Structure of Chromosomes, Cell Cycle and Cell Division

ICSE Class 10 · Biology · Chapter 2

What this chapter covers

Why cells divide
Structure of chromosome: chromatin, chromatid and centromere
Gene and the structure of DNA
The cell cycle
Interphase: G1, S and G2
The mitotic phase
Mitosis and its stages
Meiosis as a reduction division
Homologous chromosomes and crossing over
Significance of mitosis and meiosis
Differences between mitosis and meiosis

Key points

Chromatin is the uncoiled network of DNA and histone proteins; it condenses into chromosomes before division.
A duplicated chromosome has two sister chromatids joined at the centromere (primary constriction), where spindle fibres attach.
DNA is a double helix of nucleotides (deoxyribose sugar, phosphate, base); A pairs with T and G pairs with C.
A gene is a segment of DNA that codes for a protein and controls a characteristic.
The cell cycle has interphase (G1 growth, S DNA replication, G2 preparation) and a short M phase (karyokinesis and cytokinesis).
Mitosis has four stages: prophase, metaphase, anaphase and telophase, and gives two identical diploid cells.
Animal cells divide the cytoplasm by a cleavage furrow; plant cells form a cell plate.
Meiosis is a reduction division: one DNA copy, two divisions, four haploid cells; stages are not required.
Homologous chromosomes come one from each parent; crossing over between their non-sister chromatids creates new gene combinations.
Mitosis serves growth, repair and asexual reproduction; meiosis serves gamete formation, constant chromosome number and variation.

Questions answered

Define chromatin and state how it differs from a chromosome.

Chromatin is the thread-like network of DNA and histone proteins in the nucleus of a non-dividing cell. During division chromatin coils and condenses into short, thick, distinct chromosomes that are visible under a light microscope.

What is a centromere? State its role in cell division.

A centromere is the constricted region where the two sister chromatids of a chromosome are joined. Spindle fibres attach to it, and the chromosome is pulled to the pole by it during anaphase.

Distinguish between sister chromatids and homologous chromosomes.

Sister chromatids are two identical copies of one chromosome made during the S phase and joined at a centromere. Homologous chromosomes are a pair of similar chromosomes, one from each parent, carrying genes for the same traits but possibly different versions of them.

A DNA sample has 18% guanine. Calculate the percentages of the other three bases.

Cytosine = 18%. A + T = 100% − 36% = 64%, so adenine = 32% and thymine = 32%.

Define a gene.

A gene is a segment of DNA on a chromosome that carries the instruction for a particular protein and so controls a particular characteristic.

Name the three sub-phases of interphase and state one event in each.

G1: the cell grows and makes proteins and organelles. S: DNA is replicated. G2: the cell makes proteins for the spindle and prepares for division.

Why is interphase wrongly called the "resting stage"?

The nucleus looks inactive because the chromatin is uncoiled, but the cell is busy growing, replicating its DNA and preparing for division.

In which stage of mitosis (a) do chromosomes line up at the equator, (b) does the nuclear membrane reappear, (c) does the centromere split?

(a) Metaphase. (b) Telophase. (c) Anaphase.

Past board exam questions

ICSE 2027 sample paper · 1 mark

Rahul fell down while riding a bicycle and injured his knee. The injury healed within a few days due to a kind of cell division. Name the type of cell division responsible for the healing of wound.

Show answer

Mitosis

ICSE 2027 sample paper · 2 marks

Rahul fell down while riding a bicycle and injured his knee. The injury healed within a few days due to a kind of cell division. How many chromosomes are present in each cell of the human body? What will be number of chromosomes in the daughter cells during the healing process?

Show answer

Each human body cell has 46 chromosomes (23 pairs). The daughter cells formed by mitosis during healing also have 46 chromosomes (23 pairs).

ICSE 2027 sample paper · 3 marks

Draw a neat, labelled diagram of an animal cell having 4 chromosomes in the metaphase stage.

Show answer

Diagram: an animal cell with no nuclear membrane; two centrioles (asters) at opposite poles with spindle fibres between them; 4 chromosomes, each with two sister chromatids joined at the centromere, lined up on the equatorial (metaphase) plate and attached to spindle fibres at the centromere. Labels: cell membrane, centriole/aster, spindle fibre, chromosome, chromatid, centromere, equatorial plate.

ICSE 2026 board paper · 1 mark

Which is the resting but metabolically active stage of the cell cycle?

Show answer

Interphase.

ICSE 2026 board paper · 3 marks

Draw a neat, labelled diagram of an animal cell showing the Prophase stage of Mitosis with four chromosomes.

Show answer

Diagram: a rounded animal cell (cell membrane, no cell wall) in which — • four condensed, thread-like chromosomes are visible, each made of two sister chromatids joined at a centromere; • the nuclear membrane is breaking down (shown as a broken/dotted outline) and the nucleolus is disappearing; • two pairs of centrioles are moving towards opposite poles, with asters (radiating rays) around them, and spindle fibres beginning to form between them. Labels: cell membrane, centriole, aster, spindle fibres, chromosome (chromatid, centromere), disappearing nuclear membrane, cytoplasm.

ICSE 2026 board paper · 2 marks

Differentiate between Mitosis in plant cell and animal cell based on Cytokinesis.

Show answer

Plant cell: cytokinesis takes place by the formation of a cell plate in the middle of the cell, which grows outwards (centrifugally) to the cell wall. Animal cell: cytokinesis takes place by furrowing (constriction) of the cell membrane, which begins at the periphery and deepens inwards (centripetally) until the cell divides.

ICSE 2025 compartment paper · 1 mark

The number of chromosomes in the somatic cells of a human body before mitosis and after mitosis is:

23, 23
26, 46
23, 46
46, 46

Show answer

46, 46

ICSE 2025 board paper · 1 mark

A phase of cell division where the spindle fibres disappear is:

Telophase
Prophase
Metaphase
Anaphase

Show answer

Telophase