BiologyICSE Class 10 · Chapter 2

Structure of Chromosome ICSE Class 10: Chromatin, Chromatid, Gene and DNA

Updated 14 October 2026 · Fact-checked

A chromosome is a thread-like structure in the nucleus made of DNA and protein, carrying genes. In a dividing cell it has two identical chromatids joined at the centromere. DNA is a double helix of two strands whose bases pair A with T and G with C; a gene is a segment of DNA controlling a trait.

What Structure of Chromosome is about

The instructions for building and running a living body are stored in the nucleus, on structures called chromosomes. In a non-dividing cell they are spread out as a fine network called chromatin; when the cell divides, the chromatin coils up into compact, visible chromosomes so that it can be shared out accurately.

The CISCE 2027 syllabus asks for the basic structure of a chromosome, with an elementary understanding of the terms chromatin, chromatid, gene, structure of DNA and centromere.

Think of it as a hierarchy, from large to small: nucleus → chromosomes → DNA (double helix) → genes (segments of DNA) → bases (A, T, G, C). Each step explains the next chapter, Genetics, where genes on homologous chromosomes are passed on according to Mendel's laws.

Weightage: CISCE publishes no chapter or unit weightage for ICSE Biology. Structure of Chromosome is in the 1. Basic Biology (ii) part of the 2027 syllabus and can be asked in both Section A and Section B of the 80-mark paper.Source: CISCE ICSE Biology (Science Paper 3) syllabus, examination year 2027

Key concepts

Chromatin and chromosomes

Chromatin: a network of fine, thread-like material in the nucleus of a non-dividing cell, made of DNA wound around proteins (histones). It takes up basic stains strongly (hence 'chroma', colour).
Chromosome: a condensed, rod-like body formed when chromatin coils tightly during cell division. Each chromosome carries genes in a linear order.

Every species has a fixed number of chromosomes: humans have 46 (23 pairs) in body cells, of which 22 pairs are autosomes and 1 pair is sex chromosomes (XX in females, XY in males).

Worked example

Why are chromosomes visible only during cell division?

Try it first, then show the solutionSolution

In a non-dividing cell, DNA is uncoiled as fine chromatin threads.
During division, the chromatin coils and condenses into thick, short chromosomes that can be seen under a light microscope.

Answer: Chromatin condenses into chromosomes only during division.

Chromatids and centromere

After DNA replication (S phase), each chromosome consists of two identical sister chromatids.

Chromatid: one of the two identical, thread-like halves of a replicated chromosome.
Centromere: the constricted region where the two sister chromatids are held together. During division, spindle fibres attach to the chromosome at the centromere; in anaphase the centromere splits and each chromatid becomes a daughter chromosome.

The position of the centromere gives a chromosome its shape (it divides the chromosome into two arms).

Worked example

Name the part of a chromosome to which spindle fibres attach, and state what happens to it in anaphase.

Try it first, then show the solutionSolution

Spindle fibres attach at the centromere.
In anaphase the centromere splits, separating the sister chromatids.

Answer: Centromere; it divides.

DNA: the double helix

DNA (deoxyribonucleic acid) is the hereditary material. Its structure was proposed by Watson and Crick as a double helix: two long strands twisted around each other like a spiral (twisted) ladder.

The sides of the ladder: alternating deoxyribose sugar and phosphate groups.
The rungs: pairs of nitrogenous bases joined by hydrogen bonds.
Each unit of sugar + phosphate + base is a nucleotide; DNA is a long chain of nucleotides.

Worked example

Name the model of DNA put forward by Watson and Crick, and the three parts of each nucleotide.

Try it first, then show the solutionSolution

A double helix: two strands coiled around a common axis.
Each nucleotide: deoxyribose sugar, phosphate, nitrogenous base.

Answer: Double helix; sugar (deoxyribose), phosphate, nitrogenous base.

Base pairing in DNA

DNA has four bases: adenine (A), thymine (T), guanine (G) and cytosine (C).

They pair in only one way:

A pairs with T
G pairs with C

This is complementary base pairing. If one strand reads A-T-G-C-C-A, the other must read T-A-C-G-G-T. Because of this rule, the amount of A equals the amount of T, and the amount of G equals the amount of C, in any double-stranded DNA. Complementary pairing is what lets DNA be copied exactly before cell division.

