Chromatid and Centromere: Structure of a Chromosome (ICSE Class 10 Biology)
Updated 15 October 2026 · Fact-checked
A chromosome seen during cell division has two identical sister chromatids, formed when its DNA was copied in the S phase. They are held together at a narrow region called the centromere (primary constriction), where spindle fibres attach. In anaphase the centromere splits and each chromatid becomes a separate daughter chromosome.
Understand Chromatid and Centromere: Structure of a Chromosome
From chromatin to chromosome. In a non-dividing nucleus the hereditary material is a loose network of thin threads called chromatin, made of DNA wound round proteins. When the cell prepares to divide, the chromatin coils and condenses into short, thick, rod-shaped chromosomes that can be seen with a light microscope.
Parts of a chromosome at metaphase (the stage at which it is usually drawn):
- Chromatids: the two identical, thread-like halves of the chromosome, lying side by side. They are called sister chromatids because they are exact copies made by DNA replication in the S phase. Each chromatid contains one DNA double helix.
- Centromere: the narrow, constricted region (the primary constriction) where the two sister chromatids are held together. Spindle fibres attach here during cell division (through small disc-like structures on its sides).
- Arms: the parts of the chromosome on either side of the centromere. The position of the centromere decides whether the two arms are equal or unequal, which gives each chromosome its typical shape.
What happens to chromatids and centromeres in mitosis.
- S phase: one chromatid per chromosome becomes two sister chromatids joined at one centromere.
- Metaphase: chromosomes line up at the equator, spindle fibres attached at the centromeres.
- Anaphase: each centromere splits; the sister chromatids separate and each becomes a daughter chromosome with its own centromere.
- Daughter cells: each chromosome again has one chromatid until the next S phase.
Counting rule. Number of chromosomes = number of centromeres. Number of chromatids = 2 × chromosomes after the S phase and before anaphase. A human body cell (46 chromosomes) therefore has 92 chromatids at metaphase.
Chromosome number. Each species has a fixed number. Human body cells have 46 chromosomes in 23 pairs: 22 pairs of autosomes and 1 pair of sex chromosomes (XX in females, XY in males). Gametes have 23.
Three words that sound alike (an examiners' favourite):
| Term | What it is | Where |
|---|---|---|
| Centromere | constriction that joins two sister chromatids; spindle fibres attach here | on the chromosome |
| Centrosome | region near the nucleus of an animal cell that contains the centrioles | in the cytoplasm |
| Centriole | one of a pair of small cylindrical bodies inside the centrosome; they move to the poles and form the spindle in animal cells | inside the centrosome |
Chromatid versus chromatin versus chromosome. Chromatin is the uncoiled form in a non-dividing nucleus; a chromosome is the coiled form during division; a chromatid is one of the two identical halves of a replicated chromosome.
Draw-and-label checklist: a chromosome at metaphase. An X-shaped figure: two parallel rod-shaped chromatids, slightly curved, touching at a narrow waist. Show the waist clearly pinched. Optionally draw coiled threads inside one arm. Labels: chromatid (point to one rod only), sister chromatids (bracket both), centromere (at the waist), arm (one end), chromatin / DNA thread (inside). Title: Structure of a chromosome.
Chromatid and Centromere: Structure of a Chromosome: key facts
- Parts of a chromosome
Chromatid one of two identical halves of a replicated chromosome Centromere primary constriction holding the sister chromatids, site of spindle attachment Arm part of the chromosome on each side of the centromere - Draw and label all three.
- Counting
Chromosomes count the centromeres Chromatids at metaphase twice the chromosome number Human body cell at metaphase 46 chromosomes, 92 chromatids - After anaphase each chromatid is a chromosome.
- Centro- words
Centromere joins sister chromatids Centrosome contains the centrioles Centriole forms spindle and asters in animal cells - Mixing these up costs marks every year.
- Human chromosomes
Body cell 46 (23 pairs) Autosomes 22 pairs Sex chromosomes 1 pair, XX or XY Gamete 23 - Links to Genetics.
How to answer Chromatid and Centromere: Structure of a Chromosome questions
This topic is tested by 1-mark naming items (centromere, chromatid), labelling a chromosome, and counts of chromosomes and chromatids at different stages.
- 1Read the description carefully: 'joins two chromatids' → centromere; 'one half of a duplicated chromosome' → chromatid; 'contains centrioles' → centrosome.
- 2For counts, note the stage: before S or after anaphase, chromatids equal chromosomes; between S and anaphase, chromatids are double.
- 3In a diagram, the centromere must be a visible narrowing, and the label line must touch it exactly.
- 4When defining, include the word 'identical' or 'sister' for chromatids and 'constriction' for centromere.
Chromatid and Centromere: Structure of a Chromosome 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.
Sister chromatids as photocopies
Suppose a teacher photocopies a worksheet and staples the original and the copy together at one corner before handing the pair to a monitor, who later tears them apart and gives one to each of two groups.
Match each part of this story to a chromosome in mitosis.
Try it first, then show the solutionSolution
- The original worksheet copied exactly matches DNA replication in the S phase.
- The original and the copy are like two identical sister chromatids.
- The staple holding them together matches the centromere.
- Tearing the staple and giving one sheet to each group matches anaphase, when the centromere splits and each chromatid goes to a different daughter cell.
Answer: Copy = sister chromatid; staple = centromere; tearing apart = anaphase splitting of the centromere.
Memory hook: Copy, staple, separate.
Tricks to remember Chromatid and Centromere: Structure of a Chromosome
Short memory aids. Say each one aloud once, then use it on a question the same day.
MERE is where they MEet
Centro-MERE: the point where sister chromatids MEet. Centro-SOME: SOME centrioles live here.
Use it when: Centromere versus centrosome.
Chromatid = 'chroma-twin'
A chromatid always has an identical twin until anaphase.
Use it when: Defining a chromatid.
Count the waists
Each pinched waist (centromere) is one chromosome.
Use it when: Counting chromosomes in a diagram.
Chromatid and Centromere: Structure of a Chromosome: solved examples
Ordered from board level to HOTS. Try each one before you open the solution.
Example 1: Name the structure · 2 marks · Short answerBoard level
Name the following: (a) the point at which two duplicated chromatids are joined, (b) the network of DNA and protein threads in a non-dividing nucleus. (Part (a) was asked as an MCQ in 2024.)
Try it first, then show the solutionSolution
- (a) Centromere (1 mark).
- (b) Chromatin (1 mark).
Answer: (a) Centromere (b) Chromatin.
Example 2: Three centro- terms · 3 marks · Short answerHarder board level
Distinguish between centromere, centrosome and centriole with reference to their location and function.
Try it first, then show the solutionSolution
- Centromere: on the chromosome; joins sister chromatids and is the site of spindle-fibre attachment (1 mark).
- Centrosome: in the cytoplasm near the nucleus of an animal cell; contains the centrioles (1 mark).
- Centriole: a pair of small cylinders inside the centrosome; they move to opposite poles and organise the spindle and asters during division (1 mark).
Answer: Centromere joins chromatids (on chromosome); centrosome holds centrioles (cytoplasm); centrioles form the spindle (inside centrosome).
Example 3: Chromatid counts · 2 marks · Short answerHarder board level
A plant cell has 14 chromosomes. How many (a) centromeres and (b) chromatids does it have at metaphase? How many chromosomes move to each pole at anaphase?
Try it first, then show the solutionSolution
- (a) One centromere per chromosome: 14; (b) two chromatids each: 28 (1 mark).
- At anaphase the 28 chromatids become 28 chromosomes, 14 going to each pole (1 mark).
Answer: (a) 14 (b) 28; 14 to each pole.
Example 4: Chromosome diagram · 2 marks · Long answerBoard level
Draw a labelled diagram of a chromosome as seen at metaphase. Label: chromatid, centromere, arm.
Try it first, then show the solutionSolution
- An X-shaped figure of two parallel rods joined at a visible narrow waist (1 mark).
- Labels: one chromatid, the centromere at the waist, one arm (1 mark).
Answer: Two sister chromatids joined at a centromere, with one arm labelled.
Example 5: Why the centromere matters · 3 marks · Case-basedHOTS / competency
Suppose a chromosome loses its centromere because of damage. Predict what will happen to it in the next mitosis and to the daughter cells. Give reasons.
Try it first, then show the solutionSolution
- Spindle fibres attach to a chromosome only at its centromere, so this chromosome cannot be attached (1 mark).
- It will not be lined up at the equator or pulled to a pole in anaphase, so its chromatids may be left behind or both go into one cell (1 mark).
- One or both daughter cells will lack that chromosome and its genes, so they may not function normally (1 mark).
Answer: It cannot attach to spindle fibres, so it is not shared properly; daughter cells may lack it and its genes.
Where students lose marks in Chromatid and Centromere: Structure of a Chromosome
The slips examiners see most often, and what to write instead.
Mistake: Writing centrosome for centromere
CISCE examiners' comments (2024): candidates wrote centrosome in place of centromere for the point where duplicated chromosomes are joined; examiners advised bringing out clear differences between centrosome, centromere and centriole.
Do this: Learn the three-row table above.
Usually costs: 1 mark.
Mistake: Confusing centromere with chromatid or chromosome
CISCE examiners' comments (2025): in a 'name the structure' question candidates confused centromere with chromatid, chromosome or centrosome.
Do this: Centromere is a region (a constriction); chromatid is a strand; chromosome is the whole body.
Usually costs: 1 mark.
Mistake: Doubling the chromosome count at metaphase
Students double chromosomes instead of chromatids after the S phase.
Wrong working
Human cell at metaphase: 92 chromosomes.
Right working
46 chromosomes, 92 chromatids.
Do this: Count centromeres for chromosomes.
Usually costs: 1 mark.
Exam tips
- Define chromatid with the words 'identical' and 'replicated chromosome'.
- Call the centromere the 'primary constriction' in a definition for full marks.
- Label lines must point exactly to the narrow region for the centromere.
- Always state the stage when giving chromatid numbers.
Practice questions on Chromatid and Centromere: Structure of a 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
The two identical halves of a duplicated chromosome are called:
(a) Chromomeres (b) Chromatids (c) Centromeres (d) CentriolesHint
Sister ___.
Answer
(b) Chromatids.
Q2 · 1 mark · Very short answerBoard level
How many autosomes are present in a human body cell?
Hint
Total pairs minus the sex pair.
Answer
44 (22 pairs).
Q3 · 1 mark · Assertion-reasonHarder board level
Assertion (A): The number of chromosomes in a cell equals the number of centromeres.
Reason (R): Each chromosome, whether it has one chromatid or two, has only one centromere.
(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
Counting rule.
Answer
(a).
Q4 · 2 marks · Short answerHarder board level
A human skin cell is at metaphase. How many chromatids does it contain? How many chromatids will each daughter chromosome have just after anaphase?
Hint
46 × 2.
Answer
92 chromatids; each daughter chromosome has one chromatid.
Q5 · 2 marks · Short answerHarder board level
State two differences between chromatin and chromosome.
Hint
State of coiling; when seen.
Answer
Chromatin is a loose, uncoiled network of thin threads; a chromosome is a tightly coiled, short, thick rod. Chromatin is present in a non-dividing nucleus; chromosomes are visible during cell division.
Q6 · 2 marks · Short answerHarder board level
Write the function of the centromere during (a) metaphase and (b) anaphase.
Hint
Attachment, then splitting.
Answer
(a) Spindle fibres attach to it, and it lines the chromosome up at the equatorial plate. (b) It splits, so that the sister chromatids separate and move to opposite poles.
Q7 · 2 marks · Short answerHOTS / competency
Suppose a student draws a chromosome as a single straight rod with a label 'centromere' at one end and asks why marks were cut. Give two corrections.
Hint
Two chromatids; centromere is a constriction joining them.
Answer
A metaphase chromosome must be drawn with two sister chromatids side by side, not one rod; and the centromere must be shown as a narrow constriction where the chromatids are joined (it is not simply 'the end').
Q8 · 3 marks · Case-basedHOTS / competency
Read and answer.
Suppose a cell of an animal has 12 chromosomes in G1.
(a) How many centromeres does it have in G2? (1)
(b) How many chromatids does it have in G2? (1)
(c) How many chromosomes will each of its gametes have, and why? (1)Hint
S doubles chromatids, not centromeres.
Answer
(a) 12. (b) 24. (c) 6, because gametes are formed by meiosis, which halves the chromosome number.
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 the primary constriction of a chromosome called?Show answer
Centromere.
2.When do sister chromatids form?Show answer
In the S phase of interphase (DNA replication).
3.When does the centromere split?Show answer
In anaphase.
4.How many chromatids does a cell with 2n = 8 have at metaphase?Show answer
16.
5.Which structure contains the centrioles?Show answer
The centrosome.
Chromatid and Centromere: Structure of a Chromosome: frequently asked questions
What is the difference between a chromatid and a chromosome?
A chromosome is the whole structure; after DNA replication it consists of two identical chromatids joined at a centromere. Each chromatid becomes a chromosome after the centromere splits in anaphase.
What is the function of the centromere?
It holds the sister chromatids together and is the point where spindle fibres attach, so chromosomes can be moved to the poles during division.
Are centromere and centrosome the same?
No. The centromere is a constriction on a chromosome; the centrosome is a region in the cytoplasm of animal cells that contains the centrioles.
How many chromosomes do human body cells have?
46, in 23 pairs: 22 pairs of autosomes and one pair of sex chromosomes.