BiologyICSE Class 10 · Structure of Chromosome

Chromatin and Genes: How DNA Is Packed and Carries Traits (ICSE Class 10 Biology)

Updated 15 October 2026 · Fact-checked

Chromatin is the thread-like network of DNA wound round proteins (histones) in a non-dividing nucleus; it coils tightly into chromosomes during division. A gene is a segment of DNA at a fixed position (locus) on a chromosome that carries the information for one character. Genes are the units of heredity passed to offspring.

Chromatin and chromosomes are the same DNA and histone material; it condenses for division, and a gene is one segment of the DNA.

Illustration drawn for QuestionBuddy.

Understand Chromatin and Genes

Chromatin. Stain a non-dividing nucleus and you see a fine, tangled network of threads that takes up basic dyes strongly. This is chromatin ('chroma' means colour). It is made of:

DNA, the hereditary material, and
proteins, mainly basic proteins called histones, round which the DNA is wound.

The DNA is wrapped round groups of histones like thread round a series of beads, so chromatin looks like 'beads on a string' under an electron microscope.

Why packing is needed. The DNA of a single human cell, if stretched out, would be about 2 metres long, yet it fits into a nucleus only a few thousandths of a millimetre across. Winding round histones, then coiling again and again, packs it in.

Chromatin to chromosome. When the cell prepares to divide, chromatin coils and condenses further into short, thick, separate chromosomes, which are easy to move and share between daughter cells. After division they uncoil back into chromatin. So chromatin and chromosomes are the same material in two states:

Feature	Chromatin	Chromosome
State	loosely coiled, thin threads	tightly coiled, thick rods
When seen	non-dividing (interphase) nucleus	during cell division
Visibility	network, individual threads not distinct	separate bodies, can be counted
Activity	genes can be read and used	packed for transport; little gene use

Genes.

A gene is a segment (length) of DNA on a chromosome that carries the information for a particular character (trait), usually by coding for a particular protein.
Genes are the units of heredity: they pass from parents to offspring through gametes.
Each gene occupies a fixed position on a chromosome, called its locus (plural loci).
Each chromosome carries many genes in a linear order, like beads on a thread; a human cell has thousands of genes on its 46 chromosomes.
In a body cell genes occur in pairs, one on each homologous chromosome, at the same locus. The two forms of a gene that control contrasting expressions of the same character (for example, tall and dwarf) are called alleles.

How a gene acts (elementary). The order of bases in a gene is a code. The cell reads the code to make a protein (for example, an enzyme that makes a pigment). The protein produces the trait (for example, flower colour). A change in the base sequence (a mutation) may change the protein and hence the trait.

Flow chart: from DNA to trait. Gene (a sequence of bases in DNA) → protein (often an enzyme) → character (trait) seen in the organism.

Flow chart: levels of packing. DNA double helix → wound round histone proteins → chromatin fibre → further coiled → chromosome (two chromatids at metaphase); a gene is a short stretch of the DNA within it.

Draw-and-label checklist: DNA, gene and chromosome together. Draw a metaphase chromosome (X shape, centromere marked). From one arm, draw a zoom-out box showing a coiled thread (chromatin), then a further box showing the DNA double helix. Bracket one short stretch of the helix and label it gene. Labels: chromosome, chromatid, centromere, chromatin fibre, histone (bead), DNA double helix, gene, locus (position of the bracket on the chromosome).

Chromatin and Genes: key facts

Definitions: Chromatin DNA plus histone proteins in a loose network
Chromosome condensed chromatin seen in dividing cells
Gene segment of DNA carrying information for one character
Locus fixed position of a gene on a chromosome
Allele one of the alternative forms of a gene at the same locus; Learn word for word; each is a 1-mark question.
Size order: 1.Nucleotide
2.gene
3.DNA molecule
4.chromosome
5.nucleus
6.cell; A gene is part of DNA; a chromosome is DNA plus protein.
Gene to character: $G e n e \to p r o t e in (e n z y m e) \to c ha r a c t er$; A mutation in the gene can change the character.

Chromatin	DNA plus histone proteins in a loose network
Chromosome	condensed chromatin seen in dividing cells
Gene	segment of DNA carrying information for one character
Locus	fixed position of a gene on a chromosome
Allele	one of the alternative forms of a gene at the same locus

How to answer Chromatin and Genes questions

Questions here are definitions (1 mark), differences (2 marks), the relationship between DNA, genes and chromosomes, and sequencing terms by size.

1Define with a category and a function: 'A gene is a segment of DNA (category) that carries information for a particular character (function).'
2For chromatin versus chromosome, compare the same features: state of coiling, when seen, visibility.
3For 'relationship' questions, use the containment chain: chromosome contains DNA; DNA contains genes; genes are sequences of bases.
4In sequence questions, order from smallest to largest unless told otherwise, and check that each item is part of the next.