Worked example

In a DNA sample, 30% of the bases are adenine. What percentages of T, G and C are present?

Try it first, then show the solutionSolution

A pairs with T, so T = 30%.
A + T = 60%, so G + C = 40%.
G = C, so each is 20%.

Answer: T 30%, G 20%, C 20%.

Genes

A gene is a segment of DNA on a chromosome that carries the information for a particular character (trait), usually by coding for a protein. Genes are the units of heredity, passed from parents to offspring.

Each gene has a fixed position on a chromosome, called its locus.
A chromosome carries hundreds to thousands of genes in a line.
Homologous chromosomes carry genes for the same characters at the same loci; the different forms of a gene are alleles (see Genetics).

Worked example

Arrange in order of size, largest first: gene, nucleus, chromosome, DNA molecule, base pair.

Try it first, then show the solutionSolution

Nucleus contains chromosomes.
Each chromosome is one long DNA molecule (with proteins).
A gene is a segment of that DNA, made of many base pairs.

Answer: Nucleus > chromosome > DNA molecule > gene > base pair.

Structure of Chromosome in real life

Each concept, met in an everyday situation. Read the situation, answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Complementary base pairing

Suppose a forensic laboratory has read one strand of a short DNA fragment from a sample as A-T-T-G-C-A-G and needs the matching strand to compare with a database.

Write the complementary strand and explain the rule used.

Try it first, then show the solutionSolution

A pairs with T; G pairs with C.
A-T-T-G-C-A-G pairs with T-A-A-C-G-T-C.

Answer: T-A-A-C-G-T-C.

Memory hook: Every A finds a T; every G finds a C.

Fixed chromosome number

Suppose a doctor examining a newborn's chromosomes under a microscope counts 47 instead of 46.

At which stage of cell division would the chromosomes be counted, and why is an extra chromosome significant?

Try it first, then show the solutionSolution

Chromosomes are counted in metaphase, when they are most condensed.
Each chromosome carries many genes.
An extra chromosome means extra copies of many genes, which can disturb normal development.

Answer: Metaphase; an extra chromosome adds many extra genes.

Memory hook: Count at metaphase; every chromosome is a bundle of genes.

DNA in every cell

Suppose police collect a single hair root from a crime scene and say it contains the same DNA as a blood sample from a suspect.

Explain why a hair root and a blood sample from the same person carry the same DNA.

Try it first, then show the solutionSolution

All body cells come from the zygote by mitosis.
Mitosis copies the chromosomes exactly into each daughter cell.
So every nucleated body cell carries the same DNA.

Answer: Mitosis gives every body cell an identical copy of the DNA.

Memory hook: One zygote, one DNA, copied into every cell.

Topics in this chapter

Structure of Chromosome: key facts

Base pairing rule: $A = T, G = C$; A pairs with T; G pairs with C.
Base percentages: $% A + % T + % G + % C = 100$; With %A = %T and %G = %C.
Human chromosome set: $46 = 22 pairs autosomes + 1 pair sex chromosomes$; XX female, XY male.
Nucleotide: $nucleotide = deoxyribose + phosphate + base$; The repeating unit of DNA.

Structure of Chromosome: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

Structure of a chromosome (metaphase)

Draw a labelled diagram of a chromosome.

Draw

An X-shaped body: two parallel rod-like chromatids.
A narrow constriction where they are joined.
Optional: show the coiled chromatin inside the arms.

Label

chromatid
centromere
arm
chromatin (DNA + protein)

Watch out: Make both chromatids the same length and shape: they are identical.

DNA double helix

Draw and label the structure of DNA.

Draw

Two ribbon-like strands twisting around each other.
Horizontal rungs between the strands.
Label a few rungs A-T and G-C.

Label

sugar-phosphate backbone
base pair
hydrogen bonds
double helix

Watch out: Show A opposite T and G opposite C.

How the board asks Structure of Chromosome

The question patterns to expect, and the method that works for each.

