4.4 Meiosis

Short Introduction Meiosis is a type of cell division which gives rise to four daughter cells, each having half the number of chromosomes of the parent cell. It takes place only in cells involved in sexual reproduction. Such parent cells are usually diploid, which by this reduction division produce haploid daughter cells.

Main Idea

Meiosis is a special type of cell division that reduces the chromosome number by half, producing four haploid daughter cells. It is essential for sexual reproduction, as it produces gametes (sperm and eggs) with half the normal chromosome number. When two gametes fuse during fertilization, the original chromosome number is restored. Meiosis consists of two successive divisions: Meiosis I (reduction division) and Meiosis II. It also introduces genetic variation through crossing over and independent assortment.

Core Concept

Definition and Terminology generated by DeepSeek to PDF

Meiosis is a type of cell division which gives rise to four daughter cells, each having half the number of chromosomes of the parent cell. It takes place only in cells involved in sexual reproduction. Such parent cells are usually diploid (2n) which by this reduction division produce haploid (n) daughter cells. Diploid (2n): Cells which have two sets of chromosomes. Haploid (n): Cells with half the number of chromosomes. Chromosomes in a diploid cell are in homologous pairs. Haploid cells carry one member of each homologous pair. Phases of Meiosis Meiosis is a continuous process but for convenience is divided into: 1. Interphase I 2. Meiosis I 3. Interphase II 4. Meiosis II Interphase I Before the onset of the division process, the cell forms a copy of its genome by replication. That is why chromosomes appear with two chromatids during the division process. Meiosis I (Reduction Division) It is the reduction division which for daughter cells reduces the number of chromosomes to half. Firstly, karyokinesis forms two haploid nuclei, which is followed by cytokinesis. Karyokinesis in meiosis I is divided into: Prophase I, Metaphase I, Anaphase I, Telophase I. Prophase I: It is the longest phase of meiosis. Chromosomes begin to appear as thicker fibres due to coiling of chromatin. Homologous chromosomes line up point to point against each other and form pairs. Their centromeres are in the same position. This pairing process is called synapsis. Each pair is called a bivalent. The non-sister chromatids of homologous chromosomes join each other at some points along their length. Such points are called chiasma. Each chiasma is a site for the exchange of some genetic material between chromatids. The exchange of segments of non-sister chromatids during prophase I is called crossing over. Sister chromatids are two exactly similar copies attached by a centromere, while the chromatids of two homologous chromosomes are non-sister chromatids. To the end of prophase I, chromosomes are fully condensed and paired chromosomes repel each other. However, they are still attached at chiasma. Nucleoli and nuclear envelope disappear. Spindle apparatus is formed. Metaphase I: (Note: The provided text jumps from Prophase I to Interphase II. However, based on standard meiosis, during Metaphase I, bivalents align at the equator and spindle fibres attach to centromeres. The text does not explicitly detail Metaphase I, Anaphase I, or Telophase I in the provided excerpt.) Interphase II generated by DeepSeek to PDF

This phase varies in length but there is no further DNA replication during this phase. Meiosis II It is similar to mitosis and is divided into: Prophase II, Metaphase II, Anaphase II, Telophase II. Prophase II: Chromatin coils again so chromosomes appear as thick fibres. Spindle apparatus is organized. Nucleoli disappear and nuclear envelope disintegrates. Metaphase II: Chromosomes line up separately around the equator of the spindles. Spindle fibres are attached to the centromere of the chromosome. Like mitosis, here each chromosome is attached by a fibre from both poles. Anaphase II: Centromeres divide and spindle fibres pull the chromatids to the opposite poles. Telophase II: Chromatids reach their poles and uncoil. The spindle apparatus disappears. Nuclear envelopes are formed around chromatin at each pole. Cytokinesis Cleavage furrow in animal cells and cell wall formation in plant cells give four daughter cells at the end of the meiosis process. All cells formed by meiosis are haploid as they have half the number of parental chromosomes. They also have changed genetic makeup because of crossing over between homologous chromosomes during meiosis I. Significance of Meiosis 1. Maintenance of Chromosomes in Sexual Reproduction: During the life cycle of sexually reproducing organisms, diploid germ cells undergo meiosis to produce haploid daughter cells which act as gametes. Fusion of such haploid gametes maintains the chromosome number specific for each species. Humans have 46 chromosomes in their somatic cells. The haploid gametes (eggs and sperms) formed by meiosis have 23 chromosomes. In fertilization, a 23-chromosome sperm fuses with an egg also having 23 chromosomes. The original chromosome number of 46 is restored in the zygote. In plants, meiosis takes place during spore formation. 2. Genetic Variations: Variations are the differences among the members of the same species. They are necessary for the survival of species in always-changing environments. It is because of meiosis that new combinations of genes appear in gametes and then in the zygote. Crossing over and independently formed combinations of chromosomes in gametes are two important events of meiosis. Both provide the basis of variations. generated by DeepSeek to PDF

