BiologyICSE Class 10 · Cell Cycle and Cell Division

The Cell Cycle: Interphase (G1, S, G2) and M Phase (ICSE Class 10 Biology)

Updated 15 October 2026 · Fact-checked

The cell cycle is the sequence of events from one cell division to the next. It has a long interphase, made of G1 (growth), S (DNA replication) and G2 (preparation for division), followed by a short M phase in which the nucleus divides by mitosis (karyokinesis) and the cytoplasm divides (cytokinesis).

Notice that interphase (G1, S, G2) fills almost the whole cycle, while the M phase, where the cell divides, is only a thin slice.

Illustration drawn for QuestionBuddy.

Understand The Cell Cycle: Interphase and M Phase

What the cell cycle is. A cell that divides goes round a repeating cycle: it grows, copies its DNA, prepares, and then divides into two. The cell cycle is this ordered series of events between the formation of a cell and its own division into two daughter cells. It has two main parts: interphase and the M phase (mitotic phase).

Interphase: busy, not resting. Older books called interphase the 'resting stage' because no division is seen under a microscope. In fact the cell is very active, growing and copying its DNA. Interphase takes up most of the cycle: in a typical human cell growing in culture the whole cycle takes about 24 hours, of which the division itself lasts only about one hour, so interphase is more than 95% of the time. Interphase has three sub-phases:

G1 phase (first gap): the interval after the previous division. The new cell grows, makes proteins and RNA, and forms organelles. DNA is not copied yet.
S phase (synthesis): DNA replicates. Each chromosome is copied into two sister chromatids joined at the centromere. The amount of DNA doubles, but the number of chromosomes stays the same (a 2n cell is still 2n, because two joined chromatids still count as one chromosome). In animal cells the centriole also duplicates at this time.
G2 phase (second gap): the cell continues to grow and makes the proteins needed for division, for example for the spindle.

M phase (mitotic phase). The short, dramatic part of the cycle:

Karyokinesis: division of the nucleus by mitosis (prophase, metaphase, anaphase, telophase), which shares the sister chromatids equally.
Cytokinesis: division of the cytoplasm (furrowing in animal cells, cell plate in plant cells).

At the end there are two daughter cells, each of which can enter G1 and start the cycle again.

Flow chart. G1 (growth) → S (DNA copied, 2 chromatids per chromosome) → G2 (proteins for division) → M: karyokinesis → cytokinesis → two daughter cells in G1.

Tracking DNA and chromosomes through one cycle (cell with 2n = 4):

Stage	Chromosomes	Chromatids per chromosome	DNA amount
G1	4	1	normal (call it 1 unit)
after S, in G2	4	2	doubled (2 units)
each daughter cell after M	4	1	back to 1 unit

Not every cell keeps cycling. Cells in the root and shoot tips (meristems) of plants, in the skin and in the bone marrow divide frequently. Many specialised cells, such as most nerve cells and heart muscle cells in adults, stop dividing after they mature.

Why the cycle must be ordered. If a cell divided before finishing S, a daughter would lack some DNA; if it copied DNA twice without dividing, its nucleus would carry extra DNA. The order G1 → S → G2 → M ensures that each daughter receives one complete copy of every chromosome.

Draw-and-label checklist: the cell cycle. A circle divided into four unequal sectors drawn clockwise: a large G1 sector, a large S sector (label 'DNA replication'), a smaller G2 sector and a narrow M sector (label 'mitosis + cytokinesis'). Bracket G1, S and G2 together as interphase. Draw an arrow round the circle to show direction and two small daughter cells leaving the M sector.

The Cell Cycle: Interphase and M Phase: key facts

Phases and their main event: G1: cell growth, protein and RNA synthesis, organelles formed
S: DNA replication, each chromosome becomes two sister chromatids
G2: growth continues, proteins for division made
M: karyokinesis (mitosis) then cytokinesis; G1, S and G2 together make interphase.
What changes in S phase: DNA amount doubles
Chromosome number unchanged
Chromatids per chromosome one becomes two
Centriole (animal cell) duplicates; The most-tested fact of the topic.
Typical timings (human cell in culture): Whole cycle about 24 hours
M phase about 1 hour
Interphase more than 95 per cent of the cycle; Timings differ between organisms and cell types.

DNA amount	doubles
Chromosome number	unchanged
Chromatids per chromosome	one becomes two
Centriole (animal cell)	duplicates

Whole cycle	about 24 hours
M phase	about 1 hour
Interphase	more than 95 per cent of the cycle

How to answer The Cell Cycle: Interphase and M Phase questions

Questions ask you to name the phase in which an event occurs, explain why interphase is not resting, track DNA or chromosome numbers, or draw the cycle.

1Match the event to the phase: growth → G1; DNA replication → S; proteins for division → G2; nuclear division → M (karyokinesis); cytoplasmic division → cytokinesis.
2For number questions, separate 'chromosome number' (count centromeres; unchanged in S) from 'DNA amount' (doubles in S, halves back at division).
3For 'why not resting', give two activities with their sub-phases: growth in G1 and G2, DNA replication in S.
4In a diagram, make interphase much larger than M, label all four phases and show the direction of the cycle.

The Cell Cycle: Interphase and M Phase 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.

Interphase is preparation

Suppose a school is moving to a new building. For a month the teachers pack, label and copy every file; the actual move takes one Saturday.

Match the school's activities with the phases of the cell cycle and explain why the 'packing month' is not a rest.

Try it first, then show the solutionSolution

Growing and getting supplies ready matches G1 (cell growth, proteins and organelles).
Copying every file so both buildings get a full set matches S (DNA replication, two sister chromatids).
Labelling boxes and arranging trucks matches G2 (proteins for the spindle and division).
The one-day move matches the short M phase (karyokinesis and cytokinesis).

Answer: G1 growth, S copying, G2 preparation, M the move itself; interphase is the long, busy preparation that makes the short division possible.

Memory hook: Grow, Synthesise, Get ready, Move.

Tricks to remember The Cell Cycle: Interphase and M Phase

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

G-S-G-M: Grow, Synthesise, Get ready, Move
The four phases in order, each with its job.
Use it when: Naming the phase of any event.
S for Synthesis, S for Sisters
In S phase DNA is synthesised and the sister chromatids appear.
Use it when: When do chromosomes become two-chromatid?
DNA doubles, number holds
S doubles the DNA but not the chromosome count.
Use it when: Avoiding the commonest numerical trap.

The Cell Cycle: Interphase and M Phase: solved examples

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

Example 1: Name the phase · 2 marks · Short answerBoard level

Name the phase of the cell cycle in which: (a) DNA is replicated, (b) the cytoplasm divides, (c) the cell makes proteins needed for the spindle, (d) the newly formed cell grows in size.

Try it first, then show the solutionSolution

(a) S phase; (b) cytokinesis (end of M phase) (1 mark).
(c) G2 phase; (d) G1 phase (1 mark).

Answer: (a) S (b) Cytokinesis (c) G2 (d) G1.

Example 2: Not a resting phase · 2 marks · Short answerBoard level

Interphase was once called the 'resting phase'. Give two reasons why this name is misleading.

Try it first, then show the solutionSolution

The cell grows and makes proteins, RNA and organelles in G1 and G2 (1 mark).
DNA is replicated in the S phase, so every chromosome gets two sister chromatids ready for division (1 mark).

Answer: The cell is actively growing (G1, G2) and replicating its DNA (S); only visible division is absent.

Example 3: DNA versus chromosome number · 3 marks · Short answerHarder board level

A cell in G1 has 8 chromosomes and 10 units of DNA. Give the number of chromosomes, the number of chromatids and the DNA amount (a) in G2, (b) in each daughter cell after the M phase.

Try it first, then show the solutionSolution

S phase doubles DNA but not chromosome number (1 mark).
(a) G2: 8 chromosomes, 16 chromatids, 20 units of DNA (1 mark).
(b) Each daughter: 8 chromosomes (single chromatid each, so 8 chromatids), 10 units of DNA (1 mark).

Answer: (a) 8 chromosomes, 16 chromatids, 20 units. (b) 8 chromosomes, 8 chromatids, 10 units.

Example 4: Timing the cycle · 3 marks · Short answerHOTS / competency

Suppose a root-tip cell takes 20 hours to complete one cell cycle, of which G1 takes 8 hours, S 6 hours and G2 4.5 hours. (a) How long is the M phase? (b) What percentage of the cycle is interphase? (c) Why are dividing cells hard to find in a slide of this root tip?

Try it first, then show the solutionSolution

(a) M = 20 − (8 + 6 + 4.5) = 1.5 hours (1 mark).
(b) Interphase = 18.5 hours; 18.5 ÷ 20 × 100 = 92.5% (1 mark).
(c) Since a cell spends only 1.5 of every 20 hours dividing, at any moment most cells on the slide are in interphase and only a few show mitotic stages (1 mark).

Answer: (a) 1.5 hours (b) 92.5% (c) Most cells are in interphase at any moment because M is so short.

Example 5: Diagram of the cycle · 3 marks · Long answerHarder board level

Draw a labelled diagram of the cell cycle showing G1, S, G2 and M phases, and mark interphase.

Try it first, then show the solutionSolution

A circle divided clockwise into G1 (largest), S, G2 and a narrow M sector (1 mark).
Labels of events: growth (G1), DNA replication (S), preparation for division (G2), mitosis and cytokinesis (M) (1 mark).
G1, S and G2 bracketed as interphase; an arrow showing the direction; two daughter cells leaving M (1 mark).

Answer: A circular diagram with G1 → S → G2 → M clockwise, interphase bracketed, events labelled and an arrow for direction.

Where students lose marks in The Cell Cycle: Interphase and M Phase

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

Mistake: Doubling the chromosome number in S phase
Students see 'DNA doubles' and double the chromosome count too.
Wrong working
2n = 46 in G1, so 92 chromosomes in G2.
Right working
46 chromosomes, each with 2 chromatids (92 chromatids) in G2.
Do this: Count centromeres: they do not split until anaphase.
Usually costs: 1 mark.
Mistake: Listing interphase as a stage of mitosis
Interphase belongs to the cell cycle, not to mitosis; writing 'interphase, prophase, metaphase…' for stages of mitosis is marked wrong.
Do this: Mitosis begins at prophase; interphase comes before it.
Usually costs: 1 mark.
Mistake: Mixing up G1 and G2
Both are 'gap' phases and students reverse their order around S.
Do this: Remember 1 comes before S and 2 comes after it.
Usually costs: 1 mark.

Exam tips

Write G1, S and G2 with their full names (gap 1, synthesis, gap 2) once in a long answer.
Always say 'DNA replicates' for S; 'chromosomes replicate' alone is weaker.
Use the word karyokinesis for nuclear division and cytokinesis for cytoplasmic division.
In numericals show a small table: stage, chromosomes, chromatids, DNA.

Practice questions on The Cell Cycle: Interphase and M Phase

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 phase of the cell cycle in which DNA content doubles is:
(a) G1 (b) S (c) G2 (d) M
Hint
Synthesis.
Answer
(b) S.
Q2 · 1 mark · Very short answerBoard level
What are the two events of the M phase?
Hint
Nucleus, then cytoplasm.
Answer
Karyokinesis (division of the nucleus) and cytokinesis (division of the cytoplasm).
Q3 · 1 mark · Assertion-reasonHarder board level
Assertion (A): After the S phase, a human cell still has 46 chromosomes.
Reason (R): Two sister chromatids joined at a centromere are counted as one chromosome.
(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
Count centromeres.
Answer
(a).
Q4 · 2 marks · Short answerHarder board level
Why must DNA replication be completed before mitosis begins?
Hint
Each daughter needs a full set.
Answer
In mitosis the sister chromatids of every chromosome are separated, one to each daughter cell; unless the DNA has been copied (two chromatids per chromosome), the daughters could not each receive a complete set of genetic information.
Q5 · 2 marks · Short answerHarder board level
A cell has 12 chromosomes in G1. How many chromatids does it have (a) in G1, (b) in G2? How many chromosomes will each daughter cell have?
Hint
S phase gives each chromosome two chromatids.
Answer
(a) 12 (one chromatid each). (b) 24 (two each). Each daughter cell: 12 chromosomes.
Q6 · 2 marks · Short answerHarder board level
Distinguish between G1 and G2 phases with respect to (a) their position in the cycle and (b) the DNA content of the cell.
Hint
Before and after S.
Answer
(a) G1 comes after a division and before S; G2 comes after S and before M. (b) In G1 the DNA is unreplicated (normal amount); in G2 it has been doubled.
Q7 · 2 marks · Short answerHOTS / competency
Suppose a chemical stops cells from leaving G1. Predict two effects on a growing root tip treated with it.
Hint
No S, no M.
Answer
No cell would replicate its DNA or divide, so no new cells would be added and the root would stop growing in length; on a slide, no mitotic stages would be seen.
Q8 · 3 marks · Case-basedHOTS / competency
Read and answer.
Suppose a student counts 200 cells in a slide of an onion root tip: 180 are in interphase and 20 show some stage of mitosis.
(a) What percentage of cells are dividing? (1)
(b) What does this tell you about the length of the M phase compared with interphase? (1)
(c) In which sub-phase would a cell with two-chromatid chromosomes but no visible division be? (1)
Hint
Fraction of cells ∝ time spent.
Answer
(a) 20 ÷ 200 × 100 = 10%. (b) The M phase is much shorter than interphase (20 : 180, about one-ninth of the time spent in interphase), so few cells are caught dividing. (c) G2 (after S, before M).

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.Which phase is the longest part of the cell cycle?Show answer
Interphase.
2.In which phase does the centriole duplicate in animal cells?Show answer
S phase (during interphase).
3.What does the 'S' in S phase stand for?Show answer
Synthesis (of DNA).
4.About how long does a human cell in culture take for one cycle?Show answer
About 24 hours.
5.A cell has 6 units of DNA in G1. How much in G2?Show answer
12 units.

The Cell Cycle: Interphase and M Phase: frequently asked questions

What are the phases of the cell cycle?

Interphase, made of G1 (growth), S (DNA synthesis) and G2 (preparation), and the M phase, made of karyokinesis (mitosis) and cytokinesis.

Does the chromosome number double in the S phase?

No. The DNA amount doubles and each chromosome gets two sister chromatids, but the number of chromosomes stays the same.

Why is interphase not a resting phase?

Because the cell grows, makes proteins and organelles, and replicates its DNA during interphase; it is only resting from visible division.

Which is shorter, interphase or M phase?

The M phase; in a typical human cell it lasts about an hour out of a 24-hour cycle.