Grade 10-11 (age 15-17)

high school Cells and Cell Division

The cells of our body keep growing, copying their DNA and splitting in two, and one round of this is called the cell cycle. Most of the cycle is interphase (G₁, S and G₂), when the cell grows and copies its DNA; the M phase, when it actually divides, is short. Mitosis makes two daughter cells with the same number of chromosomes, letting the body grow and heal, while meiosis makes four reproductive cells with half the number of chromosomes. The diagrams show how the chromosomes of a 2n = 4 cell change through the cycle and how homologous chromosomes are shared out in the two kinds of division.

What Is the Cell Cycle?

🔄 From a New Cell to Its Division

①A new cell grows, copies its DNA, prepares to divide and then splits in two

②One round of this is called the cell cycle

③A human cell takes about 24 hours per cycle: about 23 hours of interphase (G₁ + S + G₂) and only about an hour of actual division (M phase)

Following the Cell Cycle

Cell cycle phaseG₁

💡 What Happens in Each Phase

①G₁: the cell grows and makes proteins; this is the longest phase

②S: the DNA is copied and the amount of DNA doubles; the two copies stay joined as sister chromatids, so there are still 4 chromosomes

③G₂: the cell makes what it needs for division and gets ready

④M: the chromosomes condense and line up at the equator, then the chromatids are pulled apart to the two sides and two daughter cells form (prophase → metaphase → anaphase → telophase)

Mitosis and Meiosis

Comparing the Divisions

Mitosis vs Meiosis

Feature	Mitosis	Meiosis
Number of divisions	1	2 (meiosis I + II)
Daughter cells	2	4
Chromosome number of daughters	2n (same as the parent)	n (halved)
Genetic variety	none (identical)	yes (crossing over, independent assortment of homologs)
Where	body cells	reproductive organs
Purpose	growth, repair	making reproductive cells

Worked Examples

Example 1

A human body cell has 2n = 46 chromosomes. After one mitotic division, how many chromosomes does each daughter cell have?

Mitosis keeps the chromosome number the same as the parent's (2n → 2n).

The parent is 2n = 46, so each daughter cell also has 46.

▸ 46

The chromatids copied in the S phase before division are shared out one to each daughter cell, so the number stays the same.

Example 2

In a person with 2n = 46, how many chromosomes are in one sperm made by meiosis?

Meiosis halves the chromosome number (2n → n).

2n = 46, so n = 23.

▸ 23

Sperm and egg each have n = 23, so fertilization restores 2n = 46. Thanks to meiosis, the chromosome number stays constant from generation to generation.

Summary

Mitosis in Brief

2n → 2n + 2n (2 daughter cells, same as the parent)

Division for growth and repair

Meiosis in Brief

2n → n + n + n + n (4 daughter cells, chromosome number halved)

Division for making reproductive cells

CSAT-style problem, adapted

Which statement is correct about a cell that has just finished the S phase? (The parent cell is 2n = 4.)

①The amount of DNA halves

②The chromosome number rises to 8

③The amount of DNA doubles, but there are still 4 chromosomes

④It splits into two daughter cells

⑤Homologous chromosomes separate

▸ ③ The amount of DNA doubles, but there are still 4 chromosomes

In the S phase the DNA is copied, so the amount of DNA doubles.

The copied DNA stays joined as two sister chromatids, so there are still 4 chromosomes; G₁ and G₂ show the same count in the cell-cycle diagram.

🎯 Exam Points

①Mitosis: 2n → 2n, two daughter cells identical to the parent

②Meiosis: 2n → n, four daughter cells, genetically varied

③In meiosis I, homologous chromosomes separate, so 2n → n

④In meiosis II, chromatids separate, n → n, much like mitosis

⑤In the S phase the amount of DNA doubles, but the number of chromosomes stays the same

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Metabolism and Energy

Last updated 2026-10-17

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