Molecular Basis of Inheritance NEET Notes 2027: Key Points and PDF

Molecular Basis of Inheritance is Chapter 5 of NCERT Class 12 Biology and part of Unit 7, Genetics and Evolution, of the NEET UG syllabus. These notes cover DNA structure and packaging, the search for genetic material, replication, transcription, the genetic code, translation, the lac operon, the Human Genome Project and DNA fingerprinting, with a free notes PDF.

Molecular Basis of Inheritance NEET Notes

Section-wise key points from NCERT Chapter 5, comparison tables, high-yield facts and common mistakes for NEET 2027 revision.

Molecular Basis of Inheritance at a Glance

The topic list follows the NEET UG 2026 syllabus released by the National Medical Commission (NMC) on 22 December 2025, which stays the reference until the 2027 syllabus is out. The full list is on the NEET syllabus page.

What is the structure of DNA?

DNA is a long polymer of deoxyribonucleotides. Its length is given as the number of nucleotides or base pairs (bp), which is characteristic of an organism.

Polynucleotide Chain

• A nucleotide has three parts: a nitrogenous base, a pentose sugar (ribose in RNA, deoxyribose in DNA) and a phosphate group.
• Purines: adenine and guanine. Pyrimidines: cytosine, uracil and thymine. Cytosine is in both DNA and RNA; thymine (5-methyl uracil) is in DNA and uracil replaces it in RNA.
• Base + sugar (N-glycosidic linkage at the 1′ C) = nucleoside; nucleoside + phosphate at the 5′ C (phosphoester linkage) = nucleotide.
• Nucleotides join by 3′-5′ phosphodiester linkages. The chain has a free phosphate at the 5′ end and a free 3′ OH at the 3′ end; the backbone is sugar and phosphate, with bases projecting from it.
• RNA carries an extra 2′-OH group on every ribose.
• Friedrich Miescher (spelt Meischer in NCERT) first identified DNA as an acidic substance in the nucleus in 1869 and named it ‘Nuclein’.

Watson and Crick Double Helix (1953)

• Proposed by James Watson and Francis Crick using X-ray diffraction data of Maurice Wilkins and Rosalind Franklin.
• Based also on Erwin Chargaff’s rule: in double-stranded DNA the ratios A:T and G:C are constant and equal one.
• Two polynucleotide chains with a sugar-phosphate backbone and bases inside; the chains are antiparallel (one 5’→3′, the other 3’→5′).
• A pairs with T by two hydrogen bonds; G pairs with C by three. A purine always faces a pyrimidine, so the distance between strands stays uniform.
• Right-handed coil; pitch 3.4 nm with roughly 10 bp per turn, so about 0.34 nm between adjacent base pairs.
• Stacking of base pairs, in addition to hydrogen bonds, stabilises the helix.
• Central dogma (Crick): DNA → RNA → Protein. In some viruses information flows from RNA to DNA (reverse transcription).

How is DNA packaged in prokaryotes and eukaryotes?

The DNA of a typical mammalian cell (6.6 × 10⁹ bp × 0.34 × 10^-9 m/bp) would be about 2.2 metres long, far longer than a nucleus of about 10^-6 m, so it must be packaged.

• A histone octamer has eight histone molecules; negatively charged DNA wraps around it to form a nucleosome containing about 200 bp of DNA.
• Nucleosomes are the repeating unit of chromatin and appear as ‘beads-on-string’ under the electron microscope.
• Chromatin fibres coil and condense at metaphase to form chromosomes; this higher-level packaging needs NHC proteins.
• Euchromatin is loosely packed, stains light and is transcriptionally active. Heterochromatin is densely packed, stains dark and is inactive.

How was DNA proved to be the genetic material?

Three sets of experiments moved the answer from ‘some transforming principle’ to DNA.

• S strain has a mucous (polysaccharide) coat, forms smooth shiny colonies and is virulent; R strain has no coat, forms rough colonies and does not cause pneumonia.
• Heat-killed S alone did not kill mice, showing that heat did not destroy the genetic material.
• Hershey-Chase steps: infection of E. coli by labelled phages, blending to remove viral coats, then centrifugation to separate virus particles from bacteria.

