Q.(a) The diagram below shows the sequence of amino acids in part of haemoglobin molecule.
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🔒 Start your 14-day free trial to unlock the full solution →Part (a)Concept understanding — Point Mutation
Let’s begin with something you already know: a typo.
Imagine you are typing a long paragraph on your phone. You mean to type “I will meet you at the gate,” but your thumb slips and you type “I will meet you at the gate.” One letter changed — the whole meaning stays the same. But if you had typed “I will meet you at the hate,” that single letter change flips the meaning entirely.
Now scale that idea down to the microscopic level of your own body. Every cell in you contains a long, coiled instruction manual written in a chemical language — DNA. That manual is made of four letters (A, T, G, C) arranged in precise sequences. A point mutation is exactly what it sounds like: a change in just one of those letters.
What a point mutation is, precisely
A point mutation is a change in a single nucleotide base pair in the DNA sequence. In NCERT terms, it is “a change in a single base pair of DNA.” That is the textbook definition.
There are three common types you should know:
- Substitution – one base is swapped for another (like changing A to G).
- Insertion – an extra base is added in the middle of the sequence.
- Deletion – one base is removed.
Insertions and deletions are especially important because they can shift the entire reading frame of the genetic code — a phenomenon called frameshift mutation. Substitutions may or may not change the resulting protein, depending on whether they fall in a critical spot.
The word “point” refers to the location — a single, specific point on the DNA molecule. It does not mean the mutation is small in effect. A single-letter change can be harmless, or it can cause a disease like sickle‑cell anaemia (where one base change alters the shape of haemoglobin).
Why it matters
Point mutations are the raw material of evolution. Without them, all life would be a perfect copy of its ancestors — no variation, no adaptation. Most point mutations are neutral or harmful, but a rare few give an organism an advantage, and over generations those can spread through a population.
In medicine, point mutations are the cause of many genetic disorders. In agriculture, breeders look for beneficial point mutations that give crops resistance to pests or drought. In your own body, point mutations that accumulate in cells over a lifetime can lead to cancer. …
Part (b)Concept understanding — Nucleic Acid Functions
Nucleic Acid Functions – A First Look
Think of a living organism as a giant, incredibly complex factory. Every second, this factory needs to produce thousands of different products—proteins, enzymes, hormones, structural materials—in exactly the right amounts, at exactly the right places, and at exactly the right times. How does the factory know what to build and when? It needs a master blueprint and a set of working copies that can be carried to the assembly lines.
That master blueprint is DNA (deoxyribonucleic acid). The working copies are RNA (ribonucleic acid). Together, they are the nucleic acids, and their job is to store, transmit, and execute the genetic information that makes every living thing what it is.
The Two Main Functions
1. DNA – The Permanent Blueprint (Storage and Inheritance)
DNA is the long-term, stable repository of genetic information. It is like the original architectural plan for the entire factory, locked in a secure vault. Its functions are:
- Storing genetic information: DNA contains the instructions for building every protein the organism will ever need. These instructions are written in a chemical language using four "letters" (nucleotides: A, T, G, C). The sequence of these letters is the code.
- Replication (making copies): Before a cell divides, DNA makes an exact copy of itself. This ensures that each daughter cell receives a complete set of instructions. This is why children inherit traits from their parents—the DNA blueprint is passed down.
- Transmission to offspring: DNA is the molecule of heredity. It is passed from parents to offspring, carrying the genetic information that determines everything from eye colour to susceptibility to certain diseases.
DNA never leaves the nucleus of a cell. It is too precious and too large to move around. It stays safely inside, like a reference book that cannot be taken out of the library.
2. RNA – The Working Copy (Execution of the Blueprint)
RNA is the temporary, mobile copy of specific parts of the DNA blueprint. It is like a photocopy of a single page from the master plan, which a worker can carry to the factory floor. Its functions are:
- Transcription (copying the message): A specific segment of DNA (a gene) is used as a template to make a complementary RNA molecule. This RNA copy is called messenger RNA (mRNA).
- Translation (reading the message to build a protein): The mRNA travels out of the nucleus to the ribosomes (the protein-building machines). Here, another type of RNA called transfer RNA (tRNA) reads the mRNA code and brings the correct amino acids, one by one, to build a protein chain.
- Catalysis (as a biological catalyst): Some RNA molecules, called ribozymes, can act as enzymes and speed up chemical reactions. This is a less well-known but crucial function, especially in the ribosome itself (which is partly made of RNA).
