Questions
5–8 questions per paper
Difficulty
Medium-Hard
Importance
Core — never skip
Overview
Molecular Basis of Inheritance covers the chemical nature of genetic material and the mechanisms of information flow from DNA to proteins. It is the highest-weightage unit in Class 12 Biology, requiring a deep understanding of central dogma processes. Aspirants must master the structural properties of DNA and the enzymatic machinery governing replication, transcription, and translation.
DNA Structure and Packaging
DNA is a double-helical polymer of deoxyribonucleotides, stabilized by hydrogen bonding and base stacking interactions. Understanding how this long molecule is packed into the small nucleus via histone octamers is crucial for board exams.
- Watson-Crick model: Right-handed helix, 3.4 Å distance per base pair
- Chargaff's Rule: A+G = T+C and A+T/G+C = constant
- Nucleosome: DNA wrapped around a positively charged histone octamer (H2A, H2B, H3, H4)
- Euchromatin (transcriptionally active) vs Heterochromatin (transcriptionally inactive)
DNA Replication
Replication follows a semi-conservative model where each parent strand serves as a template for a new daughter strand. Questions often focus on the specific roles of enzymes at the replication fork.
- DNA Polymerase III: Primary replicative enzyme, 5' to 3' synthesis
- Leading strand synthesized continuously, lagging strand via Okazaki fragments
- Helicase: Unwinds DNA double helix
- DNA Ligase: Joins Okazaki fragments
- Meselson-Stahl experiment confirmed semi-conservative nature using 15N isotopes
Transcription
Transcription involves copying the genetic information from one strand of DNA into RNA. It is a highly regulated process starting at a promoter and ending at a terminator sequence.
- Transcription Unit: Promoter, Structural gene, and Terminator
- Template strand (3' to 5') vs Coding strand (5' to 3')
- RNA Polymerase: Enzyme responsible for catalysis
- Prokaryotes: Single RNA polymerase for all RNAs
- Eukaryotes: RNA Pol I (rRNA), Pol II (mRNA), Pol III (tRNA)
Translation (Protein Synthesis)
The genetic code is translated from the sequence of mRNA into a sequence of amino acids to form a polypeptide chain. This process takes place on the ribosome and involves tRNA as an adaptor molecule.
- Genetic code: Triplet, Universal, Degenerate, and Non-ambiguous
- Charging of tRNA: Aminoacylation
- Initiation: Requires start codon AUG (Methionine)
- Termination: Occurs at stop codons UAA, UAG, or UGA
- Ribosome: Composed of 23S rRNA in prokaryotes acting as a ribozyme
Exam Tip
Always draw and label the replication fork or transcription unit diagram; clear, annotated diagrams often fetch full marks even if the text explanation is concise.
Common Mistakes
- Confusing the polarity of the template strand (3' to 5') with the newly synthesized mRNA strand (5' to 3').
- Forgetting that DNA polymerase requires a pre-existing 3'-OH group to add nucleotides, necessitating RNA primers.
- Incorrectly identifying RNA Polymerase roles in Eukaryotes (Pol I, II, and III).
More Revision Notes
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