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Class 12 Biology · Chapter 5

Molecular Basis of Inheritance — Complete NEET & CBSE Notes

The full story of how a molecule barely 2 nanometres wide carries the instructions for an entire organism — DNA structure, replication, the Central Dogma, transcription, translation, the genetic code, the Human Genome Project, and DNA fingerprinting — explained the simple way, with MCQs and a rapid revision sheet.

NCERT 2026–27 aligned NEET · CUET · CTET · UPSC basics ~28 min read Jnaanangkur – The Learning Hub

Overview DNA & RNA Replication Central Dogma Transcription Genetic Code Translation Gene Regulation HGP DNA Fingerprinting Scientists Common Mistakes Revision Points Exam Q&A MCQs Summary FAQs

01Chapter Overview

Molecular Basis of Inheritance explains how hereditary information is stored, copied, and expressed at the molecular level. It picks up where Mendelian genetics leaves off — moving from "genes control traits" to "here is exactly what a gene is made of and how it works." This chapter is one of the highest-weightage chapters for NEET Biology and forms the backbone of modern biotechnology.

02DNA and RNA — Structure & Function

The two nucleic acids that store and transmit genetic information.

Definition A nucleic acid is a polymer of nucleotides. Each nucleotide = nitrogenous base + pentose sugar + phosphate group. A nucleotide without the phosphate is called a nucleoside.

Nitrogenous bases

• Purines (double ring): Adenine (A), Guanine (G)
• Pyrimidines (single ring): Cytosine (C), Thymine (T — DNA only), Uracil (U — RNA only)

NCERT Fig. 6.5 style — Watson & Crick double helix model

Watson–Crick model of DNA (1953) — key features

• DNA is made of two polynucleotide chains coiled in a right-handed double helix.
• The two strands are antiparallel (one 5'→3', the other 3'→5').
• Backbone of sugar-phosphate is on the outside; bases point inward.
• Base pairing is complementary: A pairs with T (2 H-bonds), G pairs with C (3 H-bonds) — this is Chargaff's rule in action.
• Pitch of helix = 3.4 nm, containing 10 base pairs per turn; rise per base pair = 0.34 nm.

Memory trick "Pure As Gold" — Purines are Adenine and Guanine. And remember G-C is the "triple-strength" bond (3 H-bonds) — G and C are both round and curvy like the number 3.

DNA vs RNA — comparison table

Feature	DNA	RNA
Sugar	Deoxyribose	Ribose
Strands	Double stranded (usually)	Single stranded (usually)
Pyrimidine unique to it	Thymine	Uracil
Stability	More stable (no 2'-OH)	Less stable
Function	Stores genetic information	Expresses genetic information (m/t/rRNA)
Location	Nucleus, mitochondria, chloroplast	Nucleus & cytoplasm

Types of RNA and their roles

RNA	Full form	Function
mRNA	Messenger RNA	Carries genetic message from DNA to ribosome (template for translation)
tRNA	Transfer RNA	Adapter molecule; brings specific amino acid to ribosome
rRNA	Ribosomal RNA	Structural & catalytic component of ribosome

Common Confusion Some plant viruses (e.g., TMV) and retroviruses (e.g., HIV) use RNA as genetic material, not DNA — don't assume DNA is universal.

03DNA Replication

The Meselson–Stahl experiment (1958) proved DNA replicates semiconservatively — each daughter molecule has one parental (old) strand and one newly synthesised strand.

Definition Replication is the process by which a DNA molecule makes an identical copy of itself before cell division.

Brief flow of replication

1. Helicase unwinds and unzips the double helix, breaking hydrogen bonds → forms a replication fork.
2. DNA-dependent DNA polymerase adds nucleotides one by one only in the 5'→3' direction.
3. Leading strand is synthesised continuously; lagging strand is synthesised discontinuously as Okazaki fragments.
4. DNA ligase joins the Okazaki fragments into a continuous strand.
5. Result: two DNA molecules, each with one old + one new strand.

Memory trick "Lagging is always Late" — the lagging strand is synthesised in fragments and joined later by ligase.

NCERT Fig. 6.8 style — Replication fork

04The Central Dogma

Proposed by Francis Crick (1958), the Central Dogma describes the normal one-way flow of genetic information:

DNA  →  RNA  →  Protein
(replication)     (transcription)     (translation)

In retroviruses (e.g., HIV), information can also flow RNA → DNA using the enzyme reverse transcriptase — this is called reverse transcription, an exception to the usual direction.

