What is crossing over and when does it occur?

Crossing over is the exchange of genetic material between non-sister chromatids of homologous chromosomes during prophase-I (pachytene sub-stage) of meiosis. It results in genetic recombination and variation.

What is the significance of meiosis?

Meiosis maintains the chromosome number constant across generations by halving it during gamete formation. It also introduces genetic variation through crossing over and independent assortment, which is the basis of evolution.

Cell Cycle Exercise Solutions | KalviGavani

Q: What happens during the S phase of interphase?

During the S (synthesis) phase, DNA replication occurs — each chromosome duplicates to form two sister chromatids joined at the centromere. This ensures each daughter cell receives a complete set of genetic material.

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How to Use This Page

Solve each Samacheer Kalvi exercise question on your own first, then compare your answer with the solutions below. Cell Cycle covers interphase, mitosis, meiosis, and their significance — focus on stage-wise events and comparing the two types of division.

Key Concepts (Quick Recap)

Cell Cycle Overview

The cell cycle is the sequence of events from one cell division to the next. It consists of:

Interphase (longest phase, ~95% of cycle):

G1 Phase (Gap 1): Cell growth, normal metabolism, preparation for DNA replication
S Phase (Synthesis): DNA replication; each chromosome duplicates into two sister chromatids
G2 Phase (Gap 2): Protein synthesis, preparation for mitosis, organelle duplication

M Phase (Mitotic Phase) (~5% of cycle):

Mitosis (nuclear division) + Cytokinesis (cytoplasmic division)

G0 Phase (Quiescent stage): Cells that exit the cell cycle and stop dividing.

Interphase — Detailed Breakdown

Phase	Duration	Key Events
G1 Phase	8–10 hours	Cell growth; RNA and protein synthesis; histone synthesis; cell size increases
S Phase	6–8 hours	DNA replication; each chromosome duplicates to form sister chromatids; centriole duplication begins
G2 Phase	4–6 hours	Protein synthesis for mitosis; organelle duplication; ATP accumulation; preparation for M phase

Mitosis — Stages and Events

Stage	Key Events
Prophase	Chromatin condenses into chromosomes; centrioles move to poles; spindle fibres form; nuclear membrane and nucleolus disappear
Metaphase	Chromosomes align at the equatorial plate (metaphase plate); centromeres attach to spindle fibres; best stage for chromosome counting
Anaphase	Centromeres split; sister chromatids separate and move to opposite poles; chromatids become independent chromosomes
Telophase	Chromosomes decondense at poles; nuclear membrane reappears; nucleolus reappears; spindle fibres disappear

Cytokinesis: Division of cytoplasm — in plant cells, a cell plate forms from the centre outward; in animal cells, a cleavage furrow forms from the outside inward.

Meiosis — Overview

Meiosis involves two successive divisions without DNA replication between them:

Meiosis I (Reductional division) — reduces chromosome number from 2n to n:

Sub-stage	Key Events
Prophase I	Leptotene (chromosomes become visible) → Zygotene (synapsis/forming bivalents) → Pachytene (crossing over) → Diplotene (chiasmata visible) → Diakinesis (terminalisation of chiasmata)
Metaphase I	Bivalents align at equatorial plate
Anaphase I	Homologous chromosomes separate; chromatids do NOT split
Telophase I	Two haploid nuclei form

Meiosis II (Equational division) — similar to mitosis but with haploid chromosome number:

Stage	Key Events
Prophase II	Chromosomes condense again
Metaphase II	Chromosomes align at equatorial plate
Anaphase II	Centromeres split; sister chromatids separate
Telophase II	Four haploid daughter cells formed

Mitosis vs Meiosis — Comparison

Feature	Mitosis	Meiosis
Divisions	One	Two (Meiosis I and II)
Daughter cells	Two	Four
Chromosome number	2n (diploid)	n (haploid)
Genetic variation	No variation (identical cells)	Variation (crossing over, independent assortment)
Occurs in	Somatic cells	Germ cells (gamete formation)
Prophase	Simple	Complex (with synapsis and crossing over)
Significance	Growth, repair, asexual reproduction	Gamete formation, maintains chromosome number
Duration	Short	Longer

I. Choose the Best Answer (MCQs)

Q1. The longest phase of the cell cycle is (a) M phase (b) G1 phase (c) S phase (d) Interphase

Answer(d) Interphase — Interphase (G1 + S + G2) occupies about 95% of the total cell cycle time. The M phase is relatively short.

