Schooling · Class 11 · Biology · Cell Cycle and Cell Division · Chapter on a page
CBSE Class 11 · Biology · NCERT Biology
Cell Cycle and Cell Division: chapter on a page
A cut of Shishya's notes for this chapter: the first paragraph of the introduction and the key ideas, formulas and facts, common mistakes and quick revision lists, word for word. Nothing is added or reworded; the worked examples are left out (they are on the chapter page).
About the notes: Written by Shishya's AI on 27 Sept 2026 from the chapter's title and class level, in Shishya's own words — not a copy or summary of the textbook. Read the official chapter for the book's own text, activities and exercises.
This note's current text has no applied fact-check on record.
Mind map
Drawn from the same notes by a fixed rule (no AI): each label is the item's own words — its bold term, its label before a colon, a short formula as written, or its first words.
The mind map as a list
- Key ideas
- The cell cycle has two broad parts
- Interphase is subdivided into G₁…
- Mitosis consists of prophase…
- Meiosis involves two successive…
- Crossing over (exchange of segments…
- Cytokinesis differs in plant and…
- Checkpoints in G₁, G₂ and metaphase…
- Formulas and facts
- Cell cycle duration
- DNA content change
- Chromosome terminology
- Meiosis I is reductional
- Meiosis II is equational
- Chiasmata
- Significance of mitosis
- Worked examples
- Calculating DNA content after S phase
- Identifying the stage from a…
- Predicting gamete chromosome number
- Watch out for
- Thinking S phase increases the…
- Confusing homologous chromosomes…
- Believing crossing over occurs in…
- Stating that meiosis produces two…
- Mixing up the terms haploid and…
What this chapter is about
Every living organism begins as a single cell, and the growth, development and repair of tissues all depend on the ability of cells to divide. This chapter introduces the orderly sequence of events by which a cell duplicates its contents and splits into two daughter cells. You will learn about the cell cycle — the defined phases a cell passes through between one division and the next — and the two fundamentally different modes of division: mitosis, which produces genetically identical cells, and meiosis, which halves the chromosome number and generates genetic variation.
Key ideas
- The cell cycle has two broad parts: interphase (the longer phase when the cell grows and copies its DNA) and the M phase (mitosis or meiosis plus cytokinesis).
- Interphase is subdivided into G₁ (growth and normal metabolism), S (DNA synthesis, when chromosomes replicate), and G₂ (preparation for division).
- Mitosis consists of prophase, metaphase, anaphase and telophase; it produces two daughter cells with the same chromosome number as the parent (2n → 2n in a diploid organism).
- Meiosis involves two successive divisions, meiosis I and meiosis II, and produces four haploid cells (2n → n) from one diploid cell.
- Crossing over (exchange of segments between homologous chromosomes during prophase I) and independent assortment of chromosomes generate genetic diversity.
- Cytokinesis differs in plant and animal cells: animal cells divide by cleavage furrow formation, while plant cells build a cell plate from the centre outward.
- Checkpoints in G₁, G₂ and metaphase ensure that DNA is intact and chromosomes are properly attached before the cell proceeds; failure of checkpoints can lead to uncontrolled division.
Formulas and facts to remember
- Cell cycle duration: In a rapidly dividing human cell, the entire cycle takes about 24 hours; interphase occupies roughly 95 % of this time.
- DNA content change: During S phase the DNA content doubles from 2C to 4C (where C is the amount in a haploid set); after mitosis each daughter returns to 2C.
- Chromosome terminology: A replicated chromosome has two sister chromatids joined at the centromere; these separate during anaphase (anaphase II in meiosis).
- Meiosis I is reductional: Homologous chromosomes pair (synapsis) and then separate, halving the chromosome number.
- Meiosis II is equational: Sister chromatids separate, similar to mitosis but starting from haploid cells.
- Chiasmata: The X-shaped points where crossing over occurs; visible during diplotene of prophase I.
- Significance of mitosis: Growth, repair, asexual reproduction; maintains genetic consistency.
- Significance of meiosis: Formation of gametes and spores; maintains constant chromosome number across generations and introduces variation.
Common mistakes
- Thinking S phase increases the number of chromosomes → S phase doubles the DNA but the chromosome count stays the same; sister chromatids are counted as one chromosome until they separate.
- Confusing homologous chromosomes with sister chromatids → Homologous chromosomes are two separate chromosomes (one from each parent) carrying genes for the same traits; sister chromatids are identical copies of one chromosome held together at the centromere.
- Believing crossing over occurs in mitosis → Crossing over happens only in prophase I of meiosis, when homologous chromosomes are synapsed.
- Stating that meiosis produces two cells → Meiosis produces four haploid cells; two divisions occur.
- Mixing up the terms haploid and diploid when describing gametes and somatic cells → Somatic (body) cells are diploid (2n); gametes are haploid (n).
Quick revision
- Interphase = G₁ + S + G₂; cell grows and replicates DNA here.
- Mitosis: one division, two identical diploid daughters; for growth and repair.
- Meiosis: two divisions, four genetically varied haploid cells; for sexual reproduction.
- Crossing over and independent assortment create genetic variation.
- Checkpoints prevent division if DNA is damaged or chromosomes misaligned.
- Cytokinesis: cleavage furrow in animals, cell plate in plants.