NCERT Class 12 Biology Biology: Chapter 5 — MOLECULAR BASIS OF INHERITANCE
This chapter, "Molecular Basis of Inheritance," delves into the fundamental molecule of heredity, DNA. It revisits the search for genetic material, establishing DNA as the primary carrier of genetic information for most organisms, while RNA plays diverse roles. The chapter details the structure of DNA as a polymer of deoxyribonucleotides, explaining the components of a nucleotide: nitrogenous bases (purines and pyrimidines), a pentose sugar (deoxyribose for DNA), and a phosphate group. It outlines the formation of nucleosides and nucleotides, and how they link via phosphodiester bonds to form a polynucleotide chain. The chapter also introduces the concept of the RNA world and sets the stage for discussing DNA replication, transcription, translation, gene regulation, the Human Genome Project, and DNA fingerprinting, crucial for understanding genetics and molecular biology in CBSE Class 12.
Quick info
| Board | CBSE / NCERT |
|---|---|
| Class | Class 12 |
| Subject | Biology |
| Book | Biology |
| Chapter | Chapter 5 — MOLECULAR BASIS OF INHERITANCE |
| Language | English |
| PDF type | NCERT Textbook |
| Session | CBSE 2026 |
| Reading time | 4 minutes |
| Word count | 613 |
Learning outcomes
- Understand the historical search for genetic material and its identification as DNA.
- Describe the structure of a polynucleotide chain, including nucleotides, nucleosides, and phosphodiester linkages.
- Differentiate between DNA and RNA, including their components and primary functions.
- Identify the components of a nucleotide: nitrogenous base, pentose sugar, and phosphate group.
- Explain the formation of nucleosides and nucleotides through glycosidic and phosphoester linkages.
Vocabulary
| Word | Meaning |
|---|---|
| Deoxyribonucleic acid (DNA) | A nucleic acid that carries the genetic information in the structure of a long polymer of deoxyribonucleotides. |
| Ribonucleic acid (RNA) | A nucleic acid that functions as messenger, adapter, structural, or catalytic molecule. |
| Nucleotide | A monomer unit of nucleic acids, composed of a nitrogenous base, a pentose sugar, and a phosphate group. |
| Nitrogenous base | A nitrogen-containing molecule found in nucleic acids, including purines (Adenine, Guanine) and pyrimidines (Cytosine, Uracil, Thymine). |
| Pentose sugar | A five-carbon sugar; ribose in RNA and deoxyribose in DNA. |
| Phosphate group | A chemical group consisting of phosphorus and oxygen, forming part of the nucleotide backbone. |
| N-glycosidic linkage | The bond connecting a nitrogenous base to the 1' carbon of a pentose sugar. |
| Phosphoester linkage | The bond connecting a phosphate group to the 5' carbon of a pentose sugar. |
| Phosphodiester linkage | The bond connecting the 3' carbon of one nucleotide to the 5' carbon of another in a polynucleotide chain. |
| Nucleoside | A molecule formed by linking a nitrogenous base to a pentose sugar. |
| Purines | Nitrogenous bases with a double-ring structure (Adenine, Guanine). |
| Pyrimidines | Nitrogenous bases with a single-ring structure (Cytosine, Uracil, Thymine). |
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Practice questions
- What are the three components of a nucleotide? Answer: A nucleotide consists of a nitrogenous base, a pentose sugar, and a phosphate group.
- Name the two types of nitrogenous bases found in nucleic acids. Answer: The two types are Purines (Adenine and Guanine) and Pyrimidines (Cytosine, Uracil, and Thymine).
- What is the difference between ribose and deoxyribose sugar? Answer: Ribose is found in RNA, while deoxyribose is found in DNA. Deoxyribose lacks an oxygen atom at the 2' carbon compared to ribose.
- How are two nucleotides linked to form a dinucleotide? Answer: Two nucleotides are linked by a 3'-5' phosphodiester linkage.
- Which pyrimidine base is present in DNA but not in RNA? Answer: Thymine is present in DNA, while Uracil is present in RNA in place of Thymine.
Practice MCQs
Q1. Which of the following is the genetic material in most organisms?
Explanation: DNA (Deoxyribonucleic acid) is the primary genetic material in the vast majority of organisms, carrying the hereditary information.
Q2. Adenine and Guanine are examples of:
Explanation: Adenine and Guanine are nitrogenous bases that have a double-ring structure, classifying them as purines.
Q3. In RNA, Thymine is replaced by which pyrimidine base?
Explanation: Uracil is a pyrimidine base found in RNA that substitutes for Thymine, which is found in DNA.
Q4. A nucleoside is formed by the linkage of:
Explanation: A nucleoside is formed when a nitrogenous base is linked to a pentose sugar via an N-glycosidic bond.
Q5. The bond linking a phosphate group to the 5' carbon of a pentose sugar is called:
Explanation: A phosphate group is attached to the 5' carbon of the pentose sugar through a phosphoester linkage to form a nucleotide.
Q6. Which of the following is NOT a function of RNA mentioned in the text?
Explanation: While RNA can act as genetic material, adapter, and structural molecule, its role as a primary replication catalyst is not explicitly stated in this introductory section.
Frequently asked questions
What is the primary role of DNA in living systems?
DNA acts as the genetic material in most organisms, carrying the instructions for development, functioning, growth, and reproduction.
What are the main differences between DNA and RNA?
DNA uses deoxyribose sugar and Thymine, while RNA uses ribose sugar and Uracil. DNA is typically double-stranded, while RNA is usually single-stranded.
What are the three components that make up a nucleotide?
A nucleotide is composed of a nitrogenous base, a pentose sugar (deoxyribose for DNA, ribose for RNA), and a phosphate group.
How are nucleotides joined together to form a polynucleotide chain?
Nucleotides are joined by 3'-5' phosphodiester linkages, forming a long polymer chain.
What is a nucleoside?
A nucleoside is formed when a nitrogenous base is attached to a pentose sugar through an N-glycosidic linkage.
What is the significance of the Human Genome Project mentioned in the chapter?
The Human Genome Project marked a new era of genomics by determining the complete nucleotide sequence of the human genome, leading to advancements in understanding genetic diseases and biology.
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NCERT Class 12 Biology — Biology — Chapter 5 — MOLECULAR BASIS OF INHERITANCE. Verified by NCERT Help Editorial Team. Reviewed on 29 Jul 2026. Last updated 10 Aug 2026.