Question type	How it is asked	Method
Label a chromosome	A diagram of a replicated chromosome: label chromatid, centromere, and state a function.	Two arms joined at the constriction; the constriction is the centromere.
DNA structure	Name the structure (double helix), its proposers, and the base pairs; complete a complementary strand.	A-T, G-C; spell thymine and adenine correctly.
Terms	Section A Question 2: chromatin, gene, centromere, chromatid, nucleotide.	One precise sentence each.
Odd one out	Pick the odd term (e.g. ribosome, centromere, chromatid, gene) and name the category.	Group by 'parts of a chromosome' or 'components of DNA'.
Base percentage	Given the percentage of one base, find the others.	Use A = T and G = C.

Tricks to remember Structure of Chromosome

Short memory aids. Say each one aloud once, then use it on a question the same day.

Apple in the Tree, Car in the Garage
A with T, C with G.
Use it when: Base pairing.
Centro-MERE meets, centro-SOME spins
Centromere joins chromatids; centrosome (with centrioles) organises the spindle.
Use it when: Not mixing the two terms.
Chromatin is the thread, chromosome is the coil
Thread in a resting nucleus; coiled rod in a dividing one.
Use it when: Defining chromatin and chromosome.
Ladder picture
Sides = sugar + phosphate; rungs = base pairs; twist the ladder = double helix.
Use it when: Describing DNA.

Structure of Chromosome: board-level solved questions

Ordered from board level to HOTS. Cover the solution, write your own, then compare step by step.

Example 1: Centromere · 1 mark · MCQBoard level

Which term is correctly matched with its description?
(a) chromatid: the point where spindle fibres attach
(b) centromere: the constriction joining two sister chromatids
(c) centrosome: one half of a replicated chromosome
(d) gene: the protein around which DNA is wound

Try it first, then show the solutionSolution

A chromatid is one half of a replicated chromosome; the centrosome holds the centrioles; DNA is wound around histone proteins; a gene is a segment of DNA.
The centromere is the constriction that joins the sister chromatids (and where spindle fibres attach), so (b) is correct.

Answer: (b)

Example 2: Base pairs · 1 mark · Assertion-reasonBoard level

Assertion (A): In double-stranded DNA, the amount of adenine equals the amount of thymine.
Reason (R): Adenine always pairs with thymine by hydrogen bonds.
(a) Both (A) and (R) are true, and (R) is the correct explanation for (A).
(b) Both (A) and (R) are true, but (R) is not the correct explanation for (A).
(c) (A) is true and (R) is false.
(d) (A) is false and (R) is true.

Try it first, then show the solutionSolution

Every A on one strand faces a T on the other, so their amounts are equal.

Answer: (a)

Example 3: Base pairing · 2 marks · Short answerBoard level

A short DNA segment has 6 base pairs, 4 of them made with guanine. (a) Name the two kinds of base pair in DNA and the bond holding each pair together. (b) How many adenine bases does the segment contain?

Try it first, then show the solutionSolution

(a) Adenine pairs with thymine, and guanine with cytosine, joined by hydrogen bonds.
(b) 4 pairs are G-C, so 6 − 4 = 2 pairs are A-T, each with one adenine: 2 adenine bases.

Answer: (a) A-T and G-C; hydrogen bonds (b) 2

Example 4: Base percentages · 2 marks · Short answerHarder board level

A DNA sample has 18% cytosine. Find the percentages of guanine, adenine and thymine.

Try it first, then show the solutionSolution

G = C = 18%; G + C = 36%.
A + T = 64%; A = T = 32%.

Answer: G 18%, A 32%, T 32%.

Example 5: Chromatin versus chromosome · 2 marks · Short answerHarder board level

Give two differences between chromatin and chromosomes.

Try it first, then show the solutionSolution

Chromatin: uncoiled, thread-like network; chromosomes: condensed rod-like bodies.
Chromatin: seen in non-dividing nucleus; chromosomes: visible during cell division.

Answer: As above.

Example 6: Gene definition · 2 marks · Short answerHarder board level

Define a gene and state where it is located.

Try it first, then show the solutionSolution

A gene is a segment of DNA that determines a particular character (unit of heredity).
It is located at a fixed position (locus) on a chromosome.

Answer: As above.

Example 7: Complementary strand · 3 marks · Short answerHarder board level

One strand of DNA has the base sequence G-C-T-A-A-G-T. (a) Write the complementary strand. (b) How many hydrogen-bonded base pairs are formed? (c) Name the sugar in DNA.

Try it first, then show the solutionSolution

(a) C-G-A-T-T-C-A.
(b) Seven base pairs.
(c) Deoxyribose.