Comparison of Mitosis and Meiosis (Table 5.7) Mitosis Meiosis It occurs in somatic cells. It occurs in germ line cells. Cell divides only once. Cell divides twice. It produces two daughter cells. Four daughter cells are produced. Daughter cells produced are diploid. All daughter cells are haploid. Daughter cells become part of somatic body.Daughter cells form gametes. All chromosomes remain independent of each other. Homologous chromosomes pair with each other. No chiasmata formation. Chiasmata are formed. There is no crossing over during mitosis process.Crossing over takes place during meiosis I. Daughter cells are genetically alike. Daughter cells are genetically different from each other and parent cell. In most cases daughter cells can divide again.Gametes produced by meiosis cannot divide any more.

Detailed Explanation

Meiosis is a specialized form of cell division that halves the chromosome number to produce gametes for sexual reproduction. It is a two-part process: Meiosis I and Meiosis II. 1. Interphase I: Before meiosis begins, the cell replicates its DNA. Each chromosome now consists of two sister chromatids. 2. Meiosis I (Reduction Division): This division separates homologous chromosomes. Prophase I: The longest and most complex phase. Chromosomes condense. Homologous chromosomes pair up (synapsis) forming bivalents. Non-sister chromatids exchange genetic material at points called chiasmata, a process called crossing over. This is a major source of genetic variation. The nuclear envelope breaks down, and the spindle forms. (Metaphase I, Anaphase I, Telophase I are not detailed in the provided text, but in standard meiosis, bivalents align at the equator, homologous chromosomes are pulled to opposite poles, and two haploid nuclei form.) 3. Interphase II: A brief resting phase with no DNA replication. This ensures that the cells remain haploid. 4. Meiosis II: This division separates sister chromatids, similar to mitosis. Prophase II: Chromosomes condense again, and the spindle forms. Metaphase II: Chromosomes align at the equator; spindle fibres attach to centromeres. Anaphase II: Centromeres split, and chromatids are pulled to opposite poles. Telophase II: Chromatids reach the poles, decondense, and nuclear envelopes reform. 5. Cytokinesis: The cytoplasm divides, resulting in four haploid daughter cells. Because of crossing over and independent assortment, these four cells are genetically distinct from each other and from the parent cell. 6. Significance: Maintenance of chromosome number: Meiosis produces haploid gametes. Their fusion at fertilization restores the diploid number. In humans, 23 + 23 = 46. generated by DeepSeek to PDF

Genetic variation: Crossing over and independent assortment create new combinations of genes, which are essential for evolution and adaptation to changing environments.

Intermediate Level

Meiosis is a reduction division that produces four haploid daughter cells from a diploid parent cell. It is divided into Interphase I, Meiosis I, Interphase II, and Meiosis II. Interphase I involves DNA replication. Meiosis I (reduction division) consists of Prophase I (longest phase; synapsis and crossing over occur, forming bivalents and chiasmata), Metaphase I, Anaphase I, and Telophase I. Interphase II has no DNA replication. Meiosis II is similar to mitosis and consists of Prophase II, Metaphase II, Anaphase II, and Telophase II. Cytokinesis produces four haploid cells. Meiosis is significant for maintaining the chromosome number in sexual reproduction (e.g., humans have 46 chromosomes; gametes have 23; fusion restores 46) and for creating genetic variation through crossing over and independent assortment of chromosomes. The text provides a comparison table highlighting that mitosis occurs in somatic cells and produces two diploid cells, while meiosis occurs in germ line cells and produces four haploid cells with genetic variation.

