Molecular Basis of Inheritance traces how genetic information is stored in DNA, copied, and expressed: the Watson-Crick double helix, the experiments that proved DNA is the genetic material, semiconservative replication, transcription, the genetic code, and translation, ending with gene regulation via the lac operon and genomics. It is one of the highest-weightage chapters in NEET Biology, so mastering its processes and key numbers is essential for scoring marks.
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DNA Structure (Watson-Crick)
DNA is an antiparallel, right-handed double helix of deoxyribonucleotides.
Purines A,G (double ring) pair with pyrimidines T,C (single ring).
- Strands antiparallel: one 5’→3′, other 3’→5′
- Backbone joined by 3′-5′ phosphodiester bonds
- Right-handed helix, base + sugar = nucleoside; + phosphate = nucleotide
Helix Dimensions & Chargaff
The B-DNA numbers NEET asks directly in numericals.
Chargaff’s rule: A = T and G = C, so A+G = T+C.
- Length = (number of bp) × 0.34 nm
- Turns = (number of bp) ÷ 10
- If C = 22% then G = 22%, so A = T = (100 − 44)/2 = 28%
Nucleosome & Chromatin
2.2 m of DNA fits in a nucleus by wrapping on histone spools.
H1 is a linker histone, NOT part of the octamer. Human cell ~6.6×109 bp.
- Negative DNA wraps positive histones (rich in lysine & arginine)
- Beads-on-string → solenoid → chromatin fibre → chromosome
- Euchromatin = loose/active; Heterochromatin = dense/inactive
Search for Genetic Material
Three experiments proved DNA, not protein, is the genetic material.
Only 32P (DNA) entered bacteria; 35S coat stayed outside.
- Griffith (1928): transforming principle in Streptococcus pneumoniae
- Avery-MacLeod-McCarty: transforming principle is DNA (DNase destroys it)
- Hershey-Chase (1952) used bacteriophages → DNA is genetic material
DNA Replication
Replication is semiconservative: each daughter keeps one old strand.
Meselson-Stahl (1958): 15N→14N in E. coli, hybrid DNA after 1 generation (CsCl).
- Leading strand continuous; lagging strand = Okazaki fragments joined by DNA ligase
- Starts at origin (ori); occurs in S phase in eukaryotes
- dNTPs supply both bases and energy; polymerase proofreads
Transcription
RNA polymerase copies one DNA strand into RNA, 5’→3′, no primer.
Template strand read 3’→5′; coding (sense) strand NOT copied.
- Transcription unit = promoter + structural gene + terminator
- Eukaryotes: Pol I → rRNA, Pol II → mRNA (hnRNA), Pol III → tRNA
- hnRNA processing: splicing (remove introns) + 5′ cap + 3′ poly-A tail
The Genetic Code
Codons are mRNA base triplets that specify amino acids.
Degenerate (many codons→1 aa) but unambiguous, universal, comma-less.
- Triplet, non-overlapping, read continuously
- 3 stop codons code no amino acid
- Amino acids = (coding nucleotides ÷ 3) − 1 (for the stop codon)
Translation
Ribosomes read mRNA codon-by-codon to build a polypeptide.
UTRs flank the coding region but are not translated.
- Initiation: ribosome assembles at start codon AUG
- Elongation: peptide bonds join amino acids as ribosome moves
- Termination: at a stop codon, polypeptide is released
lac Operon (Jacob & Monod)
Bacteria make lactose-digesting enzymes only when lactose is present.
Inducible system, negative regulation; lactose is substrate AND switch.
- Structural genes z (β-galactosidase), y (permease), a (transacetylase)
- i gene makes repressor that binds operator and blocks transcription
- Add lactose → repressor off → RNA polymerase transcribes the operon
HGP & DNA Fingerprinting
Reading the human genome and identifying individuals by repeat DNA.
DNA fingerprinting (Alec Jeffreys) compares VNTRs (minisatellites).
- HGP 1990–2003; Chromosome 1 most genes, Y fewest
- Methods: ESTs (expressed genes) and sequence annotation (whole genome)
- Fingerprinting steps: isolation → digestion → electrophoresis → blotting → probe → autoradiography
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- Principles of Inheritance and Variation Class 12 Notes
- Evolution Class 12 Notes
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Frequently Asked Questions
It is the study of DNA as the genetic material, including its double-helix structure, how it replicates, and how the information in genes is expressed into proteins through transcription and translation.
Semiconservative replication means each daughter DNA molecule keeps one old parental strand and one newly made strand. Meselson and Stahl proved this in 1958 using heavy nitrogen (15N) and CsCl density-gradient centrifugation in E. coli, getting hybrid DNA after one generation.
It is part of the NEET syllabus from Class 12 Biology and is one of the most heavily weighted chapters, frequently giving 4 to 6 questions, including numericals on DNA length, base pairing (Chargaff), and experiments like Hershey-Chase.
Transcription is copying one DNA strand into RNA by RNA polymerase, happening in the nucleus in eukaryotes. Translation is reading the mRNA codons on ribosomes to build a polypeptide, happening in the cytoplasm.
The genetic code is the set of 64 mRNA triplet codons that specify amino acids, with AUG as start and UAA, UAG, UGA as stop codons. It is called degenerate because most amino acids are coded by more than one codon, yet each codon is unambiguous (codes only one amino acid).