Cell Cycle and Cell Division
Welcome to HSLC Guru! This page provides complete English-medium notes, summaries, and question answers for Class 11 Biology Chapter 10 — Cell Cycle and Cell Division as per the ASSEB (Assam State School Education Board) syllabus. Cells multiply through carefully regulated cycles. In this chapter you will learn about the phases of the cell cycle, the events of mitosis and meiosis, the significance of each type of division, and how chromosome number is maintained or halved. Use this resource to revise quickly, prepare for board examinations, and strengthen your conceptual understanding of how life perpetuates itself at the cellular level.
Chapter Summary
The cell cycle is the sequence of events by which a cell duplicates its contents and divides into two daughter cells. It consists of two main stages — interphase (the long preparatory phase) and the M phase (the actual division phase). Interphase is further divided into three sub-phases: G1 phase (gap 1, where the cell grows and synthesises RNA and proteins), S phase (synthesis, where DNA replication takes place and chromosome number is doubled at the chromatid level), and G2 phase (gap 2, where the cell prepares organelles and proteins required for division). A typical animal cell completes one cycle in about 24 hours, of which roughly 23 hours are spent in interphase. Some cells, such as neurons, exit the cycle and enter the quiescent stage (G0), where they remain metabolically active but do not divide.
Mitosis is the equational division that occurs in somatic cells and produces two daughter cells genetically identical to the parent cell, each retaining the diploid chromosome number. It proceeds through four karyokinetic stages — prophase (chromatin condenses, nuclear envelope and nucleolus disappear, spindle begins to form), metaphase (chromosomes align at the equatorial plate with kinetochores attached to spindle fibres), anaphase (centromeres split and sister chromatids move to opposite poles), and telophase (chromatids reach the poles, nuclear envelope reforms, chromosomes decondense). Karyokinesis is followed by cytokinesis, which divides the cytoplasm — by furrowing in animal cells and by cell-plate formation in plant cells. The significance of mitosis lies in growth, repair, regeneration of tissues, healing of wounds, and asexual reproduction in some organisms.
Meiosis is the reductional division that takes place in germ cells and produces four haploid gametes from a single diploid parent cell. It involves a single round of DNA replication followed by two successive divisions — meiosis I (reductional) and meiosis II (equational). The most distinctive stage is prophase I, which is sub-divided into five sub-stages: leptotene (chromosomes appear as long threads), zygotene (synapsis or pairing of homologous chromosomes occurs forming bivalents and the synaptonemal complex), pachytene (crossing over takes place at recombination nodules; tetrads become visible), diplotene (homologues begin to separate but remain attached at chiasmata), and diakinesis (terminalisation of chiasmata, nuclear envelope breakdown). In metaphase I bivalents align at the equator; in anaphase I homologous chromosomes (not chromatids) separate; in telophase I the nuclear envelope may reform briefly. Meiosis II resembles mitosis and separates sister chromatids, producing four haploid cells.
The significance of meiosis is enormous. It maintains the constant chromosome number of a species across generations by halving the chromosome number in gametes; fertilisation then restores the diploid state. It introduces genetic variation through crossing over and the random assortment of homologous chromosomes, providing the raw material for evolution and natural selection. It is essential for gamete formation in sexually reproducing organisms. Major differences between mitosis and meiosis include — mitosis occurs in somatic cells while meiosis occurs in germ cells; mitosis produces two diploid daughter cells while meiosis produces four haploid cells; there is no synapsis or crossing over in mitosis whereas both occur in meiosis I; daughter cells of mitosis are genetically identical while those of meiosis are genetically different.
Question Answers (1 Mark)
Q1. What is the cell cycle?
Answer: The cell cycle is the orderly sequence of events by which a cell duplicates its genome, synthesises other cellular components, and divides into two daughter cells.
Q2. Name the two main phases of the cell cycle.
Answer: The two main phases are Interphase and M phase (Mitotic phase).
Q3. In which phase of interphase does DNA replication occur?
Answer: DNA replication occurs in the S (Synthesis) phase of interphase.
Q4. What is the G0 phase?
Answer: The G0 phase or quiescent stage is a resting phase in which the cell is metabolically active but does not divide further.
Q5. Define cytokinesis.
Answer: Cytokinesis is the division of the cytoplasm of a parent cell into two daughter cells following karyokinesis.
Q6. What is synapsis?
Answer: Synapsis is the pairing of homologous chromosomes during the zygotene sub-stage of prophase I in meiosis.
Q7. In which stage does crossing over occur?
Answer: Crossing over occurs during the pachytene sub-stage of prophase I of meiosis.
