Cell Cycle and Cell Division – Class 11 Biology Chapter 10 Notes & NCERT PDF

Class 11BiologyChapter 10NCERT book: Biology - Class 11

Notes and a simple summary of Chapter 10 of the NCERT Class 11 Biology book, with the official chapter PDF, flashcards, an MCQ quiz and an AI tutor for your doubts.

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Chapter 10: Cell Cycle and Cell Division

Introduction

All organisms, even the largest, begin life as a single cell. Cells reproduce by dividing into two; repeated cycles of growth and division let one cell form a structure of millions of cells. This chapter explains the cell cycle, the stages of mitosis, the special reduction division – meiosis, and the significance of each.

Key Concepts

10.1 Cell Cycle

Cell cycle: the sequence of events by which a cell duplicates its genome, synthesises other constituents and eventually divides into two daughter cells.

Cell growth (cytoplasmic increase) is continuous, but DNA synthesis occurs only in one specific stage. These events are under genetic control.

Human cells in culture divide about once every 24 hours; yeast can complete the cycle in about 90 minutes.

Two basic phases: Interphase and M phase (mitosis phase).

#### 10.1.1 Phases of Cell Cycle

In the 24-hour human cell cycle, cell division proper lasts only about an hour; interphase lasts more than 95% of the cycle.

M phase starts with nuclear division (karyokinesis) and usually ends with cytoplasmic division (cytokinesis).

Interphase ("resting phase") is actually the time of cell growth and DNA replication. It has three phases:

- G1 (Gap 1): between mitosis and start of DNA replication; cell metabolically active and grows, but no DNA replication.

- S (Synthesis): DNA replication; DNA doubles from 2C to 4C, but chromosome number stays 2n. In animal cells, DNA replication begins in the nucleus and the centriole duplicates in the cytoplasm.

- G2 (Gap 2): proteins synthesised for mitosis; growth continues.

G0 (quiescent stage): cells that stop dividing exit G1 and remain metabolically active but do not proliferate unless needed (e.g., heart cells).

In animals, mitosis occurs in diploid somatic cells, with exceptions such as male honey bees (haploid cells dividing by mitosis). Plants show mitosis in both haploid and diploid cells.

Onion root tip cells have 16 chromosomes.

10.2 M Phase (Mitosis)

The most dramatic period; since parent and progeny have the same chromosome number, it is called equational division. Karyokinesis has four stages – but it is a continuous process.

#### 10.2.1 Prophase

Follows S and G2; marked by initiation of chromatin condensation.

Chromosomes seen as two chromatids attached at the centromere.

Duplicated centrosome moves to opposite poles; each radiates microtubules called asters; asters + spindle fibres = mitotic apparatus.

At the end of prophase, Golgi complex, ER, nucleolus and nuclear envelope are not visible.

#### 10.2.2 Metaphase

Starts with complete disintegration of the nuclear envelope; condensation complete – best stage to study chromosome morphology.

Kinetochores (disc-shaped structures on centromeres) are sites where spindle fibres attach.

All chromosomes lie at the equator, sister chromatids connected to opposite poles; the plane is the metaphase plate.

#### 10.2.3 Anaphase

Each chromosome splits; daughter chromatids (now daughter chromosomes) move to opposite poles, centromere leading and arms trailing.

Key events: centromeres split and chromatids separate; chromatids move to opposite poles.

#### 10.2.4 Telophase

Chromosomes at poles decondense and lose individuality.

Nuclear envelope forms around each cluster → two daughter nuclei; nucleolus, Golgi complex and ER reform.

#### 10.2.5 Cytokinesis

Animal cells: a furrow in the plasma membrane deepens and divides the cytoplasm.

Plant cells (inextensible wall): wall formation starts in the centre and grows outward; the precursor is the cell-plate, which becomes the middle lamella.

Mitochondria and plastids are distributed between daughter cells.

If karyokinesis is not followed by cytokinesis, a multinucleate syncytium forms, e.g., liquid endosperm in coconut.

10.3 Significance of Mitosis

Usually restricted to diploid cells, but some lower plants and social insects have haploid cells dividing by mitosis.

Produces diploid daughter cells with identical genetic complement.

Responsible for growth of multicellular organisms.

Restores the nucleo-cytoplasmic ratio disturbed by cell growth.

Cell repair: upper epidermis, gut lining and blood cells are constantly replaced.

Divisions in apical and lateral cambium give continuous plant growth throughout life.

10.4 Meiosis

Specialised division in diploid cells that halves the chromosome number, producing haploid gametes; fertilisation restores diploidy. Occurs during gametogenesis.

Key features:

- Two sequential divisions (meiosis I and II) but only one cycle of DNA replication.

- Meiosis I begins after replication produces identical sister chromatids in S phase.

- Pairing of homologous chromosomes and recombination between non-sister chromatids.

- Four haploid cells at the end of meiosis II.

#### 10.4.1 Meiosis I

Prophase I – longer and more complex; five sub-stages:

Leptotene: chromosomes become gradually visible; compaction continues.

Zygotene: homologous chromosomes pair – synapsis; synaptonemal complex forms; the pair is a bivalent or tetrad.

Pachytene (longest of the first three): four chromatids clearly visible as tetrads; recombination nodules appear; crossing over between non-sister chromatids, catalysed by the enzyme recombinase; recombination completed by end of pachytene.

Diplotene: synaptonemal complex dissolves; homologues separate except at crossovers – X-shaped chiasmata. In oocytes of some vertebrates diplotene can last months or years.

