Class 9 Biology Chapter 5 Molecular Basis of Inheritance: Important Questions & Answers (2025–26)

The CBSE Class 9 Biology paper (2025–26) allocates 13–15% of total marks to genetics and heredity across Chapters 4 and 5. Chapter 5 alone contributes 8–10 marks in the annual exam. The question distribution typically follows: 1–2 one-mark MCQs (definition or identification), 1–2 two-mark questions (short mechanisms), 1 three-mark question (detailed process explanation), and 1 five-mark long-answer (essay on central dogma or gene regulation). Additionally, case-study or diagram-based HOTS questions are increasingly common, testing your ability to apply concepts to real scenarios (e.g., identifying a mutation's effect on a protein sequence). Unlike rote chapters, Chapter 5 demands conceptual clarity: you must understand *why* DNA is double-helical, *how* replication preserves genetic information, and *how* the genetic code translates DNA instructions into proteins. Practising these curated questions trains you to think in sequences and causation, not just memorize terms. Consistent drill through these patterns—available daily on platforms like cbsetutor.ai—ensures you recognize question variations instantly during the exam.

**Question 1:** Which bases are found in DNA but not in RNA? (a) Adenine and guanine (b) Thymine only (c) Cytosine only (d) Uracil and thymine **Answer:** (b) Thymine only. DNA contains thymine; RNA contains uracil in its place. Both share adenine, guanine, and cytosine. **Question 2:** The semi-conservative mode of DNA replication was proved by: (a) Watson and Crick (b) Meselson and Stahl (c) Hershey and Chase (d) Nirenberg and Matthaei **Answer:** (b) Meselson and Stahl. In 1958, they used ¹⁵N-labelled heavy nitrogen to track DNA strands across generations, showing each new DNA molecule contains one original and one new strand. **Question 3:** The genetic code is universal because: (a) All organisms use the same 64 codons (b) The same codons code for the same amino acids across almost all organisms (c) DNA replication is identical in all cells (d) Transcription occurs in the same way everywhere **Answer:** (b) The same codons code for the same amino acids across almost all organisms. This near-universality (with rare exceptions) reflects a shared evolutionary origin and allows genes to be transferred across species. **Question 4:** Which of the following is the site of transcription in a eukaryotic cell? (a) Cytoplasm (b) Nucleus (c) Ribosome (d) Mitochondrion **Answer:** (b) Nucleus. mRNA is synthesized in the nucleus by RNA polymerase II, then exported to the cytoplasm for translation at ribosomes. **Question 5:** A codon consists of: (a) One nucleotide (b) Two nucleotides (c) Three nucleotides (d) Four nucleotides **Answer:** (c) Three nucleotides. Each triplet (e.g., AUG, CAG) codes for one amino acid or signals start/stop during translation.

**Question 1:** Define DNA replication. Why is it called semi-conservative? **Answer:** DNA replication is the process by which a DNA molecule creates an identical copy of itself. It is called semi-conservative because each newly synthesized DNA molecule contains one original (parental) strand and one newly synthesized strand. During replication, the double helix unwinds, and each strand serves as a template. DNA polymerase adds complementary nucleotides, so the two resulting DNA molecules are exact copies of the original, preserving genetic information accurately. **Question 2:** Distinguish between transcription and translation in one sentence each. **Answer:** *Transcription* is the synthesis of mRNA from a DNA template by RNA polymerase in the nucleus. *Translation* is the synthesis of a protein from mRNA by ribosomes in the cytoplasm, using tRNA to deliver amino acids. **Question 3:** What is the genetic code? Give one example of a codon and the amino acid it codes for. **Answer:** The genetic code is the system of triplet codons (three-nucleotide sequences) that specify which amino acids are incorporated into proteins during translation. For example, the codon AUG codes for methionine and also serves as the start signal. Another example: GGC codes for glycine. **Question 4:** Why is DNA described as the "blueprint of life"? **Answer:** DNA is the blueprint of life because it contains all the genetic instructions (genes) needed to build and maintain an organism. It directs the synthesis of proteins, which perform all cellular functions. Through DNA replication, genetic information is passed to daughter cells and offspring, ensuring continuity and heredity. **Question 5:** What is the role of tRNA in translation? **Answer:** tRNA (transfer RNA) acts as an adapter molecule during translation. Each tRNA has an anticodon at one end that pairs with a specific codon on mRNA, and a binding site at the other end that carries the corresponding amino acid. tRNA delivers amino acids to the ribosome in the correct sequence, allowing proteins to be assembled accurately.

**Question 1:** Explain the process of DNA replication with the help of a diagram description. Mention the role of DNA polymerase. **Answer:** DNA replication begins when the double helix unwinds, separating the two strands. Each strand is then used as a template. DNA polymerase, an enzyme, catalyses the addition of complementary nucleotides to each template strand, following the base-pairing rules (A with T, G with C). The new nucleotides are added at the 3′ end of the growing strand. On the leading strand, synthesis is continuous; on the lagging strand, it occurs in short segments (Okazaki fragments) that are later joined by ligase. The result is two identical DNA molecules, each containing one original strand and one newly synthesized strand. This semi-conservative replication ensures genetic fidelity. **Question 2:** Describe the process of transcription. What is mRNA and why is it temporary? **Answer:** Transcription is the synthesis of mRNA from a DNA template. RNA polymerase binds to the promoter region and unwinds the DNA. It reads the template strand (3′ to 5′) and synthesizes mRNA (5′ to 3′) using complementary base-pairing rules (A pairs with U in RNA instead of T). Once transcription ends at a terminator sequence, the mRNA is released. mRNA (messenger RNA) carries genetic instructions from the nucleus to ribosomes in the cytoplasm. It is temporary because it is degraded by enzymes after its message is translated, preventing the same protein from being made repeatedly and allowing cells to respond to changing needs. **Question 3:** Explain the role of codons and anticodons in translation. Use an example. **Answer:** During translation, mRNA is read in sets of three nucleotides called codons. Each codon specifies which amino acid should be added to the growing protein chain. tRNA molecules have anticodons—sequences complementary to and paired with mRNA codons. For example, if an mRNA codon is 5′-AUG-3′, the corresponding tRNA anticodon is 3′-UAC-5′. The tRNA carrying methionine (the amino acid for AUG) binds to the mRNA at the ribosome. This codon–anticodon pairing ensures that amino acids are added in the correct order, guaranteeing accurate protein synthesis and maintaining the genetic code's fidelity. **Question 4:** What is gene regulation? Explain its importance in a multicellular organism like humans. **Answer:** Gene regulation is the control of when and how often a gene is expressed (transcribed and translated). Not all genes are active in all cells at all times. In humans, different genes are active in different cell types: muscle cells express genes for muscle proteins, while pancreatic cells express genes for insulin. Gene regulation allows: (1) cell differentiation—different cell types emerge from the same genome by activating different genes; (2) response to environment—cells adjust protein production based on oxygen, nutrients, or hormones; (3) metabolic efficiency—energy is saved by producing only needed proteins. Without regulation, all cells would be identical, and organisms could not maintain homeostasis or respond to changing conditions.

