Q.(a)
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🔒 Start your 14-day free trial to unlock the full solution →Part (a)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 (b)Concept understanding — Pedigree Analysis
Pedigree Analysis: A Family Tree for Traits
Imagine you're looking at a family photograph album. You notice that your grandfather, your uncle, and your cousin all have the same unusual hairline or the same colour blindness. You start wondering: Is this just a coincidence, or is it something that runs in the family? How does this pattern pass from one generation to the next?
That curiosity is exactly where pedigree analysis begins.
What is a Pedigree?
A pedigree is simply a family tree that records the presence or absence of a particular trait — like a disease, a physical feature, or a genetic condition — across several generations. Instead of writing names and stories, biologists use a standard set of symbols to map out who has the trait and who doesn't, and how they are related.
The NCERT textbook defines it as a method to study the inheritance of a trait in several generations of a family. It is a tool, not a formula. You don't calculate anything; you observe patterns.
Why Do We Need It?
Humans cannot be bred in a laboratory like fruit flies or pea plants. You cannot ask two cousins to have children just to see if a disease is inherited. So pedigree analysis becomes the only ethical, practical way to study how traits pass through human families.
It helps in:
- Predicting the likelihood that a child might inherit a genetic disorder
- Tracing whether a trait is dominant or recessive
- Identifying carriers — people who carry a gene for a disease but do not show it themselves
- Offering genetic counselling to families with a history of inherited conditions
The Basic Symbols (What You Need to Know)
In a pedigree chart:
- Males are represented by squares
- Females are represented by circles
- A horizontal line connecting a square and a circle shows a marriage (mating)
- A vertical line descending from that couple leads to their children
- Shaded (filled) symbols indicate individuals who show the trait
- Unshaded (empty) symbols indicate individuals who do not show the trait
- A half-shaded symbol often indicates a carrier (someone who has the gene but does not express the trait)
In NCERT, you will see that a pedigree is drawn from the top (oldest generation) to the bottom (youngest). The first generation is labelled I, the second II, and so on. Individuals within a generation are numbered from left to right (e.g., I-1, I-2, II-3).
How to Read a Pedigree: The Core Idea
You are essentially looking for a pattern. Ask yourself:
- Does the trait appear in every generation? If yes, it might be dominant — because a dominant trait only needs one copy of the gene to show up, so it rarely skips a generation.
- Does the trait skip a generation? If yes, it might be recessive — because a recessive trait needs two copies (one from each parent), and carriers can pass it on without showing it themselves.
- Does the trait affect mostly males? If yes, it might be sex-linked (carried on the X chromosome). For example, colour blindness and haemophilia are much more common in males because they have only one X chromosome.
The most important rule from NCERT: A recessive trait can skip a generation, but a dominant trait cannot. If you see a child with a trait whose parents do not have it, the trait is almost certainly recessive. The parents are carriers.
A Simple Example (Without Numbers)
Suppose you see a pedigree where:
- A grandfather has a condition (shaded square)
- His daughter does not have it (empty circle)
- But his grandson (the daughter's son) has it (shaded square)
What does this tell you? The trait skipped a generation (from grandfather to grandson, missing the daughter). That suggests it is recessive. Also, it appears only in males in this case, so it might be X-linked recessive — the daughter is a carrier, and she passed the gene to her son. …
Part (a)
(i) Frederick Griffith worked with Streptococcus pneumoniae, which has a virulent smooth (S, capsulated) strain and an avirulent rough (R, non-capsulated) strain. Injecting mice: live S killed them; live R did not; heat-killed S did not; but heat-killed S + live R killed the mice, and live S bacteria were recovered from them. Some 'transforming principle' from the dead S bacteria had transformed live R into virulent S - the first evidence that a substance can transfer heredity. …
Part (a): Griffith's experiments on Streptococcus pneumoniae revealed bacterial transformation - a 'transforming principle' from dead S bacteria converted live R bacteria into virulent S; Avery, MacLeod and McCarty identified that principle as DNA. Part (b): Haemophilia is inherited as an X-linked recessive trait, shown by three crosses and expressed far more often in males.
Part (a)
(i) Griffith's experiments. Streptococcus pneumoniae occurs as two strains: a smooth (S) strain with a mucous (polysaccharide) capsule that is virulent, and a rough (R) strain without a capsule that is avirulent. Griffith injected mice as follows:
- Live S -> mouse died.
- Live R -> mouse lived.
- Heat-killed S -> mouse lived.
