CBSE Class 12 Biology Exemplar NCERT Solutions: Molecular Basis of Inheritance

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NCERT Solutions PDF Class 12 PDF

This resource provides comprehensive NCERT Solutions for CBSE Class 12 Biology Exemplar, Chapter 6: Molecular Basis of Inheritance. It covers fundamental concepts of molecular genetics, including the structure of DNA and RNA, the process of replication, transcription, and translation, and the genetic code. The solutions explain the linkage of nucleotides, the difference between nucleosides and nucleotides, the classification of sugars, the significance of base pairing in DNA structure, the charges on DNA and histones, and the roles of promoter and terminator sites in transcription. It also delves into genetic disorders like sickle-cell anaemia and the function of initiation codons like AUG. These solutions are designed to help students grasp complex topics, clarify doubts, and prepare effectively for their board examinations by offering step-by-step explanations and accurate answers.

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Quick info

BoardCBSE
ClassClass 12
SubjectBiology Exemplar
Session2026
LanguageEnglish
TypeNCERT Solutions
ChapterChapter 6

Chapter summary

Chapter 6 of the CBSE Class 12 Biology Exemplar focuses on the Molecular Basis of Inheritance. The NCERT Solutions provided here cover key concepts such as nucleotide linkages (phosphodiester bonds), the distinction between nucleosides and nucleotides, the classification of sugars (pentoses), the structural implications of purine-pyrimidine base pairing in DNA (uniform width), the electrostatic charges on DNA (negative) and histones (positive), and the locations of promoter and terminator sites in transcription. It also addresses specific genetic conditions like sickle-cell anaemia and the function of the AUG codon.

Learning outcomes

  • Understand the phosphodiester bonds linking nucleotides in a DNA strand.
  • Differentiate between a nucleoside and a nucleotide.
  • Identify pentoses as the class of sugars for ribose and deoxyribose.
  • Explain how purine-pyrimidine base pairing maintains uniform width in DNA.
  • Describe the charges on DNA and histone proteins.
  • Locate promoter and terminator sites relative to the transcription unit.
  • Explain the molecular basis and inheritance of sickle-cell anaemia.
  • Recognize the role of AUG as an initiation codon.

Topics covered

Paper topics

  • Nucleotide structure and linkage
  • Nucleosides vs. Nucleotides
  • Pentose sugars in nucleic acids
  • DNA double helix structure
  • Purine-pyrimidine base pairing
  • Electrostatic charges on DNA and histones
  • Transcription: Promoter and Terminator sites
  • Sickle-cell anaemia: molecular basis and inheritance
  • Genetic code: AUG codon function

Important topics

  • Nucleotide linkage and DNA structure
  • Purine-pyrimidine base pairing
  • Transcription initiation and termination
  • Molecular basis of genetic disorders (Sickle-cell anaemia)
  • The genetic code and start codons

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Questions and Solutions

Multiple Choice Questions (MCQs) - Q. 1

In a DNA strand, the nucleotides are linked together by which type of bonds?
  1. glycosidic bonds
  2. phosphodiester bonds
  3. peptide bonds
  4. hydrogen bonds
Solution: The nucleotides within a single strand of DNA are connected by phosphodiester bonds. These bonds form between the 3' carbon atom of one deoxyribose sugar and the 5' carbon atom of the next deoxyribose sugar, creating a strong sugar-phosphate backbone. Glycosidic bonds link the base to the sugar, peptide bonds are found in proteins, and hydrogen bonds link the two strands of the DNA double helix together.

Answer: (b) phosphodiester bonds

Multiple Choice Questions (MCQs) - Q. 2

A nucleoside differs from a nucleotide. It lacks which component?
  1. base
  2. sugar
  3. phosphate group
  4. hydroxyl group
Solution: A nucleoside is composed of a nitrogenous base covalently bonded to a pentose sugar (ribose or deoxyribose) via an N-glycosidic linkage. A nucleotide is formed when a phosphate group is esterified to the 5'-hydroxyl group of the pentose sugar in a nucleoside, forming a phosphodiester bond. Therefore, a nucleotide contains a phosphate group, while a nucleoside does not.

Answer: (c) phosphate group

Multiple Choice Questions (MCQs) - Q. 3

Both deoxyribose and ribose belong to a class of sugars called:
  1. trioses
  2. hexoses
  3. pentoses
  4. polysaccharides
Solution: Both deoxyribose and ribose are five-carbon sugars. Sugars are classified based on the number of carbon atoms they contain. Trioses have three carbons, hexoses have six carbons, and polysaccharides are complex carbohydrates made of many sugar units. Since both ribose and deoxyribose have five carbon atoms, they are classified as pentoses.

Answer: (c) pentoses

Multiple Choice Questions (MCQs) - Q. 4

The fact that a purine always pairs with a pyrimidine base through hydrogen bonds leads to, in the DNA double helix:
  1. the antiparallel nature
  2. the semiconservative nature
  3. uniform width throughout DNA
  4. uniform length in all DNA
Solution: In the DNA double helix, Adenine (a purine) always pairs with Thymine (a pyrimidine) via two hydrogen bonds, and Guanine (a purine) always pairs with Cytosine (a pyrimidine) via three hydrogen bonds. Since a purine (double-ring structure) always pairs with a pyrimidine (single-ring structure), the distance between the two sugar-phosphate backbones remains constant along the entire length of the DNA molecule. This specific base pairing rule ensures a uniform width of approximately 2 nanometers for the DNA double helix.