**Question 1:** Describe the central dogma of molecular biology. Explain how DNA, RNA, and protein are related through this process. **Full Solution:** The central dogma states: DNA → RNA → Protein. It describes the flow of genetic information in living cells. *Step 1 – Replication (DNA → DNA):* Before a cell divides, DNA replicates semi-conservatively. DNA polymerase synthesizes new strands complementary to each original strand. Two identical DNA molecules result, each with one old and one new strand. This ensures genetic information is preserved and passed to daughter cells. *Step 2 – Transcription (DNA → RNA):* In the nucleus, RNA polymerase reads a gene (a segment of DNA) and synthesizes mRNA. The template DNA strand (3′ to 5′) is read, and mRNA (5′ to 3′) is synthesized with complementary bases (A→U, T→A, G→C, C→G). The mRNA is then processed (capping, tailing, splicing) and exported to the cytoplasm. This mRNA is a temporary copy of genetic instructions. *Step 3 – Translation (RNA → Protein):* At the ribosome, mRNA is read codon-by-codon (triplet nucleotides). Each codon pairs with a tRNA anticodon, and the tRNA delivers its amino acid to the growing polypeptide chain. As the ribosome moves along mRNA, amino acids are linked by peptide bonds. When a stop codon is reached, translation ends, and the protein is released. *Significance:* This process ensures genetic information encoded in DNA is accurately expressed as functional proteins, which carry out all cellular processes. Mutations at any step can alter protein function, affecting the organism. **Question 2:** Explain the genetic code with reference to its features. Why is it said to be degenerate and universal? **Full Solution:** The genetic code is the triplet system of codons that specifies which amino acid is incorporated during protein synthesis. *Features of the Genetic Code:* 1. *Triplet nature:* Each codon consists of three consecutive nucleotides on mRNA (e.g., CAG, AUG, UGA). 2. *Specificity:* Each codon codes for only one amino acid (or stop signal), ensuring unambiguous instructions. 3. *Non-overlapping:* Codons are read sequentially without overlap, so one mutation affects only one amino acid, not multiple. 4. *Comma-less:* There are no "punctuation" marks; the ribosome simply reads codons in order. 5. *Universal (nearly):* The same codon codes for the same amino acid in almost all organisms (bacteria, plants, animals, fungi), suggesting a shared evolutionary origin. *Degeneracy:* The genetic code is degenerate because multiple codons code for the same amino acid. For example, leucine is coded by six different codons: UUA, UUG, CUU, CUC, CUA, CUG. This redundancy provides a buffer against mutation: if the third position ("wobble" position) changes, the same amino acid may still be specified. This reduces the harmful effects of mutations. *Universality:* The code is universal because the same codon–amino acid relationships exist across all life forms. This universality supports the theory of common descent and allows genes from one organism to be inserted into another (genetic engineering) and produce functional proteins. *Example:* The codon AUG codes for methionine in bacteria, plants, and humans. UUA codes for leucine universally. **Question 3:** Explain how mutations in DNA can affect protein synthesis. Provide examples of point mutations and their consequences. **Full Solution:** Mutations are permanent alterations in the DNA sequence. Depending on their nature and location, they can severely, mildly, or not affect protein function. *Types of Point Mutations (single nucleotide changes):* 1. *Silent (Synonymous) Mutation:* A mutation that does not change the amino acid due to degeneracy of the genetic code. - *Example:* Original codon: GCU (codes for alanine). Mutated codon: GCC (still codes for alanine). No change in protein function. 2. *Missense Mutation:* A mutation that changes one amino acid to a different one. - *Example:* Original codon: GAA (codes for glutamic acid). Mutated codon: GUA (codes for valine). This causes sickle cell anaemia in humans: the protein (haemoglobin) polymerizes under low oxygen, distorting red blood cells into sickle shapes. 3. *Nonsense Mutation:* A mutation that creates a premature stop codon, truncating the protein. - *Example:* Original codon: UAC (codes for tyrosine). Mutated codon: UAA (stop codon). The protein is incomplete and usually non-functional, causing cystic fibrosis if in the CFTR gene. *Effect on Protein Synthesis:* - If a mutation occurs in the promoter region, transcription may not initiate, producing no protein. - If a mutation alters a codon early in the gene, subsequent codons may be read out of frame (frameshift), producing a completely different and non-functional protein. - If a mutation is silent (degeneracy), the protein is normal, and there is no phenotypic change. - Missense mutations in critical regions (active sites, binding domains) usually impair function; mutations in non-critical regions may have minimal effect. *Conclusion:* Not all mutations are harmful; some are neutral or beneficial. However, mutations in coding regions can significantly alter protein structure and function, leading to genetic diseases.