Important Points

• 1. Meiosis gives rise to four daughter cells, each with half the number of chromosomes of the parent cell.
• 2. It occurs only in cells involved in sexual reproduction.
• 3. Parent cells are usually diploid (2n); daughter cells are haploid (n).
• 4. Diploid: Two sets of chromosomes; Haploid: Half the number.
• 5. Chromosomes in a diploid cell are in homologous pairs.
• 6. Phases: Interphase I, Meiosis I, Interphase II, Meiosis II.
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• 7. Interphase I: DNA replication; chromosomes have two chromatids.
• 8. Meiosis I is the reduction division; consists of Prophase I, Metaphase I, Anaphase I, Telophase I.
• 9. Prophase I: Longest phase; synapsis (pairing of homologous chromosomes) forms bivalents; chiasmata
• form; crossing over occurs (exchange of segments between non-sister chromatids).
• 10. Interphase II: No further DNA replication.
• 11. Meiosis II: Similar to mitosis; consists of Prophase II, Metaphase II, Anaphase II, Telophase II.
• 12. Cytokinesis: Results in four haploid daughter cells.
• 13. All cells formed by meiosis are haploid and have changed genetic makeup due to crossing over.
• 14. Significance 1: Maintenance of chromosome number in sexual reproduction (human 2n=46; gametes
• n=23; zygote 2n=46).
• 15. Significance 2: Genetic variations through crossing over and independent assortment of
• chromosomes.
• 16. In plants, meiosis takes place during spore formation.

Remember

• Meiosis = 4 haploid daughter cells.
• Diploid (2n) → Haploid (n).
• Phases: Interphase I → Meiosis I → Interphase II → Meiosis II.
• Meiosis I = reduction division; Prophase I = longest phase; synapsis → bivalents; chiasmata → crossing
• over.
• Interphase II = no DNA replication.
• Meiosis II = similar to mitosis; separates sister chromatids.
• Crossing over = exchange between non-sister chromatids.
• Significance: Maintenance of chromosome number; genetic variation.
• Human: 46 chromosomes in somatic cells; 23 in gametes; 46 in zygote after fertilization.
• Plants: Meiosis during spore formation.
• Mitosis vs. Meiosis: Mitosis = somatic, 2 diploid cells, no crossing over; Meiosis = germ line, 4 haploid
• cells, crossing over.

Common Mistakes

• 1. Thinking meiosis produces two daughter cells: Meiosis produces four daughter cells (mitosis produces
• two).
• 2. Thinking the chromosome number stays the same: Meiosis halves the chromosome number (diploid →
• haploid).
• 3. Confusing Meiosis I and Meiosis II: Meiosis I is the reduction division (homologous chromosomes
• separate); Meiosis II is similar to mitosis (sister chromatids separate).
• 4. Forgetting that there is no DNA replication in Interphase II: This is crucial for maintaining the haploid
• state.
• 5. Confusing crossing over with independent assortment: Crossing over is the exchange of segments
• between non-sister chromatids during Prophase I; independent assortment is the random alignment of
• homologous pairs during Metaphase I (not detailed in the text, but a key concept).
• 6. Thinking chiasmata are the same as centromeres: Chiasmata are points where non-sister chromatids
• exchange segments; centromeres hold sister chromatids together.
• 7. Thinking meiosis occurs in somatic cells: Meiosis occurs only in germ line cells (cells involved in sexual
• reproduction).
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Quick Revision

• Meiosis is a type of cell division that produces four haploid daughter cells from a diploid parent cell. It occurs only in
• cells involved in sexual reproduction. The process consists of Interphase I (DNA replication), Meiosis I (reduction
• division, including Prophase I with synapsis, bivalent formation, chiasmata, and crossing over), Interphase II (no DNA
• replication), and Meiosