DNA structure, Griffith to Hershey–Chase, replication, transcription, genetic code, translation, lac operon, Human Genome Project and DNA fingerprinting — with SVG diagrams, a codon table, a mini transcription–translation tool, MCQs, A-R and PYQs.
DNA was first isolated as 'nuclein' by Friedrich Miescher (1869). Watson and Crick used X-ray diffraction data of Maurice Wilkins and Rosalind Franklin and Chargaff's rule (A : T and G : C ratios are 1 for double-stranded DNA).
(i) Two polynucleotide chains, sugar-phosphate backbone outside, bases inside. (ii) Antiparallel polarity (5′→3′ and 3′→5′). (iii) A = T (2 H-bonds), G ≡ C (3 H-bonds). (iv) Right-handed; pitch 3.4 nm, ~10 bp per turn, 0.34 nm per bp. (v) Stacking of base pairs plus H-bonds stabilises the helix.
Central dogma (Crick): DNA → RNA → Protein. In some viruses the flow is reverse (RNA → DNA) — reverse transcription (Temin & Baltimore).
Streptococcus pneumoniae: S strain (smooth, polysaccharide coat, virulent) and R strain (rough, no coat, non-virulent). Heat-killed S + live R killed the mice, and live S bacteria were recovered. Some 'transforming principle' passed from dead S to R; its nature was not identified.
A genetic material must: (i) replicate, (ii) be chemically and structurally stable, (iii) allow slow changes (mutation) for evolution, (iv) express as Mendelian characters. Proteins fail criterion (i).
| Point | DNA | RNA |
|---|---|---|
| Stability | Stable (no reactive 2′-OH; thymine adds stability; double strand, repair) | Labile — reactive 2′-OH; catalytic |
| Mutation rate | Lower | Higher → RNA viruses evolve faster |
| Best for | Storage of genetic information | Transmission; expresses directly, dynamic roles (messenger, adapter) |
Watson and Crick proposed that the two strands separate and each acts as a template for a new complementary strand; each daughter DNA has one parental and one new strand — semiconservative replication.
Grew E. coli in ¹⁵NH₄Cl (heavy N) for many generations, then shifted to ¹⁴NH₄Cl; DNA separated by CsCl density gradient centrifugation. After 20 min (1 generation): hybrid (intermediate) DNA. After 40 min (2nd generation): equal hybrid and light DNA. (Taylor et al., 1958, confirmed it in Vicia faba using radioactive thymidine.)
Copying of genetic information from one strand of DNA into RNA (only a segment of DNA and only one strand). A pairs with U.
Why not both strands? They would code for two different RNAs/proteins from one segment, and the two RNAs would be complementary, forming double-stranded RNA that cannot be translated.
| Eukaryotic RNA polymerase | Transcribes |
|---|---|
| RNA polymerase I | rRNAs (28S, 18S, 5.8S) |
| RNA polymerase II | Precursor of mRNA — hnRNA |
| RNA polymerase III | tRNA, 5S rRNA, snRNAs |
(i) Triplet: 61 codons code amino acids, 3 are stop codons (UAA, UAG, UGA). (ii) Degenerate: some amino acids have more than one codon. (iii) Read contiguously, no punctuation. (iv) Nearly universal (UUU = Phe from bacteria to humans; exceptions in mitochondria and some protozoa). (v) AUG codes Met and is the initiator. (vi) Unambiguous — one codon specifies only one amino acid.
Enter the DNA template strand (3′→5′). mRNA is its complement (A→U, T→A, G→C, C→G), translated from the first AUG to a stop codon.
In eukaryotes: transcriptional level, processing level (splicing), transport of mRNA to cytoplasm, translational level. In prokaryotes the main control is at transcription initiation, via activators and repressors binding operators adjacent to promoters.
| Gene | Codes for |
|---|---|
| i (regulatory) | Repressor (made constitutively). i from 'inhibitor'. |
| z | β-galactosidase — hydrolyses lactose → galactose + glucose |
| y | Permease — increases permeability to β-galactosides |
| a | Transacetylase |
| Salient features of human genome | Value |
|---|---|
| Total size | 3164.7 million bp |
| Average gene | ~3000 bases; largest = dystrophin (2.4 million bases) |
| Number of genes | ~30,000 (earlier estimates 80,000–1,40,000) |
| Same in all humans | 99.9% of bases |
| Coding portion | Less than 2%; functions unknown for over 50% of genes |
| Repeated sequences | Very large portion; no direct coding function; inform chromosome structure and evolution |
| Chromosomes | Chr 1 has most genes (2968); Y has fewest (231) |
| SNPs | ~1.4 million single-nucleotide polymorphisms ('snips') |
Quick method to compare DNA of two individuals (since 99.9% of sequence is common). It uses repetitive DNA (satellite DNA — separate as small peaks in density-gradient centrifugation; micro- and mini-satellites), which does not code for protein but shows high polymorphism.
Q1. The length of E. coli DNA is 1.36 mm. Calculate the number of base pairs.
No. of bp = total length ÷ distance between consecutive bp (0.34 nm).
1.36 mm = 1.36 × 10⁻³ m; bp = (1.36 × 10⁻³) ÷ (0.34 × 10⁻⁹) = 4 × 10⁶ bp.
Q2. Coding strand is 5′-TACGTACG-3′. Write the template strand and mRNA.
Complementary and antiparallel: 3′-ATGCATGC-5′.
Same as coding strand with U for T: 5′-UACGUACG-3′.
Q3. In Meselson–Stahl experiment, what proportions of hybrid and light DNA are present after 80 minutes (E. coli divides every 20 min)?
80 ÷ 20 = 4 generations → 2⁴ = 16 DNA molecules.
Each of the 2 original ¹⁵N strands stays in one molecule → 2 hybrid.
16 − 2 = 14 light.
(A) Both true, R explains A. (B) Both true, R does NOT explain A. (C) A true, R false. (D) A false, R true.
Q. Describe the Hershey–Chase experiment. What did it prove?
Phages were grown on ³²P (labels DNA) or ³⁵S (labels protein). After infecting E. coli, blending and centrifugation, only ³²P-labelled DNA was found inside bacteria; ³⁵S protein stayed outside. So DNA is the genetic material.
Q. Why is DNA a better genetic material than RNA?
RNA has a reactive 2′-OH so it is labile and degrades easily; DNA lacks it and has thymine, is double-stranded with repair, hence chemically less reactive and structurally more stable.
Q. Describe the structure of the lac operon and how it is regulated by lactose.
Regulatory gene i (repressor) + promoter, operator + structural genes z, y, a. Without lactose, repressor binds operator and blocks RNA polymerase. Lactose (inducer) binds and inactivates the repressor, so transcription of z, y, a occurs.
Q. Write the salient features of the genetic code.
Triplet; 61 sense + 3 stop codons; degenerate; contiguous reading without punctuation; nearly universal; AUG is start codon (Met); unambiguous.
Q. What is DNA fingerprinting? Mention its applications.
Technique to identify individuals using polymorphism in repetitive (VNTR) DNA. Steps: DNA isolation, restriction digestion, gel electrophoresis, Southern blotting, hybridisation with labelled probe, autoradiography. Uses: forensics, paternity testing, population diversity studies.