- Heat-killed S + live R -> mouse died, and live S bacteria were recovered from it.
Significance: the live R bacteria had been genetically 'transformed' into virulent S by some material from the dead S bacteria. Griffith called it the 'transforming principle'. This was the first demonstration that hereditary material can pass from one organism to another, though its chemical nature was still unknown. …
Showing the 12 most recent of 22 on this concept.
- CBSE 2026Set ANNUAL1 markQ.Symbols used in human pedigree analysis is given below. Identify these symbols used in pedigree chart.
›Reveal solutionSolution
Pedigree charts use squares for males, circles for females; a filled symbol denotes an affected individual, and a horizontal line joining a square and a circle denotes mating between them.
Standard pedigree symbols include:
- Unfilled (open) square = unaffected/normal male
- Filled (solid) square = affected male
- Unfilled (open) circle = unaffected/normal female
- Filled (solid) circle = affected female
- A square joined to a circle by a horizontal line = mating (marriage) between the two individuals shown
Applying this to the given symbols:
- Symbol A is a single filled (solid black) square, which represents an affected male. …
- CBSE 2026Set ANNUAL1 markMCQQ.This pedigree symbol represents : [figure: a small printed pedigree-chart symbol — two squares (offspring symbols) placed side by side and joined by a short horizontal line, with a single line rising from the midpoint of that horizontal line up to a peak/apex above, giving the whole mark a triangular silhouette (a single point of origin branching down to the two squares).](a) Male(b) Monozygotic twins(c) Mating(d) Dizygotic twins
›Reveal solutionSolution
A horizontal bar directly joining two offspring symbols that both hang from a single line from the parents is the pedigree symbol for monozygotic (identical) twins.
In pedigree analysis, squares represent males and circles represent females, and twins are shown as two offspring symbols connected by short diagonal or vertical lines to a single point on the horizontal line coming down from the parents' mating line. The key distinguishing feature between the two kinds of twins in pedigree notation is whether there is an extra horizontal bar directly joining the two offspring symbols to each other: if such a bar is present (as described here -- the two squares are joined to each other by a short horizontal line, in addition to both connecting up to a single point of origin from the parents), it indicates that the twins arose from a single fertilised egg that split into two -- monozygotic (identical) twins. If the two offspring sy …
- CBSE 2026Set ANNUAL1 markQ.State whether True or False: Colour blind people is able to difference between Red and Green colour.
›Reveal solutionSolution
False - red-green colour-blind people cannot tell red and green apart.
Colour blindness (Daltonism) is an X-linked recessive genetic disorder in which a defect in the cone cells of the retina prevents the person from distinguishing certain colours. In red-green colour blindness, the person cannot correctly differentiat …
- CBSE 2025Set 57/4/11 markMCQQ.In the given pedigree chart, a cross between a normal couple resulted in a son who was haemophilic and a normal daughter. In course of time, when the daughter was married to a normal man, to their surprise the grandson was also haemophilic. [Pedigree chart] Choose the option that indicates the correct inheritance of trait in the above pedigree chart : (A) Autosome linked dominant trait (B) Sex-linked dominant trait (C) Autosomal recessive trait (D) Sex-linked recessive trait
›Reveal solutionSolution
The pedigree shows haemophilia skipping a generation and passing from a carrier mother to her son — the classic pattern of an X-linked recessive trait. The correct option is (D).
The concept: why sex-linked recessive fits
Haemophilia is a well-known X-linked recessive disorder. The key signature of such a trait is that it appears almost exclusively in males, and it is transmitted from a carrier mother (who is unaffected) to her sons. A father never passes his X chromosome to his sons — he gives them a Y — so an affected male always inherits the defective X from his mother. In a pedigree, this creates a pattern where the disease seems to skip a generation: an unaffected woman can carry the allele and pass it to her son, who then expresses it.
Let’s map the given family onto this logic.
Step-by-step reasoning
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The first couple: normal parents, one affected son
A normal man (XY with normal X) and a normal woman have a haemophilic son. Since the son is affected, he must have received an X chromosome carrying the haemophilia allele. His father gave him a Y, so the defective X came from his mother. The mother is phenotypically normal, so she must be a carrier — heterozygous for the recessive allele. This immediately rules out any dominant inheritance (if it were dominant, she would be affected) and any autosomal recessive pattern (where both parents would need to be carriers, but the father would then be normal — possible, but we’ll see why X-linked fits better).
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The daughter: normal phenotype, but a carrier
The couple also has a normal daughter. She received one X from her father (normal) and one X from her mother. Since her mother is a carrier, the daughter has a 50% chance of being a carrier herself. The pedigree tells us she later marries a normal man and has a haemophilic son — so she must indeed be a carrier. This is exactly what we expect for an X-linked recessive: the daughter is unaffected because she has one normal X, but she can pass the defective X to her children.
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The grandson: affected son of a carrier mother and normal father
The daughter (carrier) marries a normal man. Their son inherits his mother’s defective X and his father’s Y — and therefore has haemophilia. The father is normal, so he could not have contributed the disease allele. This pattern — affected son born to unaffected parents, where the mother is the carrier — is the hallmark of X-linked recessive inheritance.
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Why the other options fail
- (A) Autosome linked dominant: If it were autosomal dominant, at least one parent of every affected child would be affected. Here, both parents of the affected son are normal — impossible. …
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- CBSE 2025Set 57/6/11 markMCQQ.Study the pedigree chart of a family sharing the inheritance of sickle cell anemia. [Pedigree chart figure] The trait traced in the above pedigree chart is : (A) Dominant X-linked (B) Autosomal dominant (C) Recessive X-linked (D) Autosomal recessive
›Reveal solutionSolution
Sickle cell anemia is an autosomal recessive disorder, requiring two copies of the mutant allele for expression and affecting both sexes equally without sex-linkage.
Pedigree analysis is a powerful tool in human genetics that allows us to trace the inheritance pattern of a trait through generations of a family. When we examine a pedigree chart for sickle cell anemia, we're looking for clues that reveal whether the trait is dominant or recessive, and whether it's carried on an autosome (any chromosome except the sex chromosomes) or on a sex chromosome (X or Y).
Sickle cell anemia is caused by a mutation in the gene coding for the beta chain of hemoglobin. The mutant hemoglobin (HbS) causes red blood cells to assume a sickle shape under low oxygen conditions, leading to various complications. The key to identifying its inheritance pattern lies in understanding how the trait appears across generations and between sexes.
Distinguishing the pattern
To determine the correct inheritance pattern, we need to consider four possibilities:
Dominant versus Recessive:
A dominant trait appears in every generation when present — an affected individual has at least one affected parent. A recessive trait can skip generations because carriers (heterozygotes) don't show symptoms but can pass the allele to their children. Sickle cell anemia typically appears in children of two apparently healthy parents who are carriers, which immediately suggests a recessive pattern.
Autosomal versus Sex-linked:
An autosomal trait affects males and females equally because the gene is on one of the 22 pairs of autosomes. An X-linked trait shows a characteristic pattern — recessive X-linked traits predominantly affect males (who have only one X chromosome), while females are usually carriers. Dominant X-linked traits affect more females than males and show no male-to-male transmission.
NoteIn sickle cell anemia, both males and females are affected with equal frequency. If you see affected daughters born to unaffected fathers in a pedigree, the trait cannot be X-linked recessive, because fathers pass their X chromosome only to daughters, and an unaffected father cannot carry a recessive X-linked allele.
Why sickle cell anemia is autosomal recessive
The inheritance pattern of sickle cell anemia reveals several telltale features:
- Both sexes are equally affected — there's no male or female predominance, ruling out X-linkage.
- The trait can skip generations — two carrier parents (genotype HbA HbS) who are phenotypically normal can have affected children (genotype HbS HbS).
- Affected individuals typically have unaffected parents — both parents are heterozygous carriers. …
- CBSE 2025Set X11 markMCQQ.Which one among the given is a symbol used in human pedigree analysis for consanguineous mating?(a) A square (male) joined by a single horizontal line to a circle (female)(b) A square (male) joined by a double horizontal line to a circle (female)(c) A single square (male)(d) A single circle (female)
›Reveal solutionSolution
In pedigree analysis a consanguineous (related) mating is shown by a double horizontal line joining the male and female symbols.
In pedigree charts a square represents a male and a circle a female. A single horizontal line connecting them denotes a normal mating between unrelated individuals, whereas a double horizontal line denotes a consanguineous mating (between blood relativ …
- 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 …
- CBSE 2025Set ANNUAL1 markQ.Define heredity.
›Reveal solutionSolution
Heredity = passing on of hereditary characters from one generation to the next via genes.
Heredity is defined as the biological process by which physical and other characteristics (traits) of parents are transmitted to their offspring. This transmission takes place through genes, which are specific segments of the DNA molecule present on chromosomes, …
- CBSE 2024Set 57/1/11 markMCQQ.The type of bond represented by the dotted line '– – – – –' in a schematic polynucleotide chain is: [Schematic polynucleotide chain shown with P (phosphate), S (sugar) and B (base); dotted lines connect S to B.] (A) Hydrogen bond (B) Peptide bond (C) N-glycosidic linkage (D) Phosphodiester bond
›Reveal solutionSolution
The dotted line connecting sugar (S) to base (B) in a polynucleotide represents the N-glycosidic linkage, the covalent bond that attaches nitrogenous bases to the pentose sugar. The answer is (C).
A polynucleotide chain has three repeating components: a phosphate group, a pentose sugar, and a nitrogenous base. Understanding how these connect is fundamental to DNA and RNA structure.
The sugar-phosphate backbone forms the structural spine of nucleic acids, with phosphodiester bonds linking one sugar's 5′ carbon to the next sugar's 3′ carbon through a phosphate group. But the bases — the information-carrying units — must attach to this backbone somehow. That attachment is what we're identifying here.
The N-glycosidic linkage is a covalent bond between the anomeric carbon (C1′) of the pentose sugar and a nitrogen atom of the nitrogenous base. In purines (adenine, guanine), this nitrogen is N9; in pyrimidines (cytosine, thymine, uracil), it's N1. This bond is called "glycosidic" because it resembles the linkage in carbohydrates, and "N-" specifies that it involves a nitrogen atom rather than oxygen.
Let's eliminate the other options systematically:
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Hydrogen bonds (A) are weak, non-covalent interactions that hold complementary base pairs together across the two strands of a DNA double helix (A–T, G–C). They do NOT attach bases to the sugar within a single strand. The schematic shows a single polynucleotide chain, not inter-strand pairing.
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Peptide bonds (B) link amino acids in proteins through a C–N bond between a carboxyl group and an amino group. Nucleic acids contain no peptide bonds — this is a protein-specific linkage. …
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- CBSE 2024Set 57/2/11 markMCQQ.In the double helical structure of DNA molecule, the strands are : (A) identical and complementary (B) identical and non-complementary (C) anti-parallel and complementary (D) anti-parallel and non-complementary
›Reveal solutionSolution
DNA's double helix features two strands running in opposite directions (anti-parallel) with bases pairing by Watson-Crick rules (complementary). The answer is (C).
Why DNA strands must be both anti-parallel and complementary
The architecture of DNA isn't arbitrary—it's dictated by the chemistry of how nucleotides bond and how bases recognize each other. Understanding these two properties separately, then seeing why they must coexist, reveals the elegance of the double helix.
The complementarity principle
When Watson and Crick solved DNA's structure in 1953, the breakthrough was recognizing that bases pair in a specific way: adenine (A) always pairs with thymine (T) through two hydrogen bonds, while guanine (G) pairs with cytosine (C) through three hydrogen bonds. This isn't random preference—it's geometric necessity.
The purine bases (A and G, with their double-ring structure) are larger than the pyrimidine bases (T and C, single-ring). If two purines tried to pair, they'd be too bulky and distort the helix. If two pyrimidines paired, they'd be too small to bridge the gap. Only purine-pyrimidine pairs maintain the uniform diameter of the helix (about 2 nm).
Beyond size, the hydrogen-bonding patterns are specific:
- A and T have exactly the right donor and acceptor groups to form two stable H-bonds
- G and C form three H-bonds in perfect alignment
- Other combinations either can't form enough bonds or have steric clashes
This means if one strand reads 5'-ATGC-3', the other must read 3'-TACG-5' to satisfy base-pairing rules. The strands are complementary, not identical.
The anti-parallel orientation
Now consider the sugar-phosphate backbone. Each nucleotide has a deoxyribose sugar with a phosphate group attached to its 5' carbon and the next nucleotide's sugar attached via its 3' carbon. This creates directionality: one end of a strand has a free 5' phosphate, the other a free 3' hydroxyl.
In the double helix, the two strands run in opposite directions—one goes 5' → 3' while its partner goes 3' → 5'. This anti-parallel arrangement is required because:
- The geometry of base pairing only works when the glycosidic bonds (connecting base to sugar) are positioned correctly relative to each other
- The major and minor grooves of the helix form only when strands are anti-parallel
- The hydrogen bonds between bases align properly only in this configuration
If the strands were parallel (both 5' → 3'), the bases couldn't pair—the geometry would be all wrong, with the glycosidic bonds pointing in incompatible directions. …
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