Why This Matters for You
Even if you never touch a test tube, understanding nucleic acid functions helps you grasp:
- Why children resemble their parents: DNA is the hereditary material.
- How vaccines work: Many vaccines use mRNA to instruct your cells to produce a harmless piece of a virus, training your immune system.
- How genetic disorders arise: A mistake in the DNA sequence (a mutation) can lead to a faulty protein, causing diseases like sickle cell anaemia or cystic fibrosis. …
Part (a)
(i) Effect of the substitution. On the template DNA strand the triplet CTT codes (via mRNA codon GAA) for glutamic acid (Glu) at the 6th position of the beta-globin chain. If the middle T is replaced by A, the triplet becomes CAT, whose mRNA codon is GUA, coding for valine (Val). So a single base substitution changes glutamic acid -> valine in the beta-chain of haemoglobin (a point mutation). …
Part (a): changing T to A in the DNA triplet CTT turns mRNA codon GAA (glutamic acid) into GUA (valine) at position 6 of beta-globin; this point mutation causes sickle-cell anaemia, whose HbS sickles the RBCs and blocks capillaries.
Part (b): DNA is more stable than RNA because deoxyribose lacks the reactive 2'-OH group, DNA uses thymine (aiding stability and repair of deaminated cytosine), and its double-stranded helix protects the bases and allows template repair.
Part (a)
- How the base substitution affects the haemoglobin chain. In the gene for the beta-chain of haemoglobin, the template-strand triplet CTT is transcribed into the mRNA codon GAA, which is read during translation as the amino acid glutamic acid (Glu) at the sixth position of the beta-globin polypeptide. If the marked T is substituted by A in "CTT", the template triplet becomes CAT. This is transcribed into the mRNA codon GUA, which codes for valine (Val). Therefore the single base change replaces one amino acid with another: glutamic acid is replaced by valine at the 6th position of the beta-chain. Because only one base (and hence one amino acid) is changed, this is a point mutation (specifically a missense substitution).
- Name of the condition and its effects. The disorder produced is sickle-cell anaemia. Consequences of the Glu -> Val change:
- The mutant haemoglobin, HbS, tends to polymerise (aggregate) under low oxygen tension.
- This distorts the normally biconcave red blood cells into a rigid, elongated sickle shape.
- Sickled cells are fragile and are destroyed easily (haemolysis), producing anaemia; they also block small blood vessels/capillaries, causing pain, tissue damage and poor oxygen delivery. …
Showing the 12 most recent of 35 on this concept.
- CBSE 2026Set 57/3/11 markMCQQ.The smallest part of DNA molecule that can be changed by point mutation is : (A) Oligonucleotide (B) Codon (C) Gene (D) Nucleotide
›Reveal solutionSolution
A point mutation changes a single nucleotide in the DNA molecule, so the smallest unit that can be altered is the nucleotide itself.
To understand why the answer is the nucleotide, we need to step back and think about what a point mutation actually is. The word "point" is the clue — it means a single, specific spot. In the language of DNA, that spot is one nucleotide. A nucleotide is the basic building block of DNA, consisting of a sugar, a phosphate, and a nitrogenous base (adenine, guanine, cytosine, or thymine). When a point mutation occurs, it swaps out just one of these bases for another — for example, replacing adenine with guanine at a particular position along the DNA strand.
Now, let's look at the other options to see why they don't fit. An oligonucleotide is a short chain of several nucleotides linked together — it's a small fragment, but still a sequence, not a single unit. A codon is a triplet of three consecutive nucleotides that codes for a specific amino acid during protein synthesis. Changing a codon usually means altering one of its three nucleotides, but the codon itself is a group, not the smallest possible change. A gene is a much larger segment of DNA, containing many codons and regulatory regions — far too big to be the smallest unit of change.
NoteThe NCERT textbook for Class 12 Biology (Chapter 6, "Molecular Basis of Inheritance") explicitly states that point mutations arise from the substitution of a single nucleotide base pair. This confirms that the nucleotide is the fundamental unit altered. …
- CBSE 2026Set EG1 markMCQQ.Sickle cell Anaemia disease is an example of which type of mutation?(a) Point mutation(b) Euploidy(c) Deletion(d) Translocation
›Reveal solutionSolution
Sickle-cell anaemia is caused by a single-base change in the β-globin gene — a point mutation, option (a).
