Molecular Basis of Inheritance: Class 12 NCERT Biology Explained

Structure and Function of DNA in Inheritance

DNA (Deoxyribonucleic Acid) is the hereditary material in most organisms. It is a double helix formed by two complementary strands of nucleotides. Each nucleotide consists of a sugar (deoxyribose), a phosphate group, and a nitrogenous base (Adenine, Thymine, Guanine, or Cytosine).

• The bases pair specifically: Adenine (A) pairs with Thymine (T), and Guanine (G) pairs with Cytosine (C).
• This complementary base pairing is crucial for DNA replication and transcription.
• DNA stores genetic instructions required for the development, functioning, and reproduction of organisms.

Understanding DNA’s structure is foundational to grasping how inheritance occurs at the molecular level, as explained in the Class 12 NCERT syllabus.

DNA Replication: Mechanism and Enzymes Involved

DNA replication is the process by which a cell duplicates its DNA before cell division, ensuring genetic continuity.

Key features of DNA replication:

• It is semi-conservative: each new DNA molecule has one parental and one new strand.
• Replication starts at specific sites called origins of replication.

Enzymes involved:

• Helicase unwinds the double helix.
• Primase synthesizes RNA primers.
• DNA polymerase adds nucleotides complementary to the template strand.
• Ligase joins Okazaki fragments on the lagging strand.

Leading vs Lagging Strand:

• Leading strand is synthesized continuously.
• Lagging strand is synthesized discontinuously as Okazaki fragments.

Proofreading by DNA polymerase ensures high fidelity during replication.

Transcription: From DNA to RNA

Transcription is the process of synthesizing RNA from a DNA template.

• RNA polymerase binds to a specific DNA region called the promoter.
• It reads the template strand of DNA to synthesize a complementary RNA strand.
• The RNA produced is usually messenger RNA (mRNA), which carries genetic information to ribosomes.

Steps in Transcription: 1. Initiation: RNA polymerase binds to promoter. 2. Elongation: RNA strand is synthesized. 3. Termination: Transcription ends at terminator sequences.

Transcription is the first step in gene expression and is essential for protein synthesis.

Chromatin Structure and Role of Nucleosomes

DNA in eukaryotic cells is packaged into chromatin to fit inside the nucleus.

• Chromatin appears as 'beads on a string' under an electron microscope.
• Each 'bead' is a nucleosome, consisting of DNA wrapped around histone proteins.
• Nucleosomes help in DNA compaction and regulate gene expression.

This packaging is vital for protecting DNA and controlling access during replication and transcription.

Genetic Code and Translation: Protein Synthesis

The genetic code is the set of rules by which information encoded in mRNA is translated into proteins.

• Each group of three nucleotides in mRNA is called a codon.
• Codons specify amino acids or signal stop/start of translation.

Translation process:

• Occurs at ribosomes.
• Transfer RNA (tRNA) brings amino acids matching the codons.
• Amino acids join to form polypeptides, which fold into functional proteins.

This flow of genetic information from DNA to RNA to protein is central to molecular inheritance.

DNA Fingerprinting and Its Applications

DNA fingerprinting is a technique used to identify individuals based on unique DNA patterns.

• It relies on Variable Number Tandem Repeats (VNTRs), which are highly polymorphic regions.
• DNA fingerprinting is widely used in forensic science, paternity testing, and genetic diversity studies.

Understanding molecular inheritance helps explain how unique DNA sequences arise and are inherited.