05Transcription

Definition Transcription is the copying of genetic information from one strand of DNA into mRNA.

Key terms

• Template strand: the DNA strand that is actually copied (3'→5').
• Coding strand: the other DNA strand, with the same sequence as mRNA (except T→U).
• Transcription unit has three parts: a promoter, the structural gene, and a terminator.

Easy step-by-step explanation

1. Initiation: RNA polymerase binds to the promoter region.
2. Elongation: RNA polymerase moves along the template strand, adding complementary ribonucleotides (A-U, G-C).
3. Termination: RNA polymerase reaches the terminator and releases the newly formed RNA.

Post-transcriptional processing (in eukaryotes, forming mature mRNA)

• Capping: addition of methyl guanosine triphosphate cap at the 5' end.
• Tailing: addition of poly-A tail (200–300 adenine residues) at the 3' end.
• Splicing: removal of non-coding introns and joining of coding exons.

Memory trick "Exons EXit the nucleus (they are expressed); Introns stay IN (they are cut out)."

06Genetic Code

Definition The genetic code is the relationship between the sequence of nucleotides in mRNA (read as codons) and the sequence of amino acids in a protein. Cracked mainly by Har Gobind Khorana and Marshall Nirenberg.

Characteristics of the genetic code

Property	Meaning
Triplet	Each codon = 3 nucleotides; codes for 1 amino acid
Degenerate	Most amino acids are coded by more than one codon
Unambiguous	One codon codes for only one amino acid
Commaless	Codons are read continuously, no gaps between them
Universal	Same codon codes for the same amino acid across almost all organisms
Non-overlapping	One nucleotide is not shared between two codons
Start codon	AUG — codes for methionine, also initiates translation
Stop codons	UAA, UAG, UGA — do not code for any amino acid

Memory trick for stop codons "U A A, U A G, U G A — U Always Goes Away" (all three stop codons start with U).

07Translation

Definition Translation is the process of polymerising amino acids to form a polypeptide, with the order/sequence of amino acids decided by the sequence of bases in mRNA.

Easy explanation — three stages

1. Initiation: Small ribosomal subunit binds to mRNA at the start codon (AUG); initiator tRNA carrying methionine attaches.
2. Elongation: Ribosome moves codon by codon; tRNAs bring matching amino acids (anticodon pairs with codon); peptide bonds form between amino acids.
3. Termination: Ribosome reaches a stop codon; release factor releases the completed polypeptide.

NCERT Fig. 6.11 style — Translation on ribosome

08Regulation of Gene Expression — Lac Operon

The lac operon (Jacob & Monod, E. coli) is the classic NCERT example of gene regulation, consisting of a regulatory gene (i) and structural genes z, y, a under one promoter.

• In absence of lactose: repressor binds operator → transcription blocked (operon OFF).
• In presence of lactose: lactose (inducer) binds repressor → repressor releases operator → transcription proceeds (operon ON).

This is called negative regulation since the operon is controlled by a repressor.

09Human Genome Project (HGP) — Key Facts

Definition HGP was a mega international collaborative project to determine the complete sequence of the ~3.3 billion base pairs of the human genome and identify all human genes.

Fact	Detail
Duration	1990 – 2003 (13 years)
Coordinated by	US Department of Energy & National Institutes of Health (international consortium)
Genome size	~3.3 billion base pairs (bp)
Number of genes	~20,000–25,000 (far fewer than earlier estimates)
Average gene size	~3,000 bases
Chromosome with most genes	Chromosome 1
Chromosome with fewest genes	Y chromosome
% of genome coding for protein	Less than 2%
Repetitive sequences	More than 50% of genome
Similarity between all humans	99.9% identical in DNA sequence
Sequencing method	ESTs (Expressed Sequence Tags) & STS (Sequence Tag Sites) using automated DNA sequencers
Indian institute involved	CCMB, Hyderabad

Memory trick "Human Genome = 3.3 billion letters, but less than 2% is the actual 'story' (protein-coding) — the rest is footnotes and blank pages (repeats/non-coding)."

10DNA Fingerprinting

Definition DNA fingerprinting is a technique to identify variation in specific regions of DNA (satellite/repetitive DNA) that differ between individuals, developed by Alec Jeffreys. In India it was pioneered by Dr. Lalji Singh at CCMB, Hyderabad.