Chromatin and Genes in real life

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

Packing a very long thread

Suppose you must fit a 2-kilometre-long thin thread into a small tiffin box without tangling it, and still be able to pull out any metre of it when needed.

How does a cell solve the same problem with its DNA, and why must the packing be reversible?

Try it first, then show the solutionSolution

A cell's DNA is about 2 metres long but its nucleus is microscopic.
DNA is wound round histone proteins (beads), and the beaded thread is coiled again and again to form chromatin.
Loose chromatin lets the cell open the stretches (genes) it needs to read.
Before division it condenses fully into chromosomes so the copies can be separated without tangling, then uncoils again afterwards.

Answer: By winding DNA round histones and coiling it into chromatin and chromosomes; the packing must loosen so genes can be read.

Memory hook: Wind on beads, coil the string, pack for the move.

Gene, protein, trait

Suppose two rose plants are identical except that one has red petals and the other white. A gardener learns that the white plant cannot make one enzyme needed for the red pigment.

Explain the difference using the term gene.

Try it first, then show the solutionSolution

The red pigment is made by an enzyme, which is a protein.
The information for making the enzyme lies in a gene, a sequence of DNA bases.
In the white plant this gene is in a form (allele) that does not give a working enzyme.
No working enzyme means no pigment, so the petals are white.

Answer: The plants carry different alleles of the gene for the pigment enzyme; one allele gives the enzyme and red colour, the other does not.

Memory hook: Gene → enzyme → colour.

Tricks to remember Chromatin and Genes

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

Chromatin is chromosome in pyjamas
Relaxed and loose at home (interphase); dressed up and compact to travel (division).
Use it when: Chromatin versus chromosome.
Locus = location
Locus sounds like location: the address of a gene on a chromosome.
Use it when: Defining locus.
Book analogy
Nucleus = library; chromosomes = books; DNA = the text; genes = sentences; bases = letters.
Use it when: Relationship questions.

Chromatin and Genes: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Two definitions · 2 marks · Short answerBoard level

Define: (a) gene, (b) chromatin.

Try it first, then show the solutionSolution

(a) A gene is a segment of DNA, at a fixed locus on a chromosome, that carries the information for a particular character and is passed from parent to offspring (1 mark).
(b) Chromatin is the thread-like network of DNA and proteins (histones) present in the nucleus of a non-dividing cell (1 mark).

Answer: (a) Segment of DNA carrying information for a character. (b) Network of DNA and histone protein in a non-dividing nucleus.

Example 2: Chromatin versus chromosome · 3 marks · Short answerHarder board level

Give two differences between chromatin and chromosome. Are they made of different substances?

Try it first, then show the solutionSolution

Chromatin is loosely coiled and thread-like; a chromosome is tightly coiled and rod-like (1 mark).
Chromatin is seen in a non-dividing nucleus; chromosomes are seen during cell division (1 mark).
No: both are DNA plus proteins; a chromosome is simply condensed chromatin (1 mark).

Answer: Loose versus tightly coiled; interphase versus division. Same material in two states.

Example 3: Arrange in order · 1 mark · Very short answerBoard level

Arrange in increasing order of size: chromosome, nucleotide, gene, nucleus, DNA molecule.

Try it first, then show the solutionSolution

Each item is part of the next: nucleotide → gene → DNA molecule → chromosome → nucleus (1 mark).

Answer: Nucleotide → gene → DNA molecule → chromosome → nucleus.

Example 4: Relationship explained · 3 marks · Short answerHarder board level

Explain the relationship between DNA, genes and chromosomes in about four sentences.

Try it first, then show the solutionSolution

A chromosome is made of a long DNA molecule wound round proteins (1 mark).
Along the DNA are many genes in a line, each a particular segment of DNA at a fixed locus (1 mark).
Each gene carries the code (base sequence) for a protein that controls a character, so chromosomes carry the hereditary information from parents to offspring (1 mark).

Answer: Chromosome = DNA + protein; genes are segments of the DNA at fixed loci; genes code for proteins that control characters.

Example 5: How long is a gene? · 3 marks · Short answerHOTS / competency

Suppose the distance between two neighbouring base pairs in DNA is 0.34 nm. (a) What is the length of a gene that is 1500 base pairs long? (b) The DNA of one human cell has about 6.6 × 10⁹ base pairs. Estimate its total length in metres. (c) What does the answer to (b) tell you about the need for chromatin?

Try it first, then show the solutionSolution

(a) 1500 × 0.34 nm = 510 nm (1 mark).
(b) 6.6 × 10⁹ × 0.34 × 10⁻⁹ m ≈ 2.2 m (1 mark).
(c) About 2 m of DNA must fit into a nucleus only about a millionth of a metre across, so it must be wound round histones and coiled into chromatin (1 mark).

