribose + N-base + PO_{4}^{3 -}

Transcription Unit★ Board fav

Promoter \to Structural gene \to Terminator

Genetic Code Combinatorics★ Board fav

4^{3} = 64 codons; 61 sense + 3 stop (UAA, UAG, UGA)

Central Dogma

DNA transcription mRNA translation Protein

Post-transcriptional Processing (Eukaryotes)★ Board fav

hnRNA capping + splicing + tailing mature mRNA

Human Genome Facts

Bases \approx 3.164 \times 1 0^{9}; Genes \approx 20, 000 - 25, 000; Chr 1 largest, Chr Y smallest

✏️ Solved Examples

1Solved Exampleeasy4 steps

A DNA sample contains 20% adenine. Calculate the percentage of each of the other three bases using Chargaff's rule.

Apply A = T

% T = % A = 20%

2Solved Exampleboard6 steps

Describe the Meselson-Stahl experiment and explain how it proved that DNA replication is semiconservative. (5 marks)

Growth in heavy medium

E. coli in^{15} NH_{4} Cl \Rightarrow all DNA has^{15} N (heavy)

3Solved ExampleHOTS4 steps

A single mRNA is 900 nucleotides long, of which the 5' UTR is 50 nt and the 3' UTR is 100 nt. How many amino acids does the resulting protein contain?

Coding region length

900 - 50 - 100 = 750 nt

⚠️ Traps & Common Mistakes

⚠️Common Mistakes6

1
Writing uracil (U) as a base of DNA
✓DNA has A, T, G, C. RNA replaces T with U. Never mix the two lists.
2
Claiming replication is conservative or dispersive
✓Meselson-Stahl proved DNA replication is semiconservative — each daughter duplex has one old + one new strand.
3
Confusing template strand with coding (sense) strand
✓Template strand is read 3'→5' by RNA polymerase. The coding (sense) strand has the same sequence as mRNA (with T replacing U).
4
Believing all human DNA codes for protein
✓Less than 2% of the human genome codes for protein. The rest is regulatory, repetitive or non-coding RNA.
5
Mixing up the numbers of hydrogen bonds in AT and GC pairs
✓A=T uses 2 H-bonds; G≡C uses 3 H-bonds. That's why GC-rich DNA has a higher melting temperature.
6
Saying the genetic code has 20 codons because there are 20 amino acids
✓The code has 64 codons (4³): 61 sense + 3 stop. Because 61 codons specify only 20 amino acids the code is degenerate.

🎯 Practice Yourself

🎯Practice Yourself6 questions

Q1
Name the three stop codons and state their function.
Q2
State the length and pitch of B-form DNA and how many base pairs occur per turn.
Q3
Write the mRNA transcribed from the DNA template 3'-TAC GAT CAG-5'.
$3^{'} - T A C G A T C A G - 5^{'} \to 5^{'} - A U G C U A G U C - 3^{'}$
Q4
Name the enzyme that unwinds DNA at the replication fork and the enzyme that joins Okazaki fragments.
Q5
What are introns and exons? In which type of cell are they typically found?
Q6
Why is the Lac operon called an inducible operon?

📝 Notes

Molecular Basis of Inheritance

NCERT Chapter 5 tracks the molecule of inheritance — how DNA is structured, replicated, transcribed and translated, and how gene expression is regulated.

Discovery of DNA as genetic material

Griffith (1928) — transformation in Streptococcus pneumoniae.
Avery, MacLeod, McCarty (1944) — identified transforming principle as DNA.
Hershey-Chase (1952) — used ³²P and ³⁵S to prove DNA (not protein) enters the bacterium during phage infection.

Watson-Crick model (1953)

Two antiparallel polynucleotide chains form a right-handed double helix.
Pentose = deoxyribose. Phosphodiester bonds run 5' → 3'.
Bases pair specifically: A ═ T (2 H-bonds), G ≡ C (3 H-bonds).
Uniform diameter of 2 nm; 10 bp per helical turn; pitch 3.4 nm.

Chargaff's rule

For any dsDNA sample: %A = %T and %G = %C. Purine total = pyrimidine total.

Replication

Semiconservative (Meselson-Stahl).
Starts at an origin (ori) and proceeds bidirectionally.
DNA polymerase III synthesises 5' → 3' on the leading strand continuously and on the lagging strand discontinuously in Okazaki fragments joined by DNA ligase.

Transcription

Only one strand serves as template (3' → 5' read).
Prokaryotes have a single RNA polymerase for all RNAs. Eukaryotes have three (RNA pol I → rRNA, pol II → mRNA, pol III → tRNA + 5S rRNA).
Eukaryotic hnRNA is capped (7-methyl-G at 5'), tailed (poly-A at 3') and spliced to give mature mRNA.

Genetic code

Triplet — 3 bases specify 1 amino acid.
Degenerate — one amino acid may be coded by several codons.
Unambiguous — one codon codes only one amino acid.
Universal — same code across life (mitochondrial exceptions).
Non-overlapping and commaless — read continuously in frame.
Start codon = AUG (Met). Stop codons = UAA, UAG, UGA.

Translation

Ribosome holds three sites — A (aminoacyl), P (peptidyl), E (exit). Charged tRNAs deliver amino acids; peptidyl transferase forms the peptide bond (ribozyme activity of 23S rRNA).

Regulation of gene expression — the Lac operon

Jacob and Monod (1961). Components:

i (regulatory) gene — makes repressor.
p (promoter) — RNA pol binding site.
o (operator) — repressor binding site.
z, y, a — structural genes for β-galactosidase, permease and transacetylase.

Inducer (lactose derivative allolactose) binds repressor → operon transcribed.

Human Genome Project (1990-2003)

~3.164 × 10⁹ base pairs sequenced.
Estimated 20 000–25 000 genes.
Chromosome 1 largest (2968 genes); chromosome Y smallest (231 genes).
Less than 2% of DNA is protein-coding.

Quick sanity checks

If %A is known, you know all four (Chargaff).
A gene of n codons codes n amino acids only if you exclude the stop codon and count the initiator Met.
DNA melting temperature rises with GC content because G ≡ C has 3 H-bonds.