Molecular Basis of Inheritance: Class 12 NCERT Biology Guide

Discovery of DNA as the Genetic Material

Before DNA was identified as the genetic material, proteins were thought to carry hereditary information due to their complexity. The journey to discover DNA's role involved key experiments:

• Griffith’s Experiment (1928): Demonstrated a 'transforming principle' transferring virulence between bacteria strains.
• Avery, MacLeod, and McCarty (1944): Isolated DNA and showed it could transform bacteria, identifying DNA as the transforming principle.
• Hershey-Chase Experiment (1952): Used radioactive labeling to confirm DNA, not protein, enters bacterial cells to carry genetic information.

These experiments collectively established DNA as the molecule responsible for inheritance, forming the foundation of molecular genetics.

Structure of DNA: The Blueprint of Life

DNA is a double-helical molecule composed of nucleotides, each containing a sugar, phosphate group, and nitrogenous base. The four bases are:

• Adenine (A)
• Thymine (T)
• Guanine (G)
• Cytosine (C)

Key features of DNA structure:

• Two antiparallel strands forming a double helix.
• Base pairing: A pairs with T, G pairs with C via hydrogen bonds.
• Sugar-phosphate backbone provides stability.

The precise base pairing allows DNA to store genetic information reliably.

DNA Replication: Copying the Genetic Code

DNA replication is the process by which a cell copies its DNA before cell division. It is semi-conservative, meaning each new DNA molecule contains one original and one new strand.

Steps of DNA replication:

1. Initiation: Enzyme helicase unwinds the double helix. 2. Primer synthesis: RNA primase synthesizes a short RNA primer. 3. Elongation: DNA polymerase adds complementary nucleotides to the template strand. 4. Termination: DNA ligase joins Okazaki fragments on the lagging strand.

Formula for semi-conservative replication:

$$ \text{Original Strand} + \text{New Strand} = \text{Daughter DNA} $$

This ensures genetic continuity from one generation to the next.

Gene Expression: From DNA to Protein

Gene expression involves converting DNA information into functional proteins through two main processes:

• Transcription: DNA-dependent RNA polymerase binds to the promoter region of a gene and synthesizes messenger RNA (mRNA) from the template strand.
• Translation: mRNA is decoded by ribosomes to assemble amino acids into a polypeptide chain.

Important terms:

• Promoter: DNA site where RNA polymerase binds.
• Template strand: DNA strand used for mRNA synthesis.

This flow of information is summarized as:

$$ \text{DNA} \rightarrow \text{RNA} \rightarrow \text{Protein} $$

Understanding gene expression is crucial for grasping how traits develop.

Chromosome Organisation and Nucleosomes

Inside the nucleus, DNA is tightly packed into chromosomes. This packaging involves DNA wrapping around histone proteins to form nucleosomes, which look like “beads on a string” under an electron microscope.

Key points:

• Nucleosomes are the fundamental units of chromatin.
• They help in compacting DNA and regulating gene expression.
• Chromatin further coils to form chromosomes visible during cell division.

This organisation protects DNA and controls access to genetic information during transcription.

Comparison: DNA vs RNA

Understanding the differences between DNA and RNA is important for Class 12 students studying molecular biology. Here is a comparison table:

This comparison helps clarify their distinct roles in inheritance and gene expression.