Answer: (a) C-G-A-T-T-C-A (b) 7 (c) Deoxyribose

Example 8: HOTS: why the coil? · 3 marks · Short answerHOTS / competency

The DNA in one human cell would be about 2 metres long if stretched out, yet it fits in a nucleus a few micrometres wide. (a) How is this possible? (b) Why must it coil into chromosomes before the cell divides?

Try it first, then show the solutionSolution

(a) DNA winds around histone proteins and coils repeatedly into chromatin.
(b) During division the long threads must be separated equally without tangling or breaking.
Compact chromosomes can be moved by spindle fibres accurately.

Answer: Winding on proteins packs it; condensing into chromosomes lets it be shared out safely.

Case-study practice (Section B style)

Read the situation, then answer every part. In the board paper this is a 5-mark question split into 1 + 2 + 1 + 1 marks.

A labelled chromosome

Suppose a diagram shows an X-shaped structure with two identical arms on each side joined at a narrow waist, and the text says it was drawn from a cell at metaphase.

(i) · 1 mark
What are the two identical halves called?
Answer
Answer: Sister chromatids.
(ii) · 2 marks
Name the narrow waist and give its function.
Answer
Answer: Centromere: holds the chromatids together and is where spindle fibres attach.
(iii) · 1 mark
In which phase of interphase did the chromosome become double?
Answer
Answer: S phase (DNA replication).
(iv) · 1 mark
Name the chemical that forms the hereditary material of this structure and its shape.
Answer
Answer: DNA, a double helix.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Define (a) chromatid, (b) centromere. (2 marks) (2 marks)

Step	Marks
One of the two identical halves of a replicated chromosome	1
The constricted region where sister chromatids are joined and spindle fibres attach	1

A student describes DNA as 'a spiral ladder'. Rewrite this as a full description of the Watson-Crick model. (3 marks) (3 marks)

Step	Marks
Double helix: two strands twisted around each other	1
Backbone of alternating deoxyribose sugar and phosphate	1
Bases pair by hydrogen bonds: A with T, G with C	1

Where students lose marks in Structure of Chromosome

The slips examiners see most often, and what to write instead.

Mistake: Centrosome for centromere
CISCE examiners' comments, 2024: candidates wrote centrosome for the point where duplicated chromosomes are joined.
Wrong working
Duplicated chromosomes are joined at the centrosome.
Right working
They are joined at the centromere.
Do this: Centro-MERE meets; centro-SOME spins.
Usually costs: 1 mark.
Mistake: Wrong base pairs and spelling
CISCE examiners' comments, 2025: candidates paired bases wrongly and misspelt thymine as 'thiamine' and adenine as 'adenosine'.
Wrong working
Adenosine pairs with thiamine.
Right working
Adenine pairs with thymine; guanine pairs with cytosine.
Do this: Practise spelling the four bases.
Usually costs: 1-2 marks.
Mistake: Vague description of DNA
CISCE examiners' comments, 2025: for the Watson-Crick structure, candidates wrote 'spiral' or 'twisted ladder' without the term double helix.
Wrong working
DNA has a spiral ladder structure.
Right working
DNA is a double helix.
Do this: Always use the term 'double helix'.
Usually costs: 1 mark.
Mistake: Calling chromatin and chromosome the same thing
A slip worth checking in your own answer (not taken from a CISCE report): chromatin and chromosomes are the same material in different states, so a definition must say which state.
Wrong working
Chromatin is a chromosome.
Right working
Chromatin is the uncoiled network in a non-dividing nucleus; it condenses into chromosomes during division.
Do this: Name the state: uncoiled (chromatin) or condensed (chromosome).
Usually costs: 1 mark.