Q8. What are bivalents?
Answer: A bivalent is a pair of homologous chromosomes held together during synapsis in meiosis I.
Q9. Name the structure formed during synapsis.
Answer: The structure formed during synapsis is called the synaptonemal complex.
Q10. How many daughter cells are produced at the end of meiosis?
Answer: Four haploid daughter cells are produced at the end of meiosis.
Question Answers (2-3 Marks)
Q1. Distinguish between G1 and G2 phases of interphase.
Answer: The G1 phase is the gap between the end of M phase and the beginning of S phase; during this period the cell grows in size, synthesises RNA, proteins and various organelles, and remains metabolically very active. The G2 phase is the gap between the end of S phase and the start of M phase; during this period the cell synthesises proteins required for mitosis, particularly tubulin for spindle formation, and prepares for division. DNA content remains 2C in G1 and becomes 4C after S phase, persisting through G2.
Q2. What are the events of metaphase in mitosis?
Answer: During metaphase, chromosomes become the most condensed and clearly visible. Each chromosome consists of two sister chromatids held at the centromere. The chromosomes line up at the equator of the spindle apparatus, forming the metaphase plate. The kinetochores of sister chromatids attach to spindle microtubules from opposite poles, producing balanced tension. This precise alignment ensures that each daughter cell will receive one copy of every chromosome.
Q3. Differentiate between karyokinesis and cytokinesis.
Answer: Karyokinesis refers to the division of the nucleus, including condensation, alignment, separation, and reformation of chromosomes; it occurs through prophase, metaphase, anaphase, and telophase. Cytokinesis refers to the division of the cytoplasm and the formation of two separate daughter cells; it follows karyokinesis and occurs by furrow formation in animal cells and by cell-plate formation in plant cells.
Q4. Briefly describe the events of pachytene.
Answer: In pachytene, bivalents become clearly visible as four-stranded structures called tetrads. The most important event of this stage is crossing over — the exchange of genetic material between non-sister chromatids of homologous chromosomes at points called recombination nodules. This process is catalysed by the enzyme recombinase and produces new combinations of alleles, contributing significantly to genetic variation.
Q5. Why is meiosis called reductional division?
Answer: Meiosis is called reductional division because the chromosome number is reduced from diploid (2n) to haploid (n) in the daughter cells. This reduction occurs specifically during meiosis I when homologous chromosomes separate and move to opposite poles. Meiosis II is equational, separating sister chromatids without further reducing the chromosome number. The halving of chromosome number is essential for sexual reproduction, since fertilisation restores the diploid state.
Q6. What happens during anaphase of mitosis?
Answer: During anaphase, the centromeres holding sister chromatids together split simultaneously. The freed sister chromatids, now called daughter chromosomes, are pulled towards opposite poles of the cell by shortening kinetochore microtubules. Polar microtubules elongate, pushing the poles further apart. By the end of anaphase, each pole has an identical complete set of chromosomes, ensuring genetic equality of the daughter cells.
Question Answers (5-7 Marks)
Q1. Describe the various phases of the cell cycle in detail.
Answer: The cell cycle is divided into two major phases — interphase and the M phase.
Interphase is the longest part of the cell cycle and occupies about 95% of the cycle’s duration. It is sub-divided into three phases:
(i) G1 phase (Gap 1): The period between the end of mitosis and the beginning of DNA replication. The cell grows, synthesises RNA, proteins and various organelles, and decides whether or not to enter the next round of division. Cells that do not divide further enter the G0 (quiescent) phase from G1.
(ii) S phase (Synthesis phase): DNA replication takes place and the amount of DNA per cell doubles from 2C to 4C. The chromosome number, however, remains the same. In animal cells, the centriole also duplicates in the cytoplasm.
(iii) G2 phase (Gap 2): The period between the end of S phase and the start of mitosis. Proteins required for division, particularly tubulin for spindle fibres, are synthesised, and organelles are doubled.
M phase (Mitotic phase) is the actual division phase, during which both karyokinesis (nuclear division) and cytokinesis (cytoplasmic division) take place. It consists of four sub-stages — prophase, metaphase, anaphase, and telophase — followed by cytokinesis. A typical human cell completes one cycle in approximately 24 hours.
Q2. Describe the four stages of mitosis with the help of suitable diagrams.
Answer: Mitosis or karyokinesis is divided into four stages.
(i) Prophase: This is the first and longest stage. The chromatin material condenses to form distinct chromosomes, each consisting of two sister chromatids joined at the centromere. The centrioles move to opposite poles and begin organising the spindle apparatus. By the end of prophase, the nuclear envelope and nucleolus disappear.
(ii) Metaphase: Chromosomes become highly condensed and align at the equatorial plate of the cell, forming the metaphase plate. Kinetochores of sister chromatids attach to spindle fibres from opposite poles. This is the stage at which chromosomes are clearest and most often studied for karyotyping.
(iii) Anaphase: Centromeres split simultaneously and sister chromatids separate. Each chromatid, now an independent daughter chromosome, moves towards opposite poles, pulled by shortening kinetochore microtubules.
(iv) Telophase: The chromatids reach the poles and begin to decondense. The nuclear envelope reforms around each set of chromosomes, the nucleolus reappears, and the spindle disassembles. Telophase is followed by cytokinesis — by furrow formation in animal cells and by cell-plate formation in plant cells — producing two genetically identical daughter cells.
Q3. Describe the sub-stages of prophase I of meiosis.
Answer: Prophase I of meiosis is the longest and most complex phase of cell division. It is divided into five sub-stages.
(i) Leptotene: Chromosomes become gradually visible under the microscope as long, thin, thread-like structures. Compaction of chromosomes continues throughout this stage.
(ii) Zygotene: Homologous chromosomes begin to pair, a process called synapsis. The complex formed by synapsed homologues is called a bivalent or tetrad. The synapsed chromosomes are held together by a complex protein structure known as the synaptonemal complex.
(iii) Pachytene: Bivalents appear as four-stranded tetrads. Crossing over takes place at this stage — the exchange of genetic material between non-sister chromatids of homologous chromosomes at the recombination nodules. The enzyme recombinase mediates this process, leading to genetic recombination.
(iv) Diplotene: The synaptonemal complex dissolves and homologous chromosomes begin to separate, except at the points where crossing over has occurred. These X-shaped points of attachment are called chiasmata.
(v) Diakinesis: Chiasmata move towards the ends of the chromosomes, a process called terminalisation. Chromosomes become fully condensed, the nucleolus disappears, the nuclear envelope breaks down, and the meiotic spindle assembles. The cell is now ready to enter metaphase I.
Q4. Explain the significance of meiosis.
Answer: Meiosis is of immense biological importance for sexually reproducing organisms.
(i) Maintenance of chromosome number: Meiosis halves the chromosome number from diploid (2n) to haploid (n) in gametes. After fertilisation, the diploid number is restored. This ensures that the species-specific chromosome number is maintained generation after generation.
(ii) Production of gametes: Meiosis is the only mechanism by which haploid male and female gametes are produced in sexually reproducing organisms. Without meiosis, sexual reproduction would not be possible.
(iii) Genetic variation: Meiosis introduces variation through two key mechanisms — crossing over during pachytene of prophase I, which produces new combinations of alleles, and random assortment of homologous chromosomes at metaphase I, which generates 2n possible combinations.
(iv) Basis of evolution: The variation generated by meiosis provides the raw material on which natural selection acts. It is therefore essential for the evolution of species over time.
(v) Adaptation and survival: Genetic variation enables populations to adapt to changing environmental conditions, contributing to the survival and continuity of species.
Q5. Differentiate between mitosis and meiosis.
Answer: The major differences between mitosis and meiosis are listed below.
(i) Site: Mitosis occurs in somatic (body) cells whereas meiosis occurs in germ cells (reproductive cells).
(ii) Number of divisions: Mitosis involves a single division while meiosis involves two successive divisions — meiosis I and meiosis II.
(iii) Number of daughter cells: Mitosis produces two daughter cells while meiosis produces four daughter cells.
(iv) Chromosome number: Daughter cells produced by mitosis have the same chromosome number as the parent cell (diploid), whereas those produced by meiosis have half the chromosome number (haploid).
(v) Synapsis and crossing over: There is no pairing of homologous chromosomes in mitosis and no crossing over takes place. In meiosis I, homologous chromosomes pair up (synapsis) and crossing over occurs.
(vi) Genetic constitution: Daughter cells of mitosis are genetically identical to the parent cell, while those of meiosis are genetically different from the parent and from each other.
(vii) Function: Mitosis is responsible for growth, repair, regeneration, and asexual reproduction; meiosis is responsible for the formation of gametes and the introduction of genetic variation.
(viii) Duration: Mitosis is comparatively shorter; meiosis (especially prophase I) is much longer and more complex.
Multiple Choice Questions (MCQs)
Q1. DNA replication occurs in which phase of the cell cycle?
(a) G1 phase (b) S phase (c) G2 phase (d) M phase
Answer: (b) S phase
Q2. The longest phase of the cell cycle is —
(a) Prophase (b) Interphase (c) Metaphase (d) Telophase
Answer: (b) Interphase
Q3. Crossing over takes place during —
(a) Leptotene (b) Zygotene (c) Pachytene (d) Diplotene
Answer: (c) Pachytene
Q4. Synapsis occurs in —
(a) Leptotene (b) Zygotene (c) Pachytene (d) Diakinesis
Answer: (b) Zygotene
Q5. Sister chromatids separate during —
(a) Anaphase of mitosis (b) Anaphase I (c) Metaphase I (d) Prophase I
Answer: (a) Anaphase of mitosis
Q6. The X-shaped structures formed by crossing over are called —
(a) Bivalents (b) Tetrads (c) Chiasmata (d) Centromeres
Answer: (c) Chiasmata
Q7. Meiosis produces —
(a) 2 diploid cells (b) 4 diploid cells (c) 2 haploid cells (d) 4 haploid cells
Answer: (d) 4 haploid cells
Q8. In plant cells, cytokinesis takes place by —
(a) Furrow formation (b) Cell-plate formation (c) Budding (d) Fragmentation
Answer: (b) Cell-plate formation
Q9. The G0 phase is also called —
(a) Resting phase (b) Synthesis phase (c) Growth phase (d) Mitotic phase
Answer: (a) Resting phase (Quiescent stage)
Q10. Meiosis I is also known as —
(a) Equational division (b) Reductional division (c) Mitotic division (d) Cytokinesis
Answer: (b) Reductional division
Fill in the Blanks
1. DNA replication takes place during the ________ phase. (S phase)
2. The pairing of homologous chromosomes is called ________. (synapsis)
3. The X-shaped points where homologues remain attached are called ________. (chiasmata)
4. In animal cells, cytokinesis occurs by ________ formation. (cleavage furrow)
5. Meiosis produces ________ haploid daughter cells. (four)
True or False
1. Mitosis occurs in germ cells. (False — it occurs in somatic cells)
2. Crossing over takes place during pachytene. (True)
3. Meiosis II is reductional division. (False — it is equational)
4. The G0 phase is a resting stage of the cell cycle. (True)
5. Plant cells divide by furrow formation. (False — they divide by cell-plate formation)
Glossary
| Term | Definition |
|---|---|
| Cell cycle | Sequence of events by which a cell duplicates its contents and divides into two daughter cells. |
| Interphase | The longest phase of the cell cycle consisting of G1, S and G2 sub-phases. |
| G1 phase | Gap phase between M phase and S phase during which the cell grows. |
| S phase | Synthesis phase in which DNA replication takes place. |
| G2 phase | Gap phase between S phase and M phase during which the cell prepares for division. |
| G0 phase | Quiescent or resting phase in which the cell does not divide. |
| Mitosis | Equational nuclear division producing two genetically identical diploid daughter cells. |
| Meiosis | Reductional division producing four haploid daughter cells from a diploid parent cell. |
| Karyokinesis | Division of the nucleus during cell division. |
| Cytokinesis | Division of the cytoplasm following karyokinesis. |
| Prophase | First stage of mitosis in which chromosomes condense and the spindle forms. |
| Metaphase | Stage in which chromosomes align at the equatorial plate. |
| Anaphase | Stage in which sister chromatids separate and move to opposite poles. |
| Telophase | Stage in which chromatids reach the poles and the nuclear envelope reforms. |
| Synapsis | Pairing of homologous chromosomes during zygotene of prophase I. |
| Bivalent | Pair of synapsed homologous chromosomes; also called a tetrad. |
| Synaptonemal complex | Protein structure that holds synapsed homologues together. |
| Crossing over | Exchange of genetic material between non-sister chromatids of homologous chromosomes. |
| Chiasma | X-shaped point at which crossing over has occurred between homologues. |
| Terminalisation | Movement of chiasmata towards the ends of chromosomes during diakinesis. |
| Centromere | Region of a chromosome where sister chromatids are joined and to which spindle fibres attach. |
| Kinetochore | Protein complex on the centromere where spindle microtubules attach. |
| Cleavage furrow | Indentation in animal cell membrane during cytokinesis. |
| Cell plate | Structure formed at the centre of a dividing plant cell that becomes the new cell wall. |
| Haploid | Cell containing one set of chromosomes (n). |
| Diploid | Cell containing two sets of chromosomes (2n). |
| Recombinase | Enzyme that catalyses crossing over during pachytene. |
| Gamete | Haploid reproductive cell formed by meiosis. |
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