Properties of Genetic Material: DNA Versus RNA

A genetic material must (i) replicate, (ii) be chemically and structurally stable, (iii) allow slow changes (mutation) for evolution and (iv) express itself as ‘Mendelian characters’.

• RNA is the genetic material in some viruses, for example Tobacco Mosaic Virus and QB bacteriophage.
• Proteins fail the very first criterion, replication.

What is the RNA world?

RNA was the first genetic material, and essential life processes such as metabolism, translation and splicing evolved around it.

• RNA acted both as genetic material and as a catalyst, but being catalytic it was reactive and unstable.
• DNA evolved from RNA with chemical modifications that make it more stable.
• Being double stranded with a complementary strand, DNA also resists change by evolving a process of repair.

How does DNA replicate?

DNA replicates semiconservatively: each daughter molecule keeps one parental strand and gets one newly synthesised strand, as Watson and Crick proposed.

Meselson and Stahl Experiment (1958)

• E. coli was grown for many generations in ¹⁵NH₄Cl (¹⁵N is a heavy, non-radioactive isotope), then moved to ¹⁴NH₄Cl.
• DNA was separated by density in a caesium chloride (CsCl) density gradient.
• After one generation (20 minutes) all DNA had hybrid (intermediate) density; after two generations (40 minutes) DNA was half hybrid and half light.
• Taylor and colleagues (1958) used radioactive thymidine on Vicia faba (faba beans) to show chromosomes also replicate semiconservatively.

Machinery and Enzymes

• E. coli (4.6 × 10⁶ bp) completes replication in 18 minutes, about 2000 bp per second.
• Synthesis is continuous on the template with 3’→5′ polarity and discontinuous on the template with 5’→3′ polarity.
• DNA polymerases cannot initiate replication on their own.
• In eukaryotes replication takes place in the S phase; failure of cell division after replication leads to polyploidy.

What happens during transcription?

Transcription copies genetic information from one strand of DNA into RNA. Adenine pairs with uracil, and only a segment of one strand is copied.

• Only one strand is copied because two strands would code for two different RNAs and proteins, and the two RNAs would pair to form double-stranded RNA that cannot be translated.

Transcription Unit

• The coding strand codes for nothing and is displaced, but all reference points of a transcription unit are given with respect to it.
• Cistron: a DNA segment coding for a polypeptide. Structural genes are monocistronic mostly in eukaryotes and polycistronic mostly in bacteria.
• Eukaryotic genes are split: exons appear in mature RNA, while introns (intervening sequences) do not.
• Regulatory sequences are loosely called regulatory genes even though they code for no RNA or protein.

Transcription in Bacteria and Eukaryotes

• In bacteria, mRNA gives the template, tRNA brings amino acids and reads the code, and rRNA plays structural and catalytic roles.
• RNA polymerase itself only catalyses elongation; it associates transiently with the initiation factor (σ) and the termination factor (ρ).
• RNA polymerase I transcribes rRNAs (28S, 18S, 5.8S); RNA polymerase II transcribes hnRNA, the precursor of mRNA; RNA polymerase III transcribes tRNA, 5S rRNA and snRNAs.
• Capping adds methyl guanosine triphosphate at the 5′ end; tailing adds 200-300 adenylate residues at the 3′ end in a template-independent manner.
• Split genes are probably an ancient feature, and splicing reflects the dominance of the RNA world.

What are the features of the genetic code?

The genetic code links the base sequence of mRNA to the amino acid sequence of a protein.

• George Gamow, a physicist, argued that 4 bases coding for 20 amino acids need a triplet code: 4³ = 64 codons.
• Har Gobind Khorana’s chemical method made RNA with defined bases; Marshall Nirenberg’s cell-free system deciphered the code; Severo Ochoa’s enzyme (polynucleotide phosphorylase) polymerised RNA without a template.

Mutations and the Genetic Code

• Point mutation: a single base change in the beta-globin gene changes glutamate to valine and causes sickle cell anaemia.
• Inserting or deleting one or two bases shifts the reading frame from that point (frameshift insertion or deletion).
• Inserting or deleting three bases or multiples of three adds or removes whole codons; the reading frame stays unaltered after that point.

tRNA: the Adapter Molecule

• Crick postulated an adapter molecule; tRNA was then known as sRNA (soluble RNA).
• tRNA has an anticodon loop complementary to the codon and an amino acid acceptor end.
• tRNAs are specific for each amino acid; a separate initiator tRNA starts translation; there are no tRNAs for stop codons.
• Its secondary structure looks like a clover-leaf; the actual 3D structure is an inverted L.