Principle

Satellite DNA regions (short tandem repeats) show a high degree of polymorphism — the number of repeats varies between individuals — and follow Mendelian inheritance, making them ideal genetic markers for identification.

Steps involved

1. Isolation of DNA from the sample (blood, hair root, saliva, semen etc.)
2. Digestion of DNA using restriction endonucleases
3. Separation of fragments by gel electrophoresis (by size)
4. Transfer of separated fragments to a membrane (Southern blotting)
5. Hybridisation using labelled VNTR probe
6. Detection of hybridised fragments as a banding pattern (autoradiography)

Applications

• Forensic science — crime scene and paternity investigations
• Identifying disaster victims
• Determining population and genetic diversity studies
• Evolutionary studies

DNA fingerprinting — banding pattern comparison

11Important Scientists & Their Contributions

1869

Friedrich Miescher

Discovered DNA (called it "nuclein") from pus cell nuclei.

1928

Frederick Griffith

Transformation experiment in Streptococcus pneumoniae; showed a "transforming principle" exists.

1944

Avery, MacLeod & McCarty

Proved DNA (not protein) is the transforming principle / genetic material.

1952

Hershey & Chase

Blender experiment with bacteriophage confirmed DNA is the genetic material.

1953

Watson & Crick

Proposed the double helix structure of DNA (using Rosalind Franklin's X-ray data).

1950s

Erwin Chargaff

Chargaff's rule: A=T and G=C in DNA.

1952

Rosalind Franklin

X-ray diffraction images (Photo 51) that revealed DNA's helical structure.

1958

Meselson & Stahl

Proved semiconservative replication of DNA.

1958

Francis Crick

Proposed the Central Dogma of molecular biology.

1961–66

Nirenberg & Khorana

Deciphered the genetic code (codon assignments).

1961

Jacob & Monod

Proposed the lac operon model of gene regulation.

1984

Alec Jeffreys

Developed DNA fingerprinting technique.

12Common Mistakes Students Make

Watch out

• Confusing template strand (copied) with coding strand (same sequence as mRNA).
• Writing DNA polymerase as functioning 3'→5'; it actually synthesises only 5'→3'.
• Thinking the genetic code is "ambiguous" because of degeneracy — degenerate ≠ ambiguous. One codon still gives only one amino acid.
• Mixing up exons and introns — exons are expressed (coding), introns are intervening (non-coding, removed).
• Assuming all viruses have DNA — many (TMV, HIV, influenza) have RNA genomes.
• Forgetting that DNA replication is semiconservative, not conservative or dispersive.
• Mixing up Griffith's experiment (showed transformation exists) with Avery-MacLeod-McCarty (identified DNA as the agent).

13One-Line Revision Points (30)

1. DNA is a double-stranded, antiparallel, right-handed helix (Watson-Crick model, 1953).
2. A nucleotide = nitrogenous base + pentose sugar + phosphate.
3. Purines: Adenine, Guanine (double ring); Pyrimidines: Cytosine, Thymine, Uracil (single ring).
4. A=T (2 H-bonds); G≡C (3 H-bonds).
5. One helical turn of DNA = 3.4 nm = 10 base pairs.
6. Griffith (1928) discovered the "transforming principle" in pneumococcus bacteria.
7. Avery, MacLeod & McCarty (1944) proved DNA is the transforming/genetic material.
8. Hershey-Chase (1952) confirmed DNA as genetic material using bacteriophages.
9. DNA replication is semiconservative (Meselson-Stahl, 1958).
10. Replication occurs in 5'→3' direction only.
11. Leading strand: continuous synthesis; Lagging strand: discontinuous (Okazaki fragments).
12. DNA ligase joins Okazaki fragments.
13. Central Dogma: DNA → RNA → Protein (Crick, 1958).
14. Reverse transcription (RNA→DNA) occurs in retroviruses via reverse transcriptase.
15. Transcription unit has promoter, structural gene, and terminator.
16. Only one strand of DNA (template strand) is transcribed.
17. hnRNA is processed by capping, tailing, and splicing to form mature mRNA.
18. Exons are coding/expressed sequences; introns are non-coding and removed.
19. Genetic code is a triplet code — 3 nucleotides = 1 codon = 1 amino acid.
20. AUG is the start codon (codes for methionine).
21. UAA, UAG, UGA are stop/non-sense codons.
22. Genetic code is degenerate (many codons per amino acid) but unambiguous.
23. Genetic code is nearly universal across all organisms.
24. Translation converts mRNA codon sequence into a polypeptide chain.
25. tRNA is the adapter molecule with an anticodon that pairs with mRNA codon.
26. Ribosome is the site of protein synthesis; rRNA plays catalytic role.
27. Lac operon (Jacob & Monod) regulates lactose metabolism genes in E. coli.
28. Human Genome Project (1990–2003) sequenced ~3.3 billion base pairs.
29. Less than 2% of the human genome codes for proteins.
30. DNA fingerprinting uses satellite DNA (VNTRs) and was developed by Alec Jeffreys; pioneered in India by Lalji Singh.

