Molecular Basis of Inheritance is the single most tested chapter in the entire NEET Biology syllabus, regularly yielding 7 to 9 questions (28 to 36 marks) in a single exam paper. Every sentence in this NCERT chapter is packed with experimental evidence and biochemical terminology.
1. The DNA Replication Machinery
Replication is energetically demanding and catalyzed by a multi-enzyme complex in E. coli:
- DNA Polymerase III: Main polymerizing enzyme. Possesses high processivity (~2000 base pairs per second) and high fidelity (3' → 5' exonuclease proofreading). Can polymerize only in the 5' → 3' direction.
- RNA Primase: Synthesizes a short RNA primer (~10 nucleotides) providing the free 3'-OH group required by DNA polymerase to initiate synthesis.
- Helicase: Unwinds the double helix at the origin of replication (ori), creating the Y-shaped replication fork.
- Topoisomerase / DNA Gyrase: Relieves positive supercoiling strain ahead of the replication fork by making transient cuts in the phosphodiester backbone.
- DNA Ligase: Facilitates phosphodiester bond formation between adjacent Okazaki fragments on the lagging strand.
2. Transcription: The Transcription Unit
A transcription unit in DNA is defined by three components:
- A Promoter: Located towards the 5'-end (upstream) of the structural gene (with reference to the coding strand). Binding site for RNA polymerase (sigma factor in prokaryotes).
- The Structural Gene: Monocistronic in eukaryotes (codes for single polypeptide); polycistronic in prokaryotes.
- A Terminator: Located towards the 3'-end (downstream) of the coding strand; specifies termination of transcription (rho factor in prokaryotes).
NTA NEET Strand Convention Rule:
The strand with 3' → 5' polarity acts as the template strand. The other strand with 5' → 3' polarity is called the coding strand (even though it codes for nothing during transcription). All positions (upstream/downstream) are established strictly relative to the coding strand!
3. Post-Transcriptional Processing in Eukaryotes
Primary eukaryotic transcript (hnRNA) undergoes three processing modifications before export to cytoplasm:
- Splicing: Non-coding introns are removed and coding exons are joined in a defined order by the spliceosome complex (snRNPs).
- Capping: Addition of an unusual nucleotide—methyl guanosine triphosphate—to the 5'-end of hnRNA.
- Tailing (Polyadenylation): Addition of adenylate residues (200–300 residues) to the 3'-end in a template-independent manner.
4. The Lac Operon Mechanism
Elucidated by François Jacob and Jacques Monod. The operon consists of:
i gene(inhibitor): Codes for the repressor protein. Constitutively expressed (always ON).Promoter (p) & Operator (o): Regulatory sites.z gene: Encodes β-galactosidase (hydrolyzes lactose into glucose + galactose).y gene: Encodes permease (increases cell membrane permeability to β-galactosides).a gene: Encodes transacetylase.
In the absence of lactose: Repressor protein binds tightly to operator region (o), physically blocking RNA polymerase from transcribing the structural genes. Operon is OFF.
In the presence of lactose: Lactose enters via basal permease; converted to allolactose (inducer). Allolactose binds the repressor protein, causing a conformational change that prevents it from binding the operator. RNA polymerase accesses the promoter and transcribes z, y, a. Operon is ON.