Answer: (a) 510 nm (b) about 2.2 m (c) DNA must be tightly packed with proteins as chromatin.

Where students lose marks in Chromatin and Genes

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

Mistake: Defining chromatin and chromosome identically
Chromatin and chromosomes are the same material in different states, so a definition that does not say which state earns no credit.
Do this: Chromatin: loose network in a non-dividing nucleus. Chromosome: condensed body in a dividing cell.
Usually costs: 1 mark.
Mistake: Vague definition of allele
CISCE examiners' comments (2025, Genetics): candidates defined alleles vaguely as 'forms of traits or characters' without saying alternative forms of a gene at the same locus.
Do this: Allele: one of the alternative forms of a gene present at the same locus on homologous chromosomes.
Usually costs: 1 mark.
Mistake: Saying a gene is a chromosome or a protein
Students blur the levels and write that a gene is 'a part of the chromosome made of protein'.
Do this: A gene is a segment of DNA; it codes for a protein but is not itself protein.
Usually costs: 1 mark.

Exam tips

Use 'segment of DNA' in every definition of a gene.
Mention histones once when describing chromatin: it shows depth.
In relationship questions, write the chain DNA → gene → protein → character.
Keep this topic elementary; detailed packing (nucleosome numbers) is not needed.

Practice questions on Chromatin and Genes

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
The fixed position of a gene on a chromosome is called its:
(a) Allele (b) Locus (c) Centromere (d) Chromatid
Hint
Address.
Answer
(b) Locus.
Q2 · 1 mark · Very short answerBoard level
Name the basic proteins round which DNA is wound in chromatin.
Hint
Beads on a string.
Answer
Histones.
Q3 · 1 mark · Assertion-reasonHarder board level
Assertion (A): Chromosomes are visible only during cell division.
Reason (R): During division chromatin coils and condenses into short, thick bodies.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Why can't we see them in interphase?
Answer
(a).
Q4 · 2 marks · Short answerHarder board level
Distinguish between a gene and an allele.
Hint
Allele is a form of a gene.
Answer
A gene is a segment of DNA that controls a particular character (for example, height). An allele is one of the alternative forms of that gene at the same locus (for example, the tall form or the dwarf form).
Q5 · 2 marks · Short answerHarder board level
Why are genes described as 'units of heredity'?
Hint
What passes from parent to child?
Answer
Each gene controls a particular character and is passed unchanged (as a unit) from parents to offspring through the gametes, so characters are inherited gene by gene.
Q6 · 2 marks · Short answerHOTS / competency
Suppose a gene's base sequence changes so that the protein it codes for can no longer work. Predict the effect on the organism's character and name this kind of change.
Hint
Gene → protein → trait.
Answer
The character controlled by that protein will change or fail to appear (for example, a pigment may not be made). A change in a gene is called a mutation.
Q7 · 3 marks · Case-basedHOTS / competency
Read and answer.
Suppose a cell biologist sees, in one slide, cells with a fine network of threads in the nucleus, and in another, cells with distinct X-shaped bodies and no nuclear membrane.
(a) Name the material in the first slide. (1)
(b) What are the X-shaped bodies, and what is the stage of the cells? (1)
(c) Why is it an advantage for the cell to change from the first form to the second before division? (1)
Hint
Interphase vs metaphase.
Answer
(a) Chromatin. (b) Chromosomes (each of two chromatids); the cells are dividing, probably at metaphase. (c) Compact chromosomes can be separated and moved to the poles without tangling or breaking, so each daughter cell gets a complete set.
Q8 · 2 marks · Short answerBoard level
What is a gene made of, and where in the cell are most genes found?
Hint
Part of a molecule; part of a structure.
Answer
A gene is made of DNA (a sequence of nucleotides). Most genes are found on the chromosomes in the nucleus.

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 does 'chroma' mean?Show answer
Colour (chromatin stains deeply).
2.What is the unit of heredity?Show answer
The gene.
3.Where are the two alleles of a gene found in a body cell?Show answer
At the same locus on the two homologous chromosomes.
4.About how long is the DNA of one human cell when stretched out?Show answer
About 2 metres.
5.What is a change in a gene called?Show answer
Mutation.

Chromatin and Genes: frequently asked questions

What is the difference between chromatin and a chromosome?

Chromatin is the loose network of DNA and protein in a non-dividing nucleus; a chromosome is the tightly coiled form that chromatin takes during cell division.

What is a gene in simple words?

A gene is a short part of the DNA on a chromosome that carries the instructions for one character and is passed from parents to children.

What is a locus?

The fixed position of a gene on a chromosome.

Do all chromosomes carry genes?

Yes. Every chromosome carries many genes arranged in a line along its DNA.