Q2. DNA replication occurs during (a) G1 phase (b) S phase (c) G2 phase (d) M phase

Answer(b) S phase — During the S (synthesis) phase, DNA replication occurs. Each chromosome duplicates to form two sister chromatids joined at the centromere.

Q3. Crossing over occurs during which stage of meiosis? (a) Leptotene (b) Zygotene (c) Pachytene (d) Diakinesis

Answer(c) Pachytene — Crossing over (exchange of genetic material between non-sister chromatids of homologous chromosomes) occurs during the pachytene sub-stage of prophase-I.

Q4. The best stage to count the number of chromosomes is (a) Prophase (b) Metaphase (c) Anaphase (d) Telophase

Answer(b) Metaphase — In metaphase, chromosomes are aligned at the equatorial plate with centromeres on the metaphase plate, making it the best stage for counting and studying chromosome morphology.

Q5. Mitosis results in (a) Two haploid cells (b) Four haploid cells (c) Two diploid cells (d) Four diploid cells

Answer(c) Two diploid cells — Mitosis produces two daughter cells that are genetically identical to the parent cell and have the same (diploid) chromosome number.

Q6. The reduction in chromosome number occurs during (a) Mitosis (b) Meiosis I (c) Meiosis II (d) Interphase

Answer(b) Meiosis I — Meiosis I is the reductional division where homologous chromosomes separate, reducing the chromosome number from 2n to n.

Q7. In plant cells, cytokinesis occurs by (a) Cleavage furrow (b) Cell plate formation (c) Budding (d) Fragmentation

Answer(b) Cell plate formation — In plant cells, cytokinesis occurs by cell plate formation from the centre outward. In animal cells, it occurs by cleavage furrow from the outside inward.

II. Fill in the Blanks

1. The sequence of events from one cell division to the next is called the cell cycle. 2. The longest phase of the cell cycle is interphase. 3. DNA replication occurs during the S phase of interphase. 4. The phase in which cells exit the cell cycle and stop dividing is called G0 phase (quiescent stage). 5. During prophase, the nuclear membrane and nucleolus disappear. 6. Chromosomes align at the equatorial plate during metaphase. 7. Sister chromatids separate during anaphase of mitosis. 8. The nuclear membrane reappears during telophase. 9. In plant cells, cytokinesis occurs by cell plate formation. 10. In animal cells, cytokinesis occurs by cleavage furrow. 11. Crossing over occurs during pachytene stage of prophase-I. 12. Synapsis (pairing of homologous chromosomes) occurs during zygotene. 13. Meiosis produces four haploid daughter cells. 14. Meiosis I is called reductional division because it reduces the chromosome number. 15. Meiosis II is called equational division because it separates sister chromatids like mitosis.

III. True or False (with corrections for false statements)

Q1. Mitosis reduces the chromosome number by half.

AnswerFalse — Correction: Mitosis produces two diploid (2n) daughter cells that are genetically identical to the parent cell. It is MEIOSIS that reduces the chromosome number by half, producing haploid (n) cells.

Q2. Crossing over occurs during prophase-I of meiosis.

AnswerTrue — Crossing over (exchange of genetic material between non-sister chromatids) occurs during the pachytene sub-stage of prophase-I of meiosis.

Q3. The S phase is the longest phase of interphase.

AnswerTrue — The S phase (DNA replication) is typically the longest phase of interphase, lasting 6–8 hours, compared to G1 (8–10 hours may vary) and G2 (4–6 hours).

Q4. Cytokinesis in plant cells occurs by cleavage furrow.

AnswerFalse — Correction: Cytokinesis in PLANT cells occurs by CELL PLATE formation (from centre outward). Cytokinesis in ANIMAL cells occurs by cleavage furrow (from outside inward).

Q5. Meiosis II is similar to mitosis.