Practice questions on Structure of Chromosome

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
Which base pairs with adenine in DNA?
(a) Guanine
(b) Cytosine
(c) Thymine
(d) Uracil
Hint
Apple in the Tree.
Answer
(c)
Q2 · 1 mark · MCQBoard level
The structure of DNA is described as:
(a) a single helix
(b) a double helix
(c) a triple helix
(d) a ring
Hint
Watson and Crick.
Answer
(b)
Q3 · 1 mark · Very short answerHarder board level
Pick the odd one out and name the category of the rest: chromatid, centromere, ribosome, gene.
Hint
Parts of a chromosome.
Answer
Ribosome; the others are parts of (or on) a chromosome.
Q4 · 1 mark · Assertion-reasonHarder board level
Assertion (A): Chromosomes are clearly visible in a non-dividing cell.
Reason (R): Chromatin condenses only during cell division.
(a) Both (A) and (R) are true, and (R) is the correct explanation for (A).
(b) Both (A) and (R) are true, but (R) is not the correct explanation for (A).
(c) (A) is true and (R) is false.
(d) (A) is false and (R) is true.
Hint
When are they visible?
Answer
(d)
Q5 · 3 marks · Very short answerBoard level
Name: (a) the proteins around which DNA is wound; (b) the bond between base pairs; (c) the repeating unit of DNA.
Hint
One term each.
Answer
(a) Histones (b) Hydrogen bond (c) Nucleotide
Q6 · 2 marks · Short answerHarder board level
A DNA molecule has 1000 base pairs, of which 300 are A-T pairs. How many guanine bases are there in the molecule?
Hint
Count G-C pairs.
Answer
G-C pairs = 1000 - 300 = 700; each has one G, so 700 guanine bases.
Q7 · 2 marks · Short answerHarder board level
Distinguish between a chromosome and a gene.
Hint
Whole and part.
Answer
A chromosome is a whole thread-like body of DNA and protein carrying many genes; a gene is one segment of that DNA controlling a particular character.
Q8 · 2 marks · Short answerHarder board level
Explain why the two chromatids of a chromosome are genetically identical.
Hint
Copying by pairing.
Answer
They are formed by replication of one DNA molecule in the S phase; complementary base pairing makes an exact copy, so each chromatid has the same base sequence.
Q9 · 2 marks · Short answerHOTS / competency
A student writes the complementary strand of A-G-C-T as A-G-C-T. Explain the error and correct it.
Hint
Pairs, not copies.
Answer
The complementary strand pairs each base with its partner, not the same base: A-G-C-T pairs with T-C-G-A.
Q10 · 2 marks · Short answerHOTS / competency
How many chromatids are present in a human cell at metaphase of mitosis, and how many chromosomes? Explain.
Hint
Each chromosome has two chromatids.
Answer
46 chromosomes, each with two chromatids: 92 chromatids. DNA has replicated but the chromatids are still joined at the centromeres.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.What is chromatin made of?Show answer
DNA and proteins (histones).
2.Who proposed the double helix model?Show answer
Watson and Crick.
3.Base that pairs with guanine?Show answer
Cytosine.
4.Sugar in DNA?Show answer
Deoxyribose.
5.Define gene.Show answer
A segment of DNA that determines a particular character; a unit of heredity.
6.Number of autosomes in a human body cell?Show answer
44 (22 pairs).
7.What is a locus?Show answer
The fixed position of a gene on a chromosome.

Best order to study it

Learn chromatin and chromosome, and the human chromosome number.
Learn chromatid and centromere with a labelled diagram.
Learn DNA: double helix, nucleotide parts, base pairing.
Learn the gene and locus.
Practise base percentage and complementary strand questions.

Quick revision checklist

The night before the exam: can you do each of these without looking?

Can you define chromatin, chromatid, centromere and gene?
Can you describe DNA as a double helix and name its parts?
Can you write the complementary strand of any sequence?
Can you calculate base percentages from one given value?
Can you draw and label a metaphase chromosome?

Structure of Chromosome: frequently asked questions

Is RNA in the ICSE Class 10 chromosome chapter?

No. The 2027 syllabus asks for an elementary understanding of chromatin, chromatid, gene, structure of DNA and centromere.

Do I need to know how DNA replicates?

Only that DNA is copied in the S phase of interphase so that each chromosome has two identical chromatids; the mechanism is not required.

Are genes and alleles the same?

No. A gene controls a character; alleles are the alternative forms of that gene (for example, tall and dwarf).

Based on: CISCE ICSE 2027 Biology (Science Paper 3) syllabus; CISCE ICSE 2027 Biology (Science Paper 3) specimen question paper and marking scheme; CISCE Analysis of Pupil Performance, ICSE Biology (Science Paper 3), 2025; CISCE Analysis of Pupil Performance, ICSE Biology (Science Paper 3), 2024