Molecular Basis of Inheritance Class 12 NCERT PDF: Complete Guide
By ConceptScroll Team · Published on 18 June 2026 · 4 min read
The molecular basis of inheritance class 12 NCERT PDF covers essential concepts like DNA structure, replication, and gene expression. This chapter is crucial for CBSE biology exams and helps students understand how genetic information is stored and transmitted.
Overview of the Molecular Basis of Inheritance
The molecular basis of inheritance explains how genetic information is passed from one generation to the next at the molecular level. This chapter in the Class 12 NCERT Biology textbook introduces DNA as the genetic material, replacing earlier theories that proteins carried heredity. It covers the discovery of DNA, its chemical composition, and the experiments that proved DNA's role in inheritance.
Key points:
- DNA contains nucleotides made of a sugar, phosphate group, and nitrogenous base
- Four bases: Adenine (A), Thymine (T), Cytosine (C), Guanine (G)
- Chargaff's rules: A pairs with T, C pairs with G
- DNA is located mainly in the nucleus of eukaryotic cells
Understanding these basics sets the foundation for studying DNA replication, transcription, and translation.
Structure and Properties of DNA
DNA's structure is a double helix formed by two complementary strands running antiparallel. This discovery by Watson and Crick explained how genetic information is stored and copied.
Important features:
- Each strand is a polymer of nucleotides
- Sugar-phosphate backbone with bases projecting inward
- Hydrogen bonds hold complementary bases: A-T (2 bonds), C-G (3 bonds)
- The strands run in opposite directions: 5' to 3' and 3' to 5'
| Feature | Description |
|---|---|
| Shape | Double helix |
| Strands | Two complementary, antiparallel |
| Bases pairing | A-T and C-G via hydrogen bonds |
| Backbone | Sugar-phosphate |
This stable structure allows DNA to carry genetic information reliably.
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DNA Replication: The Semi-Conservative Process
DNA replication is the process by which DNA makes a copy of itself during cell division. It is semi-conservative, meaning each new DNA molecule contains one original and one new strand.
Steps of replication: 1. Initiation: Helicase unwinds the double helix, creating a replication fork. 2. Primer synthesis: Primase synthesizes RNA primers. 3. Elongation: DNA polymerase adds nucleotides complementary to the template strand in the 5' to 3' direction. 4. Termination: Replication ends when the entire molecule is copied.
Key enzymes involved:
- Helicase: unwinds DNA
- DNA polymerase: adds nucleotides
- Ligase: joins Okazaki fragments on lagging strand
Formula for base pairing during replication: $$ A \leftrightarrow T, \quad C \leftrightarrow G $$
This process ensures genetic information is accurately transmitted.
From DNA to Protein: Transcription and Translation
Gene expression involves two main processes: transcription and translation.
Transcription:
- DNA is used as a template to make messenger RNA (mRNA).
- RNA polymerase binds to the promoter region and synthesizes mRNA in the 5' to 3' direction.
- mRNA carries the genetic code from the nucleus to the cytoplasm.
Translation:
- Occurs in ribosomes where mRNA is decoded to build proteins.
- Transfer RNA (tRNA) brings amino acids matching the mRNA codons.
- The polypeptide chain forms according to the sequence of codons.
The genetic code is:
- Universal: same in almost all organisms
- Degenerate: multiple codons code for the same amino acid
Understanding these steps is vital for grasping how traits are expressed.
Role of RNA and Types of RNA in Inheritance
RNA plays a crucial role in the flow of genetic information. Unlike DNA, RNA is usually single-stranded and contains uracil (U) instead of thymine.
Types of RNA:
- mRNA (messenger RNA): Carries genetic code from DNA to ribosomes.
- tRNA (transfer RNA): Brings amino acids to ribosomes during translation.
- rRNA (ribosomal RNA): Structural and functional component of ribosomes.
RNA is essential in decoding DNA instructions into functional proteins, completing the central dogma of molecular biology: DNA → RNA → Protein.
Mutations and Their Impact on Genetic Information
Mutations are changes in the nucleotide sequence of DNA. They can occur naturally or due to environmental factors.
Types of mutations:
- Point mutations: Change in a single nucleotide
- Insertions or deletions: Addition or loss of nucleotides
Effects:
- Silent mutations: no change in protein
- Missense mutations: change in amino acid
- Nonsense mutations: create stop codon, truncating protein
Example: A point mutation changing codon from GAA (Glutamic acid) to GUA (Valine) alters the protein structure.
Mutations can lead to genetic disorders or variation, important in evolution.
Frequently asked questions
What is the molecular basis of inheritance?
It is the study of how genetic information is stored in DNA and passed to offspring.
Why is DNA called the genetic material?
Because it carries hereditary information that controls traits and is replicated during cell division.
What is semi-conservative replication?
Each new DNA molecule has one original and one newly synthesized strand.
How does transcription differ from translation?
Transcription makes mRNA from DNA; translation uses mRNA to build proteins.
What are the main types of RNA involved in inheritance?
mRNA, tRNA, and rRNA, each with specific roles in protein synthesis.
Can mutations affect protein function?
Yes, mutations can change amino acid sequences, altering protein structure and function.
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