Answer: (c) uniform width throughout DNA

Multiple Choice Questions (MCQs) - Q. 5

The net electric charge on DNA and histones is:
  1. both positive
  2. both negative
  3. DNA negative, histones positive
  4. DNA positive, histones negative
Solution: DNA is a nucleic acid and contains phosphate groups in its backbone. Each phosphate group carries a negative charge (PO<sub>4</sub><sup>3-</sup>). Therefore, the overall net electric charge on DNA is negative. Histones are basic proteins, rich in amino acids like lysine and arginine, which have positively charged side chains. These positive charges on histones are essential for their interaction with the negatively charged DNA, facilitating the compaction of DNA into chromatin.

Answer: (c) DNA negative, histones positive

Multiple Choice Questions (MCQs) - Q. 6

The promoter site and the terminator site for transcription are located at:
  1. 3' (downstream) end and 5' (upstream) end, respectively, of the transcription unit
  2. 5' (upstream) end and 3' (downstream) end, respectively, of the transcription unit
  3. the 5' (upstream) end
  4. the 3' (downstream) end
Solution: Transcription is the process of synthesizing RNA from a DNA template. A transcription unit includes a promoter, a structural gene, and a terminator. The promoter site is the region where the RNA polymerase enzyme binds to initiate transcription. It is typically located at the 5' end (upstream) of the structural gene. The terminator site signals the end of transcription and is located at the 3' end (downstream) of the structural gene.

Answer: (b) 5' (upstream) end and 3' (downstream) end, respectively, of the transcription unit

Multiple Choice Questions (MCQs) - Q. 7

Which of the following statements is the most appropriate for sickle-cell anaemia?
  1. It cannot be treated with iron supplements
  2. It is a molecular disease
  3. It confers resistance to acquiring malaria
  4. All of the above
Solution: Sickle-cell anaemia is an inherited disorder caused by a point mutation in the beta-globin gene, leading to the substitution of glutamic acid with valine at the sixth position of the beta-globin chain. This change results in abnormal haemoglobin (HbS) that causes red blood cells to sickle under low oxygen conditions. It is considered a molecular disease because it stems from a specific alteration at the molecular level (a change in a protein). Iron supplements do not correct the underlying genetic defect. Importantly, individuals who are heterozygous for the sickle-cell trait (carrying one copy of the HbS gene and one copy of the normal HbA gene) often exhibit increased resistance to malaria, as the parasite Plasmodium falciparum has difficulty surviving in their red blood cells.

Answer: (d) All of the above

Multiple Choice Questions (MCQs) - Q. 8

One of the following is true with respect to AUG:
  1. it codes for methionine only
  2. it is also an initiation codon
  3. it codes for methionine in both prokaryotes and eukaryotes
  4. All of the above
Solution: The sequence AUG is a codon within messenger RNA (mRNA). It serves a dual purpose: it codes for the amino acid methionine, and it acts as the start signal or initiation codon for protein synthesis (translation). This function is conserved across both prokaryotic and eukaryotic organisms. Therefore, all the statements provided are true regarding the AUG codon.

Answer: (d) All of the above

Common mistakes

  • Confusing phosphodiester bonds with glycosidic or peptide bonds.
  • Incorrectly identifying the component missing in a nucleoside compared to a nucleotide.
  • Misunderstanding the significance of base pairing for DNA width.
  • Confusing the directionality of promoter and terminator sites.
  • Not recognizing sickle-cell anaemia as a molecular and inherited disease.

Revision tips

  • Focus on the chemical bonds that form the DNA backbone.
  • Clearly distinguish between nucleosides and nucleotides by their components.
  • Visualize the DNA double helix and how base pairing determines its width.
  • Understand the roles of charged molecules like DNA and histones in packaging.
  • Review the genetic code and the specific function of the AUG codon.

Practice MCQs

Q1. In a DNA strand, nucleotides are linked together by which type of bonds?

Q2. What component is lacking in a nucleoside that is present in a nucleotide?

Q3. Both deoxyribose and ribose sugars belong to which class?

Q4. The pairing of a purine with a pyrimidine in the DNA double helix results in:

Q5. What is the net electric charge on DNA and histones, respectively?

Q6. The promoter site and terminator site for transcription are located at:

Q7. Which statement is most appropriate for sickle-cell anaemia?

Q8. The codon AUG is significant because it:

Frequently asked questions

What are the primary bonds that link nucleotides together in a DNA strand?

Nucleotides in a DNA strand are primarily linked by 3'-5' phosphodiester bonds, forming the sugar-phosphate backbone.

How does a nucleoside differ from a nucleotide?

A nucleoside consists of a nitrogenous base linked to a pentose sugar. A nucleotide is a nucleoside with one or more phosphate groups attached, typically at the 5' position of the sugar.

Why does the pairing of purines with pyrimidines maintain a uniform width in the DNA double helix?

Purines (Adenine, Guanine) are larger molecules than pyrimidines (Cytosine, Thymine). By pairing one purine with one pyrimidine, the distance between the two sugar-phosphate backbones remains constant, resulting in a uniform width.

What are the charges on DNA and histones, and why are they important?

DNA is negatively charged due to its phosphate groups, while histones are positively charged due to basic amino acids. This opposite charge attraction is crucial for the packaging of DNA into chromatin.

What is the significance of the AUG codon in protein synthesis?

The AUG codon is the start codon; it signals the beginning of protein synthesis and also codes for the amino acid methionine, which is incorporated at the N-terminus of most proteins.

What is sickle-cell anaemia, and why is it considered a molecular disease?

Sickle-cell anaemia is a genetic disorder caused by a single point mutation in the beta-globin gene, leading to an abnormal haemoglobin molecule (HbS). This molecular change results in the characteristic sickle shape of red blood cells under low oxygen conditions.

Important links

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