**Case Study: Sickle Cell Anaemia and the Genetic Code** Sickle cell anaemia is an inherited blood disorder caused by a single point mutation in the β-globin gene. The normal codon for the 6th amino acid is GAG, which codes for glutamic acid. In sickle cell anaemia, a mutation changes the 2nd nucleotide from A to T, producing GTG, which codes for valine instead. This single amino acid substitution causes the haemoglobin protein to polymerize under low oxygen, distorting red blood cells into characteristic sickle shapes. **Questions:** 1. **Identify the type of mutation.** Is it a point mutation? Is it silent, missense, or nonsense? **Answer:** Yes, it is a point mutation (single nucleotide substitution). Specifically, it is a *missense mutation* because it changes the codon from GAG (glutamic acid) to GTG (valine), resulting in a different amino acid being incorporated. This changes the protein's properties. 2. **Explain why this single amino acid change has such severe consequences for the protein and the organism.** **Answer:** Glutamic acid is a hydrophilic (water-loving), negatively charged amino acid, while valine is hydrophobic (water-fearing) and nonpolar. This change alters the protein's three-dimensional structure and charge distribution. Under low oxygen conditions, the mutated haemoglobin molecules stick together, forming rigid fibres that distort red blood cells into sickles. These sickle cells get stuck in blood vessels, blocking blood flow, causing pain, tissue damage, and organ dysfunction. A single amino acid in a critical region can drastically alter protein folding and function. 3. **If the mutation had resulted in a stop codon (nonsense mutation), how would the outcome differ from the sickle cell scenario?** **Answer:** A nonsense mutation would create a premature stop codon, causing the ribosome to terminate translation early. The resulting β-globin protein would be much shorter and incomplete, lacking critical structural and functional domains. Such a truncated protein would be non-functional or rapidly degraded. In contrast, the sickle cell mutation produces a full-length protein that is *structurally* altered, allowing it to polymerize abnormally. While both mutations are harmful, a nonsense mutation likely causes more severe dysfunction because the entire "tail" of the protein is missing, whereas the missense mutation affects protein function through altered shape and properties.

Mastering Chapter 5 requires more than reading textbooks once. You need repeated, targeted practice under exam-like conditions, instant feedback, and conceptual reinforcement—exactly what adaptive AI tutoring provides. CBSETUTOR.ai's intelligent platform personalizes your learning in three ways: **1. Daily Question Drills:** Each morning, the AI generates a quiz of 5–8 questions from Chapter 5, mixing all difficulty levels (1-mark MCQs, 2-mark short-answers, 3-mark mechanisms, 5-mark essays). Questions are randomized and drawn from a curated pool of 200+ verified NCERT-aligned items, so you never see the same question twice. You answer within 20 minutes, simulating exam time pressure. **2. Instant Diagnostic Feedback:** After each drill, the AI identifies weak areas instantly. If you struggled with transcription-translation sequencing, it flags that topic and suggests you review that concept before tomorrow's drill. If you confused DNA replication with transcription, it delivers a micro-lesson (2–3 minutes) clarifying the difference. This prevents misconceptions from hardening. **3. Spaced Repetition & Conceptual Linking:** The AI tracks which concepts you've mastered and which you revisit frequently. Hard topics (like the genetic code's universality or semi-conservative replication's proof) are spaced intelligently across multiple drills. You also see connection questions that link Chapter 5 to Chapter 4 (Heredity) or other chapters, building your big-picture understanding. **4. Exam Simulation Mode:** One week before your exam, the AI shifts to full-length mock tests: a realistic paper mixing Chapters 4–5 in board exam proportions. You get timed feedback and a detailed score report showing your strengths and last-minute study priorities. Start a 3-day free trial at cbsetutor.ai to experience personalized drills on Molecular Basis of Inheritance and see how AI-powered practice accelerates conceptual mastery.

Before your exam, ensure you can: ✓ **Explain DNA structure:** Double helix, complementary base-pairing (A–T, G–C), sugar-phosphate backbone, major and minor grooves. ✓ **Describe semi-conservative replication:** Meselson–Stahl experiment proof, role of DNA polymerase, leading and lagging strands, Okazaki fragments. ✓ **Outline transcription:** RNA polymerase, template strand, mRNA synthesis, promoter, terminator, role of mRNA as temporary messenger. ✓ **Explain translation:** mRNA codons, tRNA anticodons, ribosomes, amino acid addition, start (AUG) and stop codons (UAA, UAG, UGA). ✓ **Articulate the genetic code:** 64 codons, degeneracy (multiple codons → one amino acid), universality (near-identical across organisms), wobble hypothesis. ✓ **Discuss gene regulation:** Differential gene expression in different cell types, response to environmental signals, metabolic efficiency, cell differentiation. ✓ **Analyze mutations:** Point mutations (silent, missense, nonsense), frameshift, effects on protein synthesis and phenotype. Use sickle cell as an example. Practise these 18 questions (5 × 1-mark + 5 × 2-mark + 4 × 3-mark + 3 × 5-mark + 1 case study) until you can answer each in 50–60% of exam time. Consistency and conceptual depth, not memorization, are your keys to 9–10 marks on Chapter 5 in the 2025–26 board exam.