Diakinesis: terminalisation of chiasmata; chromosomes fully condensed; meiotic spindle assembled; nucleolus disappears and nuclear envelope breaks down – transition to metaphase.

Metaphase I: bivalents align on the equatorial plate; microtubules from opposite poles attach to kinetochores of homologous chromosomes.

Anaphase I: homologous chromosomes separate, while sister chromatids stay together at their centromeres.

Telophase I: nuclear membrane and nucleolus reappear; cytokinesis gives a dyad of cells.

Interkinesis: short stage between the two divisions; no DNA replication.

#### 10.4.2 Meiosis II (resembles normal mitosis)

Prophase II: begins immediately after cytokinesis; nuclear membrane disappears; chromosomes compact again.

Metaphase II: chromosomes align at the equator; microtubules attach to kinetochores of sister chromatids.

Anaphase II: centromeres split; sister chromatids move to opposite poles by shortening of microtubules.

Telophase II: nuclear envelopes form; cytokinesis → tetrad of cells (four haploid daughter cells).

10.5 Significance of Meiosis

Conserves the specific chromosome number of a species across generations in sexually reproducing organisms, even though it halves the number.

Increases genetic variability, which is important for evolution.

Diagrams to Know

Cell cycle wheel: G1, S, G2 (interphase) and M phase, with approximate durations.

Stages of mitosis: prophase, metaphase (metaphase plate, kinetochores), anaphase (centromere leading), telophase, cytokinesis (furrow vs cell plate).

Stages of meiosis I and II, showing bivalents, chiasmata, dyad and tetrad of cells.

Key-Facts Sheet

Human cell cycle ≈ 24 h; M phase ≈ 1 h; interphase > 95%. Yeast ≈ 90 min.

G1 → S (2C → 4C, 2n stays 2n) → G2 → M. G0 = quiescent.

Mitosis = equational division; meiosis = reduction division.

Prophase I sub-stages in order: Leptotene, Zygotene, Pachytene, Diplotene, Diakinesis.

Synapsis in zygotene; crossing over in pachytene (recombinase); chiasmata in diplotene; terminalisation in diakinesis.

Anaphase I separates homologues; anaphase II separates sister chromatids.

Onion root tip: 16 chromosomes (G1: 16, after S: 16, after M: 16; DNA 2C at G1, 4C after S and at G2 if 2C after M).

Exam Focus (from exercises)

Average cell cycle span of a mammalian cell (about 24 hours).

Distinguish karyokinesis and cytokinesis; plant vs animal cytokinesis.

Name the stage: chromosomes at spindle equator (metaphase); centromere splits (anaphase); pairing of homologues (zygotene); crossing over (pachytene).

Define synapsis, bivalent, chiasmata with diagrams.

Anaphase of mitosis vs anaphase I; main differences between mitosis and meiosis.

Key Terms

Cell cycle (कोशिका चक्र), Interphase (अंतरावस्था), Mitosis (समसूत्री विभाजन), Meiosis (अर्धसूत्री विभाजन), Karyokinesis (केन्द्रक विभाजन), Cytokinesis (कोशिकाद्रव्य विभाजन), Chromatid (अर्धगुणसूत्र), Centromere (गुणसूत्र बिंदु), Kinetochore (काइनेटोकोर), Spindle (तर्कु), Synapsis (सूत्रयुग्मन), Bivalent (युगली), Crossing over (जीन विनिमय), Chiasmata (काएज्मेटा), Quiescent stage (प्रशांत अवस्था)

Common Mistakes

Saying the chromosome number doubles in S phase – only DNA content doubles (2C → 4C); chromosome number stays 2n.

Calling interphase a truly "resting" phase – it is the phase of growth and DNA replication.

Mixing up anaphase I (homologues separate) and anaphase II (sister chromatids separate).

Writing the wrong order of prophase I sub-stages (remember L-Z-P-D-D).

Thinking DNA replicates during interkinesis – it does not.

💡 Key Learning Points

✓The cell cycle has interphase (G1, S, G2) and M phase; interphase takes more than 95% of a 24-hour human cell cycle.
✓DNA content doubles in S phase from 2C to 4C while the chromosome number stays the same.
✓Mitosis (prophase, metaphase, anaphase, telophase, then cytokinesis) is an equational division giving identical diploid daughter cells.
✓Meiosis has two divisions with one DNA replication, involving synapsis and crossing over, and produces four haploid cells.
✓Mitosis supports growth and repair, while meiosis conserves chromosome number across generations and increases genetic variation.

👨‍🏫 Teaching Tips

→Observe prepared slides or squash preparations of onion root tips to identify mitotic stages.
→Use pipe cleaners or coloured threads to model chromosomes through mitosis and meiosis, including crossing over.
→Teach the prophase I mnemonic 'Lazy Zebras Prefer Dozing Daily' for Leptotene, Zygotene, Pachytene, Diplotene, Diakinesis.
→Have students track chromosome number (n) and DNA content (C) through each stage in a list.
→Show animations contrasting furrow formation in animal cells with cell plate formation in plant cells.
→Make students write mitosis vs meiosis differences point by point for board-style answers.

📋 Assessment Questions

Can the student name and describe the phases of the cell cycle including G0?
Can the student state what happens to DNA content and chromosome number in each phase?
Can the student identify the stage of mitosis or meiosis from given events?
Can the student define synapsis, bivalent and chiasmata?
Can the student explain the significance of mitosis and meiosis?

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