Cell Division

Cell Division (Mitosis)

When a cell grows too large, its surface-to-volume ratio falls and exchange of materials becomes hard, so it divides at a certain size. Mitosis copies DNA in interphase, then goes through prophase, metaphase, anaphase, and telophase to make two daughter cells with the same chromosome number as the parent. In metaphase chromosomes line up at the equator and are easiest to see; the process supports growth and repair. On this page, change the division stage to watch how chromosomes move at each step.

Why Do Cells Divide?

🔬 Intuition — Big Cells Are Inefficient

①As a cell grows, volume rises faster than surface area

②Smaller surface/volume ratio → harder to exchange materials

③So cells split before getting too large

Mitosis Stages

Stage0 Interphase

📖 Stage Summary

①Interphase: DNA replication (most of the cell cycle)

②Prophase: chromosomes condense, spindle forms, nuclear membrane breaks

③Metaphase: chromosomes line up on the equator

④Anaphase: sister chromatids pulled to poles

⑤Telophase: nuclear membranes reform, cytokinesis → 2 daughter cells

Summary

Mitosis Result

Mother cell (2n) → 2 daughter cells (2n)

Same chromosome count; DNA replicates first, then divides

Cell Cycle

Interphase (G₁→S→G₂) → M phase

Interphase takes about 90% of the cycle

🎯 Exam Points

①Order: Inter→Pro→Meta→Ana→Telo

②Metaphase: chromosomes most condensed, easiest to observe

③Anaphase: chromosome count temporarily doubled

④Daughter chromosome count = mother cell

⑤For growth and regeneration

Examples and Unit Test

Example 1

After interphase, in what order does mitosis proceed, and at which stage do the chromosomes line up in a single row at the middle (the equator) of the cell?

After DNA is copied in interphase, division proceeds as prophase → metaphase → anaphase → telophase.

The chromosomes line up in a single row at the middle (equatorial plate) of the cell during metaphase, the easiest stage to observe them.

▸ Interphase → prophase → metaphase → anaphase → telophase / lined up at the equator = metaphase

Each stage of division is told apart by where the chromosomes are and how they behave.

Example 2

How do mitosis and meiosis differ in the number of divisions and the chromosome number of the daughter cells? (parent cell 2n)

Mitosis divides once and makes 2 daughter cells with the same chromosome number as the parent (2n).

Meiosis divides twice in a row and makes 4 reproductive cells with half the chromosome number (n).

▸ Mitosis = 1 division, 2 daughter cells, 2n (unchanged) / meiosis = 2 divisions, 4 cells, n (half)

Because meiosis halves the chromosome number, it stays constant from generation to generation after fertilization.

Unit Test

Which statement about cell division is correct?

①In mitosis the chromosome number is halved

②Meiosis divides only once

③In metaphase of mitosis the chromosomes line up at the equator

④Nothing happens during interphase

⑤In anaphase the chromosomes gather at the equator

▸ ③ In metaphase of mitosis the chromosomes line up at the equator

In mitosis the chromosome number stays the same (① wrong, meiosis halves it), and meiosis divides twice in a row (② wrong).

DNA is copied during interphase (④ wrong), and in anaphase the sister chromatids move to the poles while alignment at the equator is metaphase (⑤ wrong); in metaphase of mitosis the chromosomes line up at the equator (③ correct).

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Neutralization

Mendelian Genetics

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