NEET biology molecular basis of inheritance Short Notes 2027, Chapter 5 NCERT+PYQ

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l am a NEET Dropper. Today I'm sharing complete notes molecular basis of inheritance Class 12 biology Chapter 5 *Molecular Basis of Inheritance - Short Notes* 

*1. DNA: The Genetic Material*  

*Why DNA, not RNA?* DNA is more stable due to 2'-H in deoxyribose, thymine instead of uracil, and double helix structure. RNA acts as genetic material only in some viruses like TMV.

*2. Structure of DNA*  

*Double Helix Model*: Given by Watson & Crick based on Franklin's X-ray data.  

- *Backbone*: Sugar-phosphate on outside  

- *Bases*: A-T with 2 H-bonds, G-C with 3 H-bonds, inside  

- *Antiparallel*: One strand 5'→3', other 3'→5'  

- *Pitch*: 3.4 nm, 10 base pairs per turn, 0.34 nm per bp

5'  —S—P—S—P—S—  3'

        |   |   |   |

        A   T   G   C

        |   |   |   |

        T   A   C   G

3'  —S—P—S—P—S—  5'

S = Deoxyribose Sugar, P = Phosphate

*3. DNA Packaging*  

*In Eukaryotes*: DNA + Histone = Nucleosome. 8 histones make histone octamer. Nucleosomes form chromatin → condensed to form chromosomes.  

*Heterochromatin*: Dark, transcriptionally inactive  

*Euchromatin*: Light, transcriptionally active

*4. DNA Replication*  

*Semiconservative*: Meselson-Stahl experiment proved it using N¹⁵.  

*Main Enzymes*:  

- *Helicase*: Unwinds DNA  

- *DNA Polymerase*: Synthesizes new strand 5'→3', proofreading also  

- *Ligase*: Joins Okazaki fragments on lagging strand

*5. Central Dogma*  

DNA → Transcription → RNA → Translation → Protein  

Proposed by Crick. Retroviruses show reverse transcription: RNA → DNA.

*6. Transcription*  

*In Prokaryotes*: 1 RNA polymerase does all.  

*In Eukaryotes*: RNA Pol I = rRNA, RNA Pol II = mRNA, RNA Pol III = tRNA.  

*Steps*: Initiation by sigma factor, Elongation, Termination by rho factor.  

*Post-transcriptional changes*: Capping at 5', Tailing at 3', Splicing to remove introns.

*7. Genetic Code*  

Triplet, Degenerate, Universal, Non-overlapping.  

*Start codon*: AUG → Methionine  

*Stop codons*: UAA, UAG, UGA

*8. Translation*  

*tRNA*: Adapter molecule with anticodon loop.  

*Ribosome*: 2 sites - P site and A site.  

*Steps*: Charging of tRNA → Initiation → Elongation → Termination.

*9. Regulation: Lac Operon*  

*In E. coli*: Negative inducible operon.  

*No Lactose*: Repressor binds to operator, genes off.  

*With Lactose*: Lactose = inducer, binds repressor, genes on → β-galactosidase made.

*10. Human Genome Project*  

Launched 1990, completed 2003.  

*Findings*: 3 billion bp, ∼30,000 genes, 99.9% DNA same in all humans, <2% codes for proteins.

*11. DNA Fingerprinting*  

Technique by Alec Jeffreys. Uses VNTRs = Variable Number Tandem Repeats.  

*Steps*: Isolation → Digestion → Electrophoresis → Southern Blotting → Hybridization → Autoradiography.

*NEET Quick Points*  

1. Griffith: Transformation, Avery-MacLeod-McCarty: DNA is genetic material

2. Hershey-Chase: DNA not protein is genetic material in bacteriophage  

3. Chargaff Rule: A=T, G=C

4. Uracil replaces Thymine in RNA