14Previous Year & Expected Exam Questions

Q1. Which experiment proved that DNA (and not protein) is the genetic material? (NEET-type)

Ans: Hershey and Chase's bacteriophage blender experiment (1952), using radioactively labelled DNA (³²P) and protein (³⁵S).

Q2. Why is DNA replication called semiconservative?

Ans: Because each of the two daughter DNA molecules retains one parental (old) strand and gains one newly synthesised strand, as proven by Meselson and Stahl.

Q3. Name the enzyme that removes RNA primers and joins Okazaki fragments during replication. (CTET/State TET-type)

Ans: DNA polymerase I removes RNA primers; DNA ligase joins the Okazaki fragments.

Q4. What percentage of the human genome codes for proteins, as per HGP findings? (UPSC Prelims basics-type)

Ans: Less than 2% of the human genome codes for proteins; the rest is regulatory or repetitive/non-coding DNA.

Q5. State any two characteristics of the genetic code.

Ans: (i) It is a triplet code — three nucleotides code for one amino acid. (ii) It is nearly universal — the same codon specifies the same amino acid across almost all living organisms.

Q6. Who is regarded as the "Father of DNA fingerprinting" in India?

Ans: Dr. Lalji Singh, who developed the technique at CCMB, Hyderabad.

15MCQs with Explanations

1. In a DNA molecule, which base pair is joined by three hydrogen bonds?

1. A-T
2. G-C
3. A-G
4. T-C

Correct: B) G-C. Guanine and Cytosine are joined by three hydrogen bonds, making G-C rich DNA more thermally stable than A-T rich DNA.

2. The semiconservative nature of DNA replication was experimentally proved by:

1. Griffith
2. Hershey and Chase
3. Meselson and Stahl
4. Watson and Crick

Correct: C) Meselson and Stahl, using ¹⁵N and ¹⁴N labelled DNA in E. coli (1958).

3. Okazaki fragments are formed on the:

1. Leading strand
2. Lagging strand
3. Both strands equally
4. mRNA strand

Correct: B) Lagging strand, since DNA polymerase can synthesise only in the 5'→3' direction, forcing discontinuous synthesis here.

4. Which of the following is NOT a characteristic of the genetic code?

1. Triplet
2. Degenerate
3. Ambiguous
4. Universal

Correct: C) Ambiguous. The genetic code is unambiguous — one codon codes for only one specific amino acid.

5. The enzyme reverse transcriptase is associated with:

1. Bacteriophages
2. Retroviruses
3. Bacteria only
4. Plant viruses only

Correct: B) Retroviruses (e.g., HIV) use reverse transcriptase to synthesise DNA from an RNA template.

6. In the lac operon, lactose acts as a/an:

1. Repressor
2. Inducer
3. Promoter
4. Terminator

Correct: B) Inducer — lactose binds the repressor protein, inactivating it and allowing transcription of the operon.

7. hnRNA is converted to mRNA by which of the following processes?

1. Replication only
2. Translation only
3. Capping, tailing and splicing
4. Denaturation

Correct: C) Capping, tailing and splicing convert heterogeneous nuclear RNA (hnRNA) into mature mRNA.

8. The Human Genome Project was declared complete in the year:

1. 1990
2. 2000
3. 2003
4. 2010

Correct: C) 2003, though a working draft was announced in 2000; the project ran from 1990 to 2003.

9. DNA fingerprinting primarily makes use of variation in:

1. Structural genes
2. Satellite DNA (VNTRs)
3. rRNA genes
4. Promoter sequences

Correct: B) Satellite/repetitive DNA regions (VNTRs), which show high polymorphism between individuals.

