Molecular Basis of Inheritance: Class 12 NCERT Biology Guide

DNA: The Genetic Material in Living Organisms

DNA, or Deoxyribonucleic Acid, is the molecule responsible for storing hereditary information in most living organisms. Initially, proteins were thought to carry genetic information due to their complexity. However, experiments by Griffith, Avery, MacLeod and McCarty, and Hershey and Chase conclusively proved DNA’s role as the genetic material.

DNA is made up of nucleotides, each consisting of three components:

• A nitrogenous base (adenine, thymine, guanine, or cytosine)
• A deoxyribose sugar
• A phosphate group

The sequence of these nucleotides encodes genetic instructions. DNA’s discovery as the genetic material laid the foundation for molecular biology and genetics.

Structure of DNA: Understanding the Double Helix

The structure of DNA was discovered by Watson and Crick, revealing its iconic double helix shape. This structure consists of two strands running antiparallel, twisted around each other.

Key features include:

• Sugar-phosphate backbone: Forms the outer framework of each strand.
• Nitrogenous bases: Adenine pairs with thymine (A-T), guanine pairs with cytosine (G-C) via hydrogen bonds.
• Complementary base pairing: Ensures accurate replication and transcription.

This structure explains how DNA stores genetic information and replicates precisely.

DNA Replication: Ensuring Genetic Continuity

DNA replication is the process by which DNA makes an exact copy of itself during cell division. This is vital for passing genetic information to daughter cells.

Steps of DNA replication:

1. Initiation: The double helix unwinds at origins of replication. 2. Unwinding: Helicase enzyme breaks hydrogen bonds between base pairs. 3. Primer synthesis: RNA primase synthesizes a short RNA primer. 4. Elongation: DNA polymerase adds nucleotides complementary to the template strand in the 5' to 3' direction. 5. Termination: Replication ends when the entire molecule is copied.

Worked example:

If the template strand sequence is 3'-TACGGA-5', the newly synthesized strand will be 5'-ATGCCT-3'.

This semi-conservative replication ensures each daughter DNA has one old and one new strand.

Transcription and Translation: From DNA to Protein

The genetic code in DNA is expressed through two main processes:

• Transcription: DNA is used as a template to synthesize messenger RNA (mRNA).
• Translation: mRNA directs protein synthesis at ribosomes.

Transcription details:

• RNA polymerase binds to the promoter region on DNA.
• Only one DNA strand, called the template strand, is transcribed.
• RNA is synthesized complementary to the DNA template, replacing thymine (T) with uracil (U).

Translation details:

• mRNA codons are read in sets of three nucleotides.
• Transfer RNA (tRNA) brings specific amino acids matching codons.
• Amino acids join to form polypeptides, building proteins.

These processes convert genetic information into functional molecules essential for life.

Chromosome Structure and DNA Packaging

DNA in cells is tightly packed to fit inside the nucleus. This packaging involves several levels:

• Nucleosomes: DNA wraps around histone proteins forming bead-like structures called nucleosomes.
• Chromatin: Nucleosomes coil to form chromatin fibers.
• Chromosomes: Chromatin further condenses during cell division to form visible chromosomes.

This organization protects DNA and regulates gene expression.

Applications: DNA Fingerprinting and Genetic Studies

DNA fingerprinting is a technique used to identify individuals based on variations in their DNA sequences. It relies on regions called Variable Number Tandem Repeats (VNTRs), which differ among individuals.

Applications include:

• Forensic identification
• Paternity testing
• Genetic diversity studies

Understanding the molecular basis of inheritance helps in biotechnology, medicine, and evolutionary biology.