AnswerTrue — Meiosis II is similar to mitosis in that it involves the separation of sister chromatids. However, the chromosome number is already haploid (n) at the start of Meiosis II.

IV. Match the Following

Column A	Column B	Answer
1. Prophase	(a) Chromatids move to poles	(c) Chromosomes condense, nuclear membrane disappears
2. Metaphase	(b) Nuclear membrane reappears	(d) Chromosomes align at equatorial plate
3. Anaphase	(c) Chromosomes condense, nuclear membrane disappears	(a) Chromatids move to poles
4. Telophase	(d) Chromosomes align at equatorial plate	(b) Nuclear membrane reappears
5. S phase	(e) Crossing over	(f) DNA replication
6. Pachytene	(f) DNA replication	(e) Crossing over

V. Answer Briefly (2-mark questions)

Q1. What is the cell cycle? Name its phases.

Answer

The cell cycle is the sequence of events from one cell division to the next. It consists of two main phases: 1. Interphase (G1, S, G2) — the longest phase (~95%); cell grows and DNA replicates 2. M Phase (Mitosis + Cytokinesis) — nuclear and cytoplasmic division Some cells may enter G0 (quiescent phase) and stop dividing.

Q2. Differentiate between mitosis and meiosis.

Answer

| Feature | Mitosis | Meiosis | |---|---|---| | Divisions | One | Two | | Daughter cells | Two (2n) | Four (n) | | Genetic variation | None (identical cells) | Present (crossing over, independent assortment) | | Occurs in | Somatic cells | Germ cells | | Prophase | Simple | Complex (synapsis, crossing over) | | Significance | Growth, repair | Gamete formation |

Q3. What is crossing over? State its significance.

Answer

Crossing over is the exchange of genetic material (DNA segments) between non-sister chromatids of homologous chromosomes during the pachytene stage of prophase-I of meiosis. Significance: 1. Produces genetic variation in offspring 2. Provides raw material for evolution 3. Creates new gene combinations (recombinants) 4. Forms the basis of genetic mapping (recombination frequency)

Q4. What happens during the S phase of interphase?

Answer

During the S (synthesis) phase: 1. DNA replication occurs — each chromosome duplicates to form two sister chromatids joined at the centromere 2. Histone proteins are synthesised to package the newly formed DNA 3. The DNA content doubles from 2C to 4C (but chromosome number remains 2n) 4. Centriole duplication begins (in animal cells)

VI. Answer in Detail (5-mark questions)

Q1. Describe the stages of mitosis with their key events.

Answer

Mitosis is divided into four stages: 1. Prophase: - Chromatin condenses into distinct chromosomes (each with two sister chromatids) - Centrioles move to opposite poles and form the spindle apparatus - Nuclear membrane begins to disintegrate - Nucleolus disappears - Chromosomes become visible under a microscope 2. Metaphase: - Chromosomes align at the equatorial plate (metaphase plate) - Centromeres of all chromosomes lie on the equatorial plane - Spindle fibres attach to kinetochores of centromeres - This is the best stage for chromosome counting and karyotyping 3. Anaphase: - Centromeres split; sister chromatids separate - Chromatids (now called daughter chromosomes) move to opposite poles - Movement is due to shortening of spindle fibres (chromosomal fibres) - The cell becomes elongated 4. Telophase: - Chromosomes reach the poles and decondense into chromatin - Nuclear membrane reappears around each set of chromosomes - Nucleolus reappears - Spindle fibres disappear - Two daughter nuclei are formed Cytokinesis follows telophase: division of cytoplasm. In plant cells, a cell plate forms from centre outward. In animal cells, a cleavage furrow forms from outside inward.

Q2. Describe the stages of prophase-I of meiosis in detail.

Answer

Prophase-I is the longest and most complex phase of meiosis, divided into five sub-stages: 1. Leptotene (Thin thread stage): - Chromosomes become visible as thin, long, thread-like structures - Each chromosome consists of two chromatids, but they are not yet distinguishable - Chromomeres (bead-like structures) are visible along the chromosomes 2. Zygotene (Pairing stage): - Homologous chromosomes pair up (synapsis) - Each pair forms a bivalent (tetrad) — two chromosomes with four chromatids - Synaptonemal complex forms between homologous chromosomes 3. Pachytene (Crossing over stage): - Crossing over occurs — exchange of genetic material between non-sister chromatids - Recombination nodules form at crossing over sites - Chiasmata (points of crossing over) become visible - This stage produces genetic variation 4. Diplotene (Separation stage): - Homologous chromosomes begin to separate (desynapsis) - But they remain connected at chiasmata (singular: chiasma) - In some species, chromosomes become very active (lampbrush chromosomes) 5. Diakinesis (Terminalisation stage): - Chiasmata move towards the ends of chromosomes (terminalisation) - Chromosomes become fully condensed - Nuclear membrane and nucleolus disappear - Spindle fibres begin to form - The cell is ready for metaphase-I

Q3. Compare mitosis and meiosis in detail.

Answer

| Feature | Mitosis | Meiosis | |---|---|---| | Type of division | One | Two (Meiosis I + Meiosis II) | | Occurs in | Somatic (body) cells | Germ cells (gamete mother cells) | | Daughter cells | Two | Four | | Chromosome number | Same as parent (2n) | Half of parent (n) | | DNA replication | Once (before prophase) | Once (before Meiosis I only) | | Prophase | Simple, short | Complex (prophase-I has 5 sub-stages) | | Synapsis | Absent | Present (zygotene of prophase-I) | | Crossing over | Absent | Present (pachytene of prophase-I) | | Metaphase | Chromosomes at equatorial plate | Bivalents at equatorial plate (MI); chromosomes at plate (MII) | | Anaphase | Sister chromatids separate | Homologous chromosomes separate (AI); sister chromatids separate (AII) | | Genetic variation | None (identical daughter cells) | Present (crossing over, independent assortment) | | Significance | Growth, repair, asexual reproduction | Gamete formation, genetic diversity | | Duration | Short (30 min – 3 hours) | Long (days) |

Common Mistakes to Avoid

Saying mitosis reduces chromosome number: Mitosis produces two DIPLOID (2n) daughter cells identical to the parent. Only MEIOSIS reduces the chromosome number from 2n to n.
Confusing chromatids and chromosomes: Before S phase, each chromosome has one chromatid. After S phase, each chromosome has two sister chromatids. After anaphase, each chromatid becomes an independent chromosome.
Confusing anaphase of mitosis with anaphase-I of meiosis: In mitosis anaphase, sister chromatids separate. In meiosis anaphase-I, homologous chromosomes separate (not sister chromatids).
Confusing synapsis and crossing over: Synapsis = pairing of homologous chromosomes (zygotene). Crossing over = exchange of genetic material (pachytene). They are different events.
Forgetting that Meiosis II is equational: Meiosis I is reductional (2n → n). Meiosis II is equational (n → n), similar to mitosis but starting with haploid cells.
Confusing cytokinesis in plant and animal cells: Plant cells — cell plate from centre outward. Animal cells — cleavage furrow from outside inward.
Saying DNA replicates before Meiosis II: DNA replication occurs only BEFORE Meiosis I. There is NO DNA replication between Meiosis I and Meiosis II.

Exam Checklist

[ ] Can you name and describe all phases of the cell cycle (G1, S, G2, M, G0)?
[ ] Can you describe the four stages of mitosis with key events?
[ ] Can you describe the five sub-stages of prophase-I of meiosis?
[ ] Can you compare mitosis and meiosis (at least 8 points)?
[ ] Can you explain crossing over and its significance?
[ ] Can you explain what happens during the S phase?
[ ] Can you differentiate between anaphase of mitosis and anaphase-I of meiosis?
[ ] Can you explain cytokinesis in plant vs animal cells?
[ ] Can you state the significance of mitosis and meiosis?
[ ] Can you explain synapsis, bivalent, and chiasmata?

Frequently Asked Questions

Mitosis is a single division producing two identical diploid (2n) daughter cells, used for growth and repair. Meiosis involves two successive divisions (Meiosis I and II) producing four genetically different haploid (n) cells, used for gamete formation. Meiosis includes crossing over and reduces chromosome number by half.