10. Which scientist(s) proposed the double helix model of DNA?

1. Nirenberg and Khorana
2. Jacob and Monod
3. Watson and Crick
4. Avery and MacLeod

Correct: C) James Watson and Francis Crick (1953), using Rosalind Franklin's X-ray diffraction data.

11. The stop codons in the genetic code are:

1. AUG, UAA, UAG
2. UAA, UAG, UGA
3. AUG, UGA, UAC
4. UAC, UAG, UAA

Correct: B) UAA, UAG, UGA — these do not code for any amino acid and signal termination of translation.

12. The Hershey-Chase experiment used which radioactive isotopes to label protein and DNA respectively?

1. ³²P and ³⁵S
2. ³⁵S and ³²P
3. ¹⁴C and ³H
4. ¹⁵N and ¹⁴N

Correct: B) ³⁵S labelled protein (as sulfur is absent in DNA) and ³²P labelled DNA (as phosphorus is absent in most amino acids).

13. Which of these is a purine base?

1. Cytosine
2. Thymine
3. Guanine
4. Uracil

Correct: C) Guanine (along with Adenine) is a purine, having a double-ring structure.

14. Approximately what fraction of the human genome codes for proteins?

1. Less than 2%
2. About 25%
3. About 50%
4. More than 75%

Correct: A) Less than 2% of the human genome codes for proteins, as revealed by the Human Genome Project.

15. The technique used to separate DNA fragments by size in DNA fingerprinting is:

1. Centrifugation
2. Gel electrophoresis
3. Chromatography
4. Southern blotting only

Correct: B) Gel electrophoresis separates DNA fragments by size; Southern blotting is a later step to transfer and probe the fragments.

16. Griffith's transformation experiment used which two strains of bacteria?

1. Rough (R) and Smooth (S) strains of pneumococcus
2. Gram-positive and Gram-negative E. coli
3. Lytic and lysogenic phage strains
4. Wild-type and mutant Drosophila

Correct: A) Griffith used non-virulent Rough (R) and virulent Smooth (S) strains of Streptococcus pneumoniae.

17. Which RNA acts as an adapter molecule during translation?

1. mRNA
2. tRNA
3. rRNA
4. hnRNA

Correct: B) tRNA carries a specific amino acid and has an anticodon that pairs with the mRNA codon.

16Chapter Summary & Quick Revision Sheet

Quick Revision Sheet

Topic	One-line recall
DNA structure	Double helix, antiparallel, A=T, G≡C, 3.4 nm/turn
Genetic material proof	Griffith → Avery-MacLeod-McCarty → Hershey-Chase
Replication	Semiconservative, 5'→3', leading/lagging strands
Central Dogma	DNA → RNA → Protein (reverse in retroviruses)
Transcription	Promoter–gene–terminator; capping, tailing, splicing
Genetic code	Triplet, degenerate, unambiguous, universal, AUG start, 3 stop codons
Translation	Initiation, elongation, termination on ribosome
Gene regulation	Lac operon — negative regulation by repressor
HGP	1990–2003, 3.3 billion bp, <2% coding, ~20,000–25,000 genes
DNA fingerprinting	Satellite DNA/VNTRs, Jeffreys; forensic & paternity use

17FAQs for Students

Is Chapter 5, Molecular Basis of Inheritance, important for NEET?

Yes — it is one of the highest-scoring and most frequently tested chapters in NEET Biology, especially topics like DNA replication, transcription, translation, genetic code, and the Human Genome Project.

What is the easiest way to remember the genetic code characteristics?

Remember the acronym set: Triplet, Degenerate, Unambiguous, Commaless, Universal, Non-overlapping — practice writing these six words with one example each until they stick.

What is the difference between a gene and a genome?

A gene is a single functional unit of heredity coding for a specific product (protein/RNA), while the genome is the entire set of genetic material (all genes and non-coding DNA) present in an organism.

Why is DNA more stable than RNA?

DNA has deoxyribose sugar (lacking a reactive 2'-OH group) and is usually double-stranded, both of which make it chemically more stable than the single-stranded, ribose-containing RNA.

How many marks does this chapter usually carry in CBSE board exams?

It typically carries significant weightage in board exams, often combining short-answer, diagram-based, and long-answer questions across topics like replication, transcription, translation, and the genetic code.

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