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Molecular Basis of Inheritance — Revision Notes
Class 12 Biology. DNA to protein — structure, replication, transcription, the genetic code and gene regulation, condensed into exam-ready key points.
1. The Genetic Material — DNA
- DNA is a polymer of nucleotides; each nucleotide = nitrogen base + pentose sugar (deoxyribose) + phosphate.
- Purines: Adenine (A), Guanine (G). Pyrimidines: Cytosine (C), Thymine (T). In RNA, Uracil (U) replaces T.
- Watson & Crick (1953) double helix: two antiparallel strands, sugar-phosphate backbone outside, bases paired inside.
- Base pairing: A=T (2 H-bonds), G≡C (3 H-bonds). Chargaff's rule: A = T, G = C.
- One helical turn = 3.4 nm, ~10 base pairs per turn; distance between adjacent bases = 0.34 nm.
🔑 Hershey–Chase experiment (bacteriophage, radioactive ³²P and ³⁵S) proved DNA, not protein, is the genetic material.
2. DNA Replication
- Semi-conservative — each new molecule has one old and one new strand (proved by Meselson & Stahl using ¹⁵N in E. coli).
- Enzyme DNA polymerase synthesises the new strand only in the 5'→3' direction.
- Leading strand: continuous synthesis. Lagging strand: discontinuous, as Okazaki fragments later joined by DNA ligase.
- Replication begins at a specific site — the origin of replication (ori).
3. Transcription
- Synthesis of RNA from a DNA template by RNA polymerase.
- Template (antisense) strand: 3'→5', read by RNA polymerase. Coding (sense) strand: same sequence as mRNA (T→U).
- In prokaryotes: one RNA polymerase makes all RNA. In eukaryotes: RNA Pol I → rRNA, Pol II → mRNA (hnRNA), Pol III → tRNA.
- Eukaryotic genes are split: exons (coding) + introns (non-coding). Introns are removed by splicing; the mRNA is capped and tailed (poly-A).
4. Genetic Code
- A triplet code — three bases (codon) code for one amino acid. 64 codons in total.
- 61 codons code for amino acids; 3 stop codons: UAA, UAG, UGA.
- Start codon: AUG (also codes methionine).
- Properties: degenerate (one amino acid, several codons), unambiguous, non-overlapping, comma-less and nearly universal.
🔑 tRNA is the adapter molecule — its anticodon reads the mRNA codon and it carries the matching amino acid.
5. Translation
- Protein synthesis on ribosomes: amino acids joined by peptide bonds in the order dictated by mRNA codons.
- Stages: initiation → elongation → termination.
- The ribosome's rRNA acts as a ribozyme (peptidyl transferase) forming peptide bonds.
6. Gene Regulation — Lac Operon
- Model of prokaryotic gene regulation proposed by Jacob & Monod in E. coli.
- Components: promoter, operator, and structural genes z (β-galactosidase), y (permease), a (transacetylase); regulated by the i gene (repressor).
- Lactose is the inducer: it binds and inactivates the repressor, so RNA polymerase can transcribe the genes (operon switched ON).
- No lactose → repressor binds operator → transcription blocked (operon OFF). This is negative regulation.
7. Human Genome Project (HGP)
- Mega project (1990–2003) to sequence the entire human genome (~3.1 billion bp).
- Estimated 20,000–25,000 genes; less than 2% codes for proteins.
- Chromosome 1 has the most genes; the Y chromosome the fewest.
- DNA fingerprinting (developed by Alec Jeffreys) uses VNTRs (variable number tandem repeats) for identity/paternity testing.
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