NEET Biology Notes: Molecular Basis of Inheritance Chapter Revision with MCQs

⚡ Quick Summary: Molecular Basis of Inheritance for NEET UG

Mastering molecular inheritance notes for neet requires a strong conceptual grip on nucleic acid organization, central dogma mechanisms, gene expression control, and genetic mapping. This high-yield NCERT revision guide simplifies key mechanisms like semi-conservative DNA replication, transcription, translation, operon models, and DNA fingerprinting into actionable insights for scoring 360/360 in NEET Biology.

NEET Biology Notes on Molecular Basis of Inheritance featuring NCERT-based chapter revision, high-yield concepts, and chapter-wise MCQs by @indiabiologyneet.

Parameter	Details
NCERT Class	Class 12 Biology (Chapter 6)
NEET Weightage	5 to 7 Questions (~20–28 Marks)
Key Focus Areas	Replication Fork Enzymes, Transcription Units, Genetic Code Properties, Lac Operon Mechanism, VNTR Profiling

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Detailed Concept Breakdown & High-Yield Notes

1. Structure of Nucleic Acids & Packaging of DNA

A nucleotide consists of a nitrogenous base, a pentose sugar, and a phosphate group linked via 3'-5' phosphodiester bonds. Nitrogenous bases comprise purines (Adenine, Guanine) and pyrimidines (Cytosine, Thymine, Uracil).

Chargaff’s Rule: Applicable strictly to double-stranded DNA. [A] + [G] = [T] + [C] or ([A]+[G]) / ([T]+[C]) = 1.
DNA Packaging in Eukaryotes: Negatively charged DNA is wrapped around a positively charged basic protein octamer called histones (rich in lysine and arginine) to form a nucleosome (~200 bp DNA). Higher packaging relies on Non-Histone Chromosomal (NHC) proteins.
Euchromatin vs Heterochromatin: Euchromatin is loosely packed, lightly stained, and transcriptionally active. Heterochromatin is densely packed, darkly stained, and transcriptionally inactive.

💡 NEET High-Yield Trick / Mnemonic: To remember histone chemical properties, think "LA-Histone": Histones are rich in Lysine and Arginine, making them basic and positively charged to tightly attract acidic, negatively charged DNA (PO₄^3- backbone).

2. DNA Replication: The Semi-Conservative Mechanism

Proven experimentally by Meselson and Stahl using heavy isotope ¹⁵N in E. coli and by Taylor et al. using radioactive ³H-thymidine in Vicia faba root tips.

Origin of Replication (ori): Specific site where replication starts. DNA unwinding by Helicase forms a Replication Fork.
DNA Polymerase: Catalyzes polymerization strictly in the 5' → 3' direction.
Leading vs Lagging Strand: The 3' → 5' template strand yields a continuous leading strand. The 5' → 3' template forms a discontinuous lagging strand made of Okazaki fragments, which are joined by DNA Ligase.

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3. Transcription: Converting DNA into RNA

A transcription unit comprises three structural elements: a Promoter (5'-upstream), a Structural Gene, and a Terminator (3'-downstream).

In prokaryotes, a single RNA Polymerase synthesizes all RNA types with the help of a Sigma (σ) factor for initiation and a Rho (ρ) factor for termination.
Eukaryotes feature three distinct RNA polymerases:
- RNA Pol I: Synthesizes rRNAs (28S, 18S, and 5.8S).
- RNA Pol II: Synthesizes precursor mRNA (hnRNA).
- RNA Pol III: Synthesizes tRNA, 5S rRNA, and snRNAs.
Post-Transcriptional Modifications in Eukaryotes:
1. Splicing: Introns (non-coding sequences) are spliced out, and exons are joined together.
2. Capping: Methyl guanosine triphosphate is added to the 5'-end.
3. Tailing: Adenylate residues (~200–300) are added to the 3'-end in a template-independent manner.

4. Translation & The Lac Operon Model

Translation: Protein synthesis occurs on ribosomes. It involves charging of tRNA (aminoacylation), initiation, elongation, and termination governed by non-catalyzed release factors responding to stop codons (UAA, UAG, UGA).

Lac Operon (Francois Jacob & Jacques Monod): A polycistronic inducible system regulated coordinately.

Regulator gene (i): Codes for the repressor protein.
Structural genes:
- lac Z: Codes for β-galactosidase (breaks lactose into glucose + galactose).
- lac Y: Codes for Permease (increases cell permeability to β-galactosides).
- lac A: Codes for Transacetylase.
In the Absence of Inducer (Lactose): Repressor binds operator, blocking RNA Polymerase → Switched OFF.
In the Presence of Inducer: Allolactose binds repressor, inactivating it → RNA Polymerase transcribes genes → Switched ON.

5. DNA Fingerprinting

Developed by Alec Jeffreys. Uses Variable Number of Tandem Repeats (VNTRs) belonging to satellite DNA as probe markers due to their high degree of polymorphism.

Key Differences: Prokaryotic vs Eukaryotic Transcription

Feature	Prokaryotic Transcription	Eukaryotic Transcription
Cellular Location	Cytoplasm (coupled directly with translation)	Nucleus (transcription separated from translation)
RNA Polymerase	Single RNA Polymerase for all RNA types	Three specialized polymerases (RNA Pol I, II, III)
Gene Structure	Polycistronic genes (multiple proteins from one mRNA)	Monocistronic genes (single protein per mRNA)
RNA Processing	No processing required; transcript is functional mRNA	Requires post-transcriptional processing (Splicing, Capping, Tailing)

Top 10 High-Yield Practice MCQs for NEET UG

Q1. If a double-stranded DNA molecule contains 30% Cytosine, what will be the percentage of Thymine in this DNA?

(A) 30%
(B) 20%
(C) 40%
(D) 60%