Cells and Cell Division

Cell Division

A cell grows, copies its DNA, and divides through the cell cycle. Most of the time is spent in interphase (G₁, S, G₂), and actual division happens in M phase. Mitosis keeps the chromosome number for growth and repair, while meiosis halves it to form gametes. Use the slider to step through the cycle and the buttons to compare mitosis with meiosis.

What is the Cell Cycle?

🔄 From Birth to Division

①A cell goes through growth → DNA replication → preparation → division

②The whole process is the "cell cycle"

③Most of the time is spent in interphase (G₁+S+G₂)!

Visualizing the Cell Cycle

Cell cycle phase0

💡 Key Points of Each Phase

①G₁: cell growth, protein synthesis (longest phase)

②S: DNA replication (DNA doubles)

③G₂: prepare for division, form spindle materials

④M: actual division (prophase→metaphase→anaphase→telophase)

Mitosis vs Meiosis

Division Comparison

ChartMitosis vs Meiosis

Item	Mitosis	Meiosis
Divisions	once	twice (Meiosis I + II)
Daughter cells	2	4
Daughter cell ploidy	2n (same as parent)	n (halved)
Genetic diversity	none (identical)	yes (crossover, independent assortment)
Location	somatic cells	reproductive organs
Purpose	growth, repair	producing gametes

Worked Examples

Example 1

A human with somatic chromosome number 2n = 46 undergoes one mitotic division. How many chromosomes does one daughter cell have?

Mitosis goes 2n → 2n, keeping the chromosome number the same as the parent cell.

Since the parent has 2n = 46, each daughter cell also has 46.

▸ 46

Mitosis preserves chromosome number (2n→2n). It serves growth and repair.

Example 2

In a human (2n = 46), how many chromosomes does one sperm produced by meiosis have?

Meiosis goes 2n → n, halving the chromosome number.

Since 2n = 46, n = 23.

▸ 23

Meiosis halves gametes to n; fertilization restores 2n. This keeps the chromosome number constant across generations.

Summary

Mitosis Core

2n → 2n + 2n (2 daughters identical to parent)

Division for growth and regeneration

Meiosis Core

2n → n + n + n + n (4 daughters with halved chromosome number)

Division for producing gametes

CSAT-style

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

①The DNA amount is halved

②The chromosome number rises to 8

③The DNA amount doubles but the chromosome number stays at 4

④It splits into two daughter cells

⑤Homologous chromosomes separate

▸ ③ The DNA amount doubles but the chromosome number stays at 4

During the S phase, DNA is replicated so the DNA amount doubles.

The replicated DNA stays joined as two chromatids, so the chromosome number itself does not change (still 2n = 4).

🎯 Exam Points

①Mitosis: 2n → 2n (2 identical daughters)

②Meiosis: 2n → n (4 daughters; increased diversity)

③Meiosis I: homolog separation (2n→n)

④Meiosis II: chromatid separation (n→n; similar to mitosis)

⑤DNA replicates in S phase → DNA doubles; chromosome number unchanged

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Characteristics of Life

Metabolism and Energy

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