How does translation make a protein?

Translation polymerises amino acids into a polypeptide in the order set by the mRNA, joining them by peptide bonds.

• Charging (aminoacylation) of tRNA: amino acids are activated in the presence of ATP and linked to their cognate tRNA.
• The ribosome has structural RNAs and about 80 proteins, in a small and a large subunit; translation begins when the small subunit meets an mRNA.
• The large subunit has two sites for successive amino acids; 23S rRNA in bacteria acts as a ribozyme for peptide bond formation.
• Translational unit: mRNA sequence from the start codon (AUG) to the stop codon. Untranslated regions (UTRs) at the 5′ and 3′ ends are needed for efficient translation.
• Initiation: ribosome binds at AUG, recognised only by the initiator tRNA. Elongation: charged tRNAs pair with codons through anticodons as the ribosome moves codon to codon. Termination: a release factor binds the stop codon and releases the polypeptide.

How Is Gene Expression Regulated? the Lac Operon

In eukaryotes, regulation can occur at the transcriptional, processing (splicing), mRNA transport and translational levels. In prokaryotes, control of the rate of transcriptional initiation is the main site.

• Regulatory proteins act positively (activators) or negatively (repressors).
• An operator lies next to the promoter and usually binds a repressor; each operon has its own operator and repressor.
• An operon is a polycistronic structural gene regulated by a common promoter and regulatory genes; examples are lac, trp, ara, his and val operons.
• Francois Jacob (geneticist) and Jacques Monod (biochemist) were the first to elucidate a transcriptionally regulated system, the lac operon.

• No inducer: the repressor binds the operator and stops RNA polymerase from transcribing the operon.
• Inducer present (lactose or allolactose): the repressor is inactivated and transcription proceeds.
• A very low level of lac operon expression is always present; otherwise lactose cannot enter the cell.
• Glucose or galactose cannot act as inducers.
• Control by the repressor is negative regulation; the lac operon also has positive regulation, which is beyond the NCERT level.

Human Genome Project: Goals, Methods and Findings

The Human Genome Project (HGP), launched in 1990 and completed in 2003, was a 13-year mega project coordinated by the U.S. Department of Energy and the National Institutes of Health.

• The Wellcome Trust (U.K.) became a major partner; Japan, France, Germany, China and others also contributed.
• At the initial estimate of US $3 per bp for about 3 × 10⁹ bp, the cost would be about 9 billion US dollars; the sequence would fill 3300 books of 1000 pages with 1000 letters each.
• HGP was closely linked with the rise of bioinformatics.
• Goals: identify all the approximately 20,000-25,000 genes; sequence the 3 billion base pairs; store the data in databases; improve analysis tools; transfer technologies to other sectors such as industries; address ethical, legal and social issues (ELSI).
• Non-human organisms sequenced include bacteria, yeast, Caenorhabditis elegans, Drosophila, rice and Arabidopsis.

Methods

• Expressed Sequence Tags (ESTs): identifying all genes expressed as RNA. Sequence annotation: sequencing the whole genome, coding and non-coding, and assigning functions later.
• DNA was broken into random fragments, cloned in bacteria and yeast using BAC (bacterial artificial chromosome) and YAC (yeast artificial chromosome) vectors, and sequenced on automated sequencers based on Frederick Sanger’s method.
• Overlapping fragments were aligned using specialised computer programs; chromosome 1 was the last to be completed, in May 2006.
• Genetic and physical maps used polymorphism of restriction endonuclease recognition sites and repetitive sequences called microsatellites.

• Repeated sequences make up a very large portion of the genome; they have no direct coding function but shed light on chromosome structure, dynamics and evolution.

How does DNA fingerprinting work?

DNA fingerprinting is a quick way to compare the DNA of two individuals by looking at repetitive DNA, where a short stretch is repeated many times.