Sickle-cell anaemia results from the substitution of a single nucleotide in the gene for the β-chain of haemoglobin: the codon GAG → GTG, which replaces glutamic acid by valine at the 6th position of the β-globin polypeptide (Glu → Val). Because it changes only one base pair, it is a classic point mutation (a substitution).
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- CBSE 2026Set ZOOLOGY1 markMCQQ.Who coined the term 'mutation'?(a) Darwin(b) Lamarck(c) Hugo de Vries(d) Mendel
›Reveal solutionSolution
Hugo de Vries coined the term mutation, based on his work on Oenothera lamarckiana.
Hugo de Vries, a Dutch botanist, studied the evening primrose plant (Oenothera lamarckiana) and observed that new phenotypic variations arose suddenly, in a single generation, without any intermediate stages, and were heritable. He called these sudden, discontinuous, heritable changes 'mutations', distinguishing them from the small continuous variations that Darwin's theory of natural selection …
- CBSE 2026Set ANNUAL1 markQ.Fill in the blank: Sickle cell anemia is a classical example of ______ mutation.
›Reveal solutionSolution
Sickle-cell anaemia is caused by a point mutation in the beta-globin gene.
Sickle-cell anaemia is an autosomal recessive disorder resulting from a single base substitution in the gene coding for the beta chain of haemoglobin. The codon GAG changes to GTG, so glutamic acid at the sixth position is replaced by valine (Glu to Val). Because only one nucleotide (one base pair) is altered, this is called a point mutation. The mutant haemoglobin (HbS) …
- CBSE 2026Set ANNUAL1 markMCQQ.β - Thalassemia is controlled by a single gene HBB on –(a) Chromosome 16(b) Chromosome 13(c) Chromosome 11(d) Chromosome 14
›Reveal solutionSolution
β-thalassaemia is controlled by the HBB (beta-globin) gene, which lies on chromosome 11.
Thalassaemia is an autosomal recessive blood disorder in which the synthesis of one of the globin chains of haemoglobin is reduced. It is classified by which chain is affected:
- α-thalassaemia — defect in the α-globin genes (HBA1/HBA2), located on chromosome 16. …
- CBSE 2026Set ANNUAL1 markQ.Analyse how trisomy of chromosome 21 occurs in Down's Syndrome.
›Reveal solutionSolution
Trisomy 21 (Down's syndrome) is caused by non-disjunction of chromosome 21 during gamete formation, giving an extra copy of chromosome 21 in the zygote.
Normally each parent contributes one copy of chromosome 21, so the child has two. In Down's syndrome, during meiosis (gamete formation) the pair of chromosome 21 fails to separate properly — this is called non-disjunction. As a result, one gamete receives two copies of chromosome 21 instead of one.
…
- CBSE 2025Set 57/5/11 markMCQQ.What would happen if a gene encoding a polypeptide of 50 amino acids, 25th Codon (UAU) is mutated to "UAA" ? (A) A polypeptide of 49 amino acids will be formed. (B) A polypeptide of 25 amino acids will be formed. (C) A polypeptide of 24 amino acids will be formed. (D) A polypeptide of 50 amino acids will be formed.
›Reveal solutionSolution
Mutating codon 25 from UAU (tyrosine) to UAA (stop) introduces a premature termination signal; translation halts after incorporating the 24th amino acid, producing a truncated polypeptide of 24 amino acids.
The heart of this question lies in understanding how the genetic code directs protein synthesis and what happens when a nonsense mutation appears mid-sequence.
The Genetic Code and Stop Signals
Every three-nucleotide codon in mRNA either specifies an amino acid or signals termination. UAU codes for tyrosine, a standard amino acid. UAA, however, is one of three stop codons (UAA, UAG, UGA) that tell the ribosome "translation ends here." When the ribosome encounters a stop codon, it releases the polypeptide chain and dissociates from the mRNA—no amino acid is added at that position.
In the original gene, codon 25 reads UAU, so tyrosine is incorporated at position 25, and translation continues through all 50 codons to produce a 50-amino-acid polypeptide. The mutation changes codon 25 to UAA, converting it into a premature stop signal.
Step-by-Step Analysis
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Translation begins normally
The ribosome starts at the start codon (AUG, position 1) and moves codon by codon, adding amino acids to the growing chain. Codons 1 through 24 are unaffected by the mutation, so amino acids 1–24 are incorporated exactly as before.
