Molecular Basis of Inheritance: DNA, Its Proof, and How the Cell Reads It
A from-scratch guide to DNA's double-helix structure, the classic experiments proving DNA is the genetic material, semiconservative replication, and the central dogma of transcription, the genetic code, and translation — built for CBSE Class 12 boards, NEET and CUET.
By the PadhoDost Team · 📖 8 min read · Updated 4 August 2026
Part of Class 12 (CBSE) prep🧠 Your body's master cookbook
Imagine every cell in your body quietly carries a thick cookbook holding the recipes that make you — your eye colour, your blood group, the enzymes in your saliva. That cookbook is DNA. The recipes are genes, the language uses just four letters (A, T, G, C), and amazingly the whole book is copied almost perfectly into every new cell. In this chapter we open the cookbook: how it is written, how scientists proved it really is the recipe book of life, and how the cell reads it to actually 'cook' proteins. Padho, dost — let's turn the pages together!
The Structure of DNA: A Twisted Ladder
DNA (deoxyribonucleic acid) is a long polymer made of repeating units called nucleotides. Each nucleotide has three parts: a nitrogenous base, a pentose sugar (deoxyribose), and a phosphate group. There are four bases — the purines Adenine (A) and Guanine (G), which have two rings, and the pyrimidines Cytosine (C) and Thymine (T), which have one ring. A base joined to a sugar is a nucleoside; add a phosphate and it becomes a nucleotide. The sugars and phosphates link through phosphodiester bonds to form a strong sugar-phosphate backbone, while the bases point inwards like the rungs of a ladder.
Salient features of the Watson-Crick double helix (1953)
- ✓DNA has two polynucleotide strands coiled into a right-handed double helix.
- ✓The two strands are antiparallel — one runs 5'→3' and the other runs 3'→5'.
- ✓Bases pair by hydrogen bonds: A pairs with T (2 H-bonds) and G pairs with C (3 H-bonds) — this is complementary base pairing.
- ✓The helix has a pitch (one full turn) of about 3.4 nm with roughly 10 base pairs per turn, so adjacent base pairs are about 0.34 nm apart.
- ✓Because pairing is complementary, if you know one strand's sequence you can always write the other.
| Feature | DNA | RNA |
|---|---|---|
| Sugar | Deoxyribose | Ribose |
| Bases | A, T, G, C | A, U, G, C (U replaces T) |
| Strands | Usually double-stranded | Usually single-stranded |
| Stability | Chemically more stable | Less stable, more reactive |
| Main role | Stores genetic information | Expresses it (mRNA, tRNA, rRNA) |
Proving DNA Is the Genetic Material
The experiments, step by step
- 1Griffith (1928): Working with Streptococcus pneumoniae, he had virulent S-type (smooth, capsulated) and harmless R-type (rough) bacteria. Heat-killed S mixed with live R killed the mice, and live S bacteria were recovered — some 'transforming principle' had passed from the dead S to the living R. This is transformation.
- 2Avery, MacLeod and McCarty (1944): They purified the transforming principle and tested it with enzymes. Protein-digesting and RNA-digesting enzymes did not stop transformation, but DNase (which destroys DNA) did — showing the transforming principle is DNA.
- 3Hershey and Chase (1952): Using bacteriophages, they labelled DNA with radioactive 32P and protein with radioactive 35S. Only 32P (DNA) entered the bacteria and appeared in the next generation of phages — confirming DNA, not protein, is the genetic material.
📝 Worked example: why 32P and 35S?
Question: In the Hershey-Chase experiment, why label DNA with 32P and protein with 35S?
Step 1 — Composition of DNA: DNA contains phosphorus (in the sugar-phosphate backbone) but no sulphur.
Step 2 — Composition of protein: Proteins contain sulphur (in the amino acids cysteine and methionine) but essentially no phosphorus.
Step 3 — Therefore 32P tags only DNA and 35S tags only protein, so the two molecules can be tracked separately.
Step 4 — Result: after infection, radioactive 32P was found inside the bacteria (and in new phages), while 35S stayed outside in the empty protein coats.
Conclusion: the molecule that entered the cell and directed new phage production was DNA — hence DNA is the genetic material.
DNA Replication: Making Two From One
Semiconservative replication, step by step
- 1The double helix unwinds at a specific origin of replication (helicase separates the strands), forming a Y-shaped replication fork; each old strand now acts as a template.
- 2DNA-dependent DNA polymerase adds new nucleotides by complementary base pairing (A with T, G with C), always building in the 5'→3' direction.
- 3Because the strands are antiparallel, the leading strand is made continuously while the lagging strand is made in short pieces called Okazaki fragments.
- 4DNA ligase joins the Okazaki fragments into one continuous strand.
- 5Result: two identical DNA molecules, each with one old (parental) strand and one new strand — this is why replication is called 'semiconservative', proved by Meselson and Stahl (1958).
Central Dogma: From DNA to Protein
The central dogma, proposed by Francis Crick, states that genetic information flows DNA → RNA → protein. (In some retroviruses the flow can run in reverse, RNA → DNA, by reverse transcription.) Transcription is the copying of a gene from DNA into messenger RNA (mRNA) by the enzyme RNA polymerase. Only one strand — the template strand, read 3'→5' — is copied; the other is the coding strand. The RNA formed has the same sequence as the coding strand, except that uracil (U) takes the place of thymine (T). In eukaryotes the first transcript (hnRNA) is processed: non-coding introns are removed and coding exons are joined (splicing), a cap is added at the 5' end and a poly-A tail at the 3' end, before the mature mRNA leaves the nucleus.
Key features of the genetic code
- ✓It is a triplet code: three bases (a codon) specify one amino acid.
- ✓Of the 64 codons, 61 code for amino acids and 3 are stop (termination) codons — UAA, UAG, UGA.
- ✓AUG is the start (initiation) codon and also codes for the amino acid methionine.
- ✓The code is degenerate — most amino acids are specified by more than one codon.
- ✓It is non-overlapping, comma-less, and nearly universal — the same codons mean the same amino acids in almost all organisms.
- ✓It is unambiguous — one codon codes for one and only one amino acid.
Translation, step by step
- 1Charging (aminoacylation): each tRNA is loaded with its specific amino acid using energy from ATP; tRNA is the 'adapter' that reads a codon through its anticodon.
- 2Initiation: the ribosome assembles on the mRNA at the start codon AUG, and the first aminoacyl-tRNA binds.
- 3Elongation: the ribosome moves along the mRNA codon by codon, peptide bonds form between amino acids, and the polypeptide chain grows.
- 4Termination: when a stop codon (UAA, UAG or UGA) is reached, no tRNA matches it, so the finished polypeptide is released and the ribosome separates.
⚡ Quick check
A DNA molecule with 15N (heavy nitrogen) in both strands is allowed to replicate exactly once in a medium containing only 14N (light nitrogen). What will the two daughter molecules be like?
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