• Repetitive DNA separates as small peaks (satellite DNA) from the major peak of bulk DNA during density gradient centrifugation.
• Satellite DNA is classified by base composition, segment length and number of repeats into micro-satellites, mini-satellites and so on; it does not code for proteins but shows high polymorphism.
• DNA polymorphism: more than one variant (allele) at a locus in a human population with a frequency greater than 0.01. It arises by mutation and is more likely in non-coding DNA.
• The technique was developed by Alec Jeffreys using a satellite DNA probe called VNTR (Variable Number of Tandem Repeats), a mini-satellite whose size varies from 0.1 to 20 kb.
• Every tissue of a person (blood, hair follicle, skin, bone, saliva, sperm) shows the same polymorphism, and polymorphisms are inherited, so it is used in forensics and paternity testing.
• The banding pattern differs between individuals except monozygotic (identical) twins. PCR has made DNA from a single cell enough for analysis.
• Other uses: determining population and genetic diversities.

Most-asked Facts for NEET: Molecular Basis of Inheritance

Revise these one-line facts from the NCERT chapter just before the exam.

• Watson and Crick proposed the double helix in 1953 using X-ray data of Wilkins and Franklin.
• A=T has two hydrogen bonds; G≡C has three. Chargaff: A/T = G/C = 1.
• Helix pitch 3.4 nm, about 10 bp per turn, 0.34 nm between base pairs.
• Nucleosome: histone octamer with about 200 bp of DNA; histones are rich in lysine and arginine.
• Euchromatin is transcriptionally active; heterochromatin is inactive.
• Griffith (1928) used Streptococcus pneumoniae; Avery, MacLeod and McCarty identified DNA as the transforming principle.
• Hershey and Chase (1952): radioactive phosphorus labels DNA, radioactive sulfur labels protein; only labelled DNA entered E. coli.
• Meselson and Stahl (1958): ¹⁵N and CsCl density gradient proved semiconservative replication.
• E. coli replicates its DNA in 18 minutes at about 2000 bp per second.
• DNA polymerase works only 5’→3′; DNA ligase joins discontinuous fragments.
• RNA polymerase I: 28S, 18S, 5.8S rRNA; II: hnRNA; III: tRNA, 5S rRNA, snRNA.
• Capping adds methyl guanosine triphosphate at 5′; tailing adds 200-300 adenylates at 3′.
• 61 sense codons and 3 stop codons (UAA, UAG, UGA); AUG codes for methionine and starts translation.
• Sickle cell anaemia: glutamate replaced by valine in beta-globin through a point mutation.
• 23S rRNA is the ribozyme for peptide bond formation in bacteria.
• Lac operon genes: i (repressor), z (beta-galactosidase), y (permease), a (transacetylase); lactose or allolactose is the inducer.
• HGP: launched 1990, completed 2003; chromosome 1 has most genes (2968), Y fewest (231); about 1.4 million SNPs.
• DNA fingerprinting: Alec Jeffreys, VNTR probe, Southern blot and autoradiography.

Common Mistakes to Avoid

Watch for these mix-ups when you answer questions on this chapter.

• Writing that the template strand has the same sequence as mRNA. The coding strand does (with T in place of U); the template strand is complementary.
• Saying both DNA strands are copied during transcription. Only one strand of one segment is copied.
• Calling 15N radioactive. It is a heavy isotope separated only by density.
• Placing the promoter at the 3′ end. It lies upstream, towards the 5′ end of the coding strand; the terminator is downstream.
• Reading the i gene of the lac operon as ‘inducer’. It comes from ‘inhibitor’ and codes for the repressor.
• Naming glucose or galactose as inducers of the lac operon. Only lactose or allolactose induce it.
• Thinking a three-base insertion causes a frameshift. Only insertions or deletions of one or two bases (not multiples of three) shift the frame.
• Saying tRNAs exist for stop codons. There are none; a release factor recognises the stop codon.
• Calling heterochromatin the active chromatin. Euchromatin is loosely packed and active.
• Mixing up the HGP numbers: the goal mentions about 20,000-25,000 genes, while the salient features estimate 30,000 genes and 3164.7 million bp. Read the question for which figure it wants.

Molecular Basis of Inheritance Notes PDF and Related Pages

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• Source text: NCERT Chapter 5 (NCERT, Class 12 Biology)

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