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The ribosome reaches codon 25
Instead of reading UAU and adding tyrosine, the ribosome now reads UAA. Because UAA is a stop codon, no amino acid corresponds to it. The ribosome recognizes the termination signal.
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Translation terminates prematurely
Release factors bind to the stop codon, the polypeptide is cleaved from the tRNA, and the ribosome dissociates. The chain that has been synthesized up to this point contains only the amino acids encoded by codons 1 through 24.
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Counting the final length
Since translation stops at codon 25 (which now codes for nothing), the polypeptide consists of the 24 amino acids that were added before the stop signal. Codons 26–50 are never translated. …
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- CBSE 2025Set 57/6/11 markMCQQ.SNPs in Human Genome Project refers to : (A) Polymorphism in repetitive sequences. (B) Single-base DNA differences. (C) Single changes in nucleotide of mRNA. (D) Loss or gain of a gene function.
›Reveal solutionSolution
SNPs (Single Nucleotide Polymorphisms) are single-base DNA differences in the genome where one nucleotide varies between individuals at the same position.
The Human Genome Project, completed in 2003, was a landmark international effort to map and sequence the entire human genome—all three billion base pairs of DNA that make up our genetic blueprint. One of the most significant discoveries from this project was the identification and cataloguing of SNPs, which stands for Single Nucleotide Polymorphisms.
To understand what SNPs are, picture the human genome as an enormous instruction manual written in a four-letter alphabet: A (adenine), T (thymine), G (guanine), and C (cytosine). Now imagine comparing your manual with someone else's. Most of the text would be identical—humans share about 99.9% of their DNA sequence. But in that remaining 0.1%, you'd find millions of spots where a single letter differs. At one particular position, you might have an 'A' while another person has a 'G'. That single-letter difference is a SNP.
These variations occur roughly once every 1000 bases throughout the genome, making them the most common type of genetic variation among people. SNPs are inherited from parents and passed to offspring, which is why they serve as valuable markers for tracking disease genes and understanding human evolution and migration patterns.
NoteThe term "polymorphism" simply means "many forms"—in genetics, it refers to variations that exist in a population. SNPs are polymorphisms at the single-nucleotide level. …
- CBSE 2025Set ANNUAL1 markMCQQ.The cause of sickle cell anemia is :(a) Point mutation(b) Natural selection(c) Adaptive radiation(d) Both (B) and (C)
›Reveal solutionSolution
A single base substitution changes one amino acid in the beta-globin chain, causing sickle cell anemia.
Sickle cell anemia is a classic Mendelian (monogenic) recessive disorder caused by the substitution of a single nucleotide (GAG → GTG) in the sixth codon of the beta-globin gene on chromosome 11. This point mutation changes the amino acid at position 6 of the beta-globin chain from glutamic acid to valine. The altered haemoglobin (HbS) polymerises under lo …
- CBSE 2025Set ANNUAL1 markQ.What kind of charge is present on DNA molecule ?
›Reveal solutionSolution
The phosphate groups in the sugar-phosphate backbone give DNA its negative charge.
Each nucleotide in the DNA backbone contains a phosphate group linked to the 5' and 3' carbons of adjacent deoxyribose sugars via phosphodiester bonds. At physiological pH, these phosphate groups are ionised (each losing a proton), leaving a negatively charged oxygen. Since this repeats along the entire length of both strands, …
- CBSE 2025Set ANNUAL1 markQ.How many types of RNA are there?
›Reveal solutionSolution
RNA occurs in three functional forms — m-RNA, r-RNA and t-RNA — each with a distinct role in protein synthesis.
Unlike DNA, which is essentially one molecule per chromosome, RNA exists in the cell as three chemically similar but functionally different types:
- Messenger RNA (m-RNA) — carries the genetic code copied from DNA (transcription) to the ribosome, specifying the sequence of amino acids to be joined. …
- CBSE 2025Set ANNUAL1 markQ.Who is responsible for heredity?
›Reveal solutionSolution
Heredity is controlled by genes, the functional units of DNA carried on the chromosomes.
Each chromosome in the nucleus of a cell is made of a very long DNA molecule. Specific segments of this DNA, called genes, code for particular traits/proteins. When a cell divides, DNA replicates and an exact copy of these genes is passed on to the daughter cells, and ultimately from parents to offspring during r …
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