Class 12 Biology: Molecular Basis of Inheritance NCERT Book Free PDF (2025-26)

A molecule must meet the following four criteria to be considered a genetic material:

• It should be able to generate its own replica (Replication).
• It must be chemically and structurally stable to ensure continuity across generations.
• It must provide the scope for slow, stable changes (mutation) that are required for evolution.
• It must be able to express itself in the form of 'Mendelian Characters' through a process like transcription and translation.

DNA is preferred over RNA as the primary genetic material because it is more stable. The 2'-OH group present in every nucleotide of RNA is a reactive group, making RNA more labile and easily degradable. In contrast, DNA's double-stranded nature and the absence of this group make it chemically less reactive and structurally more stable, which is crucial for storing genetic information with high fidelity.

The Hershey and Chase experiment (1952) conclusively proved that DNA is the genetic material using bacteriophages. Here's how:

• Experimental Setup: They grew two batches of bacteriophages. One batch was grown in a medium containing radioactive phosphorus (³²P), which got incorporated into the phage DNA (as phosphorus is part of DNA but not protein). The other batch was grown in a medium with radioactive sulfur (³⁵S), which was incorporated into the phage's protein coat (as sulfur is in amino acids but not DNA).
• Infection: These radioactive phages were allowed to infect E. coli bacteria.
• Blending and Centrifugation: After infection, the cultures were agitated in a blender to separate the viral coats from the bacteria. The mixture was then centrifuged to separate the heavier bacterial cells (pellet) from the lighter viral particles (supernatant).
• Observation and Conclusion: It was observed that in the culture with ³²P-labelled phages, the radioactivity was found inside the bacterial cells (in the pellet). In the culture with ³⁵S-labelled phages, the radioactivity was found in the supernatant. This demonstrated that only the DNA from the virus entered the bacterial cell, while the protein coat remained outside. Since the bacteria then produced new phages, the genetic material responsible for this must have been DNA.

In the process of protein synthesis (translation), codons on the mRNA have specific roles:

• Start Codon (AUG): This codon has a dual function. Firstly, it signals the ribosome to initiate the process of translation. Secondly, it codes for the amino acid Methionine. Therefore, the first amino acid in most newly synthesised polypeptides is methionine.
• Stop Codons (UAA, UAG, UGA): These codons are also known as termination or nonsense codons. They do not code for any amino acid. Instead, they act as signals for the termination of polypeptide synthesis. When a ribosome encounters a stop codon on the mRNA, it releases the completed polypeptide chain.

The lac operon in E. coli is an inducible operon that regulates the metabolism of lactose. When the inducer, lactose (or allolactose), is present in the medium, the following occurs:

• Inducer Binding: The inducer binds to the repressor protein, which is produced by the regulator gene (i gene).
• Inactivation of Repressor: This binding causes a conformational change in the repressor protein, making it inactive. The inactive repressor can no longer bind to the operator region (o) of the operon.
• Transcription Initiation: With the operator region free, the RNA polymerase enzyme can now freely bind to the promoter region (p) and move along the DNA to transcribe the structural genes (z, y, and a).
• Synthesis of Enzymes: The transcription of the structural genes results in the production of a polycistronic mRNA. This mRNA is then translated to produce three enzymes:
• β-galactosidase (from z gene): Hydrolyzes lactose into glucose and galactose.
• Permease (from y gene): Increases the permeability of the cell to lactose.
• Transacetylase (from a gene): Has a role in detoxifying by-products.

In this way, the presence of lactose itself switches the operon 'on', allowing the bacterium to produce the enzymes needed to digest it. This is a crucial mechanism for energy conservation.

The genetic code has several key features that are important for the Class 12 syllabus:

• Degenerate: The code is degenerate because some amino acids are coded for by more than one codon. For instance, the amino acid Leucine is specified by six different codons (e.g., CUU, CUC, CUA, CUG). This provides a buffer against the effects of some point mutations.
• Universal: The code is nearly universal, meaning that a particular codon specifies the same amino acid in all organisms, from bacteria to humans. For example, the codon AUG codes for Methionine in all known living systems. This universality is strong evidence for common ancestry.
• Unambiguous: The code is unambiguous because one specific codon will always code for only one specific amino acid. It never codes for two different ones. For example, the codon GAG will always code for Glutamic acid and nothing else. This ensures high fidelity in protein synthesis.

Three significant goals of the Human Genome Project (HGP) as per the NCERT syllabus are:

• To identify all the approximately 20,000-25,000 genes in human DNA.
• To determine the sequences of the 3 billion chemical base pairs that make up human DNA.
• To store this vast amount of information in databases and improve tools for data analysis.

HGP is termed a 'mega project' due to its immense scale and requirements. Sequencing 3 billion base pairs was a massive undertaking, requiring international collaboration, significant funding (approximately 9 billion US dollars), and the development of new, high-speed computational technologies for data storage, retrieval, and analysis.

The semi-conservative model of DNA replication means that when a double helix replicates, each of the two daughter molecules will have one old or 'parental' strand and one newly synthesised strand.

This was experimentally proven by Matthew Meselson and Franklin Stahl in 1958:

• They grew E. coli in a medium containing ¹⁵NH₄Cl (a heavy isotope of nitrogen) for many generations. As a result, the ¹⁵N was incorporated into the bacterial DNA, making it heavy.
• They then transferred these bacteria into a medium with normal ¹⁴NH₄Cl and allowed them to replicate.
• After one generation (20 minutes), they isolated the DNA and found its density was intermediate between ¹⁵N-DNA and ¹⁴N-DNA. This showed that each new DNA molecule was a hybrid, containing one old (¹⁵N) and one new (¹⁴N) strand.
• After a second generation, they found two types of DNA: one of intermediate density and one of light (¹⁴N) density. This result was only possible if replication was semi-conservative.

A transcription unit in DNA, which is essential for initiating protein synthesis, consists of three primary regions:

• A Promoter: This is a DNA sequence located towards the 5'-end (upstream) of the structural gene. It provides the binding site for RNA polymerase, the enzyme that carries out transcription. The presence of a promoter defines the template and coding strands.
• The Structural Gene: This is the stretch of DNA that is actually transcribed into RNA. It contains the genetic code that will eventually be translated into a protein.
• A Terminator: This sequence is located towards the 3'-end (downstream) of the coding strand. It signals the end of the transcription process. Once RNA polymerase reaches the terminator sequence, it stops transcription and releases the newly formed RNA molecule.

While DNA fingerprinting is famous for its use in forensic science to identify criminals, it has several other crucial applications that are important to know for the CBSE 2025-26 exams:

• Paternity Testing: It is used as the definitive method to resolve disputes of parentage by comparing the DNA profile of a child with that of the potential parents.
• Diagnosis of Genetic Diseases: It helps in identifying inherited disorders and genetic abnormalities by analysing the DNA of individuals.
• Conservation of Wildlife: DNA fingerprinting is used to study the genetic diversity within endangered species, prevent illegal poaching by identifying the source of animal products, and manage breeding programs in zoos.
• Studying Evolutionary Biology: By comparing the DNA fingerprints of different species, scientists can determine evolutionary relationships and trace phylogenetic lineages.