Molecular Basis of InheritanceNEET Botany · Class 12 · NCERT Chapter 3

Molecular Basis of Inheritance is a Very High Weightage chapter with 5 to 8 questions in most NEET exams. Questions focus on Hershey-Chase experiment, Chargaff's rules, DNA structure (dimensions, base pairs, bonds), nucleosome, Meselson-Stahl experiment, replication enzymes, transcription (template strand, promoter), genetic code properties (degenerate, universal, start/stop codons), lac operon (inducible, structural genes, repressor), Human Genome Project (base pair count, gene count), and DNA fingerprinting (VNTRs). This is one of the highest-yield chapters in NEET Biology.

Three landmark experiments established DNA as genetic material: (1) GRIFFITH'S EXPERIMENT (1928): Frederick Griffith showed that heat-killed S-strain (virulent) bacteria could transform live R-strain (non-virulent) bacteria into S-strain. He called the unknown agent the "transforming principle." He did not identify what the molecule was. (2) AVERY, MacLEOD, McCARTY EXPERIMENT (1944): They isolated the transforming principle and tested each class of molecule. DNase (enzyme that destroys DNA) abolished transformation; RNase and Protease did not. Conclusion: DNA is the transforming principle. (3) HERSHEY-CHASE EXPERIMENT (1952): Using bacteriophage T2, they labelled protein with 35S (sulphur, which is only in protein) and DNA with 32P (phosphorus, which is only in DNA). After infection, 32P was found INSIDE bacteria (DNA entered) while 35S stayed OUTSIDE (protein coat did not enter). This conclusively proved DNA is the genetic material. NEET trap: Griffith did not prove it was DNA; Avery proved it was DNA biochemically; Hershey-Chase confirmed it definitively.

Chargaff's rules describe the base composition of double-stranded DNA: (1) A = T (adenine equals thymine in molar amounts). (2) G = C (guanine equals cytosine in molar amounts). (3) A + G = T + C (total purines = total pyrimidines). HOW TO USE: if you know the percentage of one base, you can find all others. Example: if A = 30%, then T = 30%, and G + C = 40%, so G = C = 20%. If G = 22%, then C = 22%, A + T = 56%, so A = T = 28%. These rules apply to double-stranded DNA only. In single-stranded DNA, RNA, and in the individual strands of a double helix, A does NOT necessarily equal T. Also: A + T / G + C ratio varies between species (useful for identifying organisms), but A + G / T + C = 1 always in ds-DNA. NEET frequently gives the % of one base and asks you to calculate the others.

The Watson-Crick double helix (B-form DNA) key facts for NEET: (1) TWO ANTIPARALLEL STRANDS: one runs 5'→3', the other 3'→5'. (2) BASE PAIRS: A pairs with T (2 hydrogen bonds), G pairs with C (3 hydrogen bonds). G-C pairs are stronger (GC-rich DNA melts at higher temperature). (3) BACKBONE: sugar-phosphate backbone on the outside; bases are on the inside. (4) DIMENSIONS: 0.34 nm (3.4 Å) between consecutive base pairs; 3.4 nm (34 Å) per complete turn; 10 base pairs per turn; 2 nm (20 Å) diameter. REMEMBER: 3.4 Å per bp, 34 Å per turn, 10 bp per turn. (5) GROOVES: major groove (wider, used by proteins for recognition) and minor groove (narrower). (6) RIGHT-HANDED helix (B-form). (7) Each nucleotide = phosphate + deoxyribose sugar + nitrogenous base. Purines: Adenine (A), Guanine (G) — double-ring. Pyrimidines: Thymine (T), Cytosine (C), Uracil (U in RNA) — single ring.

DNA packaging occurs in several levels of organisation: (1) NUCLEOSOME: 146 bp of DNA wound 1.65 times around a histone OCTAMER (2 copies each of H2A, H2B, H3, H4). H1 is the LINKER histone that sits outside the nucleosome at the entry and exit of DNA. The 10 nm "beads-on-a-string" fibre is the basic level. (2) 30 nm SOLENOID: 6 nucleosomes per turn of the solenoid; this requires H1 histone. (3) 300 nm LOOPED DOMAIN structure: 30nm fibre attached to protein scaffold (non-histone proteins). (4) 700 nm chromatin: further compacted. (5) 1400 nm METAPHASE CHROMOSOME: the fully condensed form seen during cell division. Key NEET facts: Histones are positively charged (rich in lysine and arginine = basic amino acids); they interact with negatively charged DNA (phosphate groups). Nucleosome = the basic unit of chromatin. Non-histone chromosomal proteins form the scaffold.

Semiconservative replication means each daughter DNA double helix retains ONE original (parental) strand and ONE newly synthesised strand. The other proposed models were: conservative (both original strands stay together; both new strands together) and dispersive (original and new DNA are randomly scattered). MESELSON-STAHL EXPERIMENT (1958): (1) E. coli was grown on 15N medium (heavy nitrogen isotope) for many generations, making all DNA "heavy" (15N-15N). (2) The bacteria were then transferred to 14N medium (light nitrogen). (3) After ONE generation: all DNA was of INTERMEDIATE density (15N-14N hybrid) — only ONE band in CsCl density gradient centrifugation. This RULED OUT conservative replication (which would give two bands: one heavy and one light). (4) After TWO generations: TWO bands appeared — 50% intermediate (15N-14N) and 50% light (14N-14N). This CONFIRMED semiconservative replication. If dispersive, all DNA would be intermediate at every generation.

DNA replication at the replication fork uses multiple enzymes working together: (1) HELICASE: unwinds the double helix by breaking hydrogen bonds; creates the replication fork. (2) SSB PROTEINS (Single-Strand Binding Proteins): stabilise the separated single strands; prevent re-annealing. (3) TOPOISOMERASE (gyrase): relieves the torsional stress (supercoiling) ahead of the replication fork. (4) PRIMASE: synthesises a short RNA primer (8-12 nucleotides) complementary to the template; necessary because DNA polymerase cannot start synthesis without a primer. (5) DNA POLYMERASE III: the main replication enzyme; adds new deoxyribonucleotides in the 5'→3' direction only; reads template 3'→5'. (6) DNA POLYMERASE I: removes the RNA primer and fills the gap with DNA. (7) DNA LIGASE: seals the nick (break) between adjacent DNA fragments (between Okazaki fragments on the lagging strand). LEADING strand: synthesised continuously (5'→3', towards the fork). LAGGING strand: synthesised discontinuously as OKAZAKI FRAGMENTS (short pieces, each starting with a primer). NEET trap: DNA polymerase can only add to an existing 3' -OH group; it cannot start a new strand. Only primase can initiate synthesis.

The genetic code translates mRNA codons into amino acids. Its properties for NEET: (1) TRIPLET: each codon is 3 nucleotides (64 possible codons from 4 bases: 4³ = 64). (2) NON-OVERLAPPING: each nucleotide belongs to only one codon; reading frame is fixed. (3) COMMALESS: no "punctuation" between codons; read continuously. (4) DEGENERATE (REDUNDANT): most amino acids are coded by MORE than one codon (64 codons but only 20 amino acids + 3 stop codons; wobble at the 3rd position). (5) UNIVERSAL: the same code is used by nearly all organisms (viruses, bacteria, plants, animals) — an argument for common ancestry. Exceptions: mitochondria and some protists use slightly different codes. (6) UNAMBIGUOUS: one specific codon codes for only one amino acid (no ambiguity). (7) START CODON: AUG (codes for methionine in eukaryotes; N-formylmethionine in prokaryotes). (8) STOP CODONS (NONSENSE): UAA, UAG, UGA — do not code for any amino acid; signal end of translation. (9) First codon deciphered: UUU = phenylalanine (by Marshall Nirenberg and H. Gobind Khorana, 1960s; Nobel Prize 1968).

The lac operon (Jacob and Monod, 1961) is the classic example of gene regulation in E. coli. It controls 3 structural genes needed to metabolise lactose. STRUCTURE: Promoter (P) + Operator (O) + Structural genes: lacZ (β-galactosidase — cleaves lactose to glucose + galactose), lacY (permease — transports lactose into cell), lacA (transacetylase — function less clear). Separately, the regulatory gene (i) produces the LAC REPRESSOR protein continuously. HOW IT WORKS: (1) NO LACTOSE present: repressor binds the operator → blocks RNA polymerase → genes are OFF. (2) LACTOSE present: allolactose (a metabolite of lactose) acts as the INDUCER — it binds the repressor → repressor changes shape → cannot bind operator → RNA polymerase proceeds → genes are ON. (3) CATABOLITE REPRESSION: when GLUCOSE is present, cAMP levels are low → CRP (catabolite activator protein) cannot bind → low transcription even if lactose is present. When glucose is absent, cAMP rises → cAMP-CRP complex binds the promoter → strong transcription. NEET KEY: The lac operon is an INDUCIBLE operon (genes normally OFF; turned ON by inducer). The trp operon is a REPRESSIBLE operon (genes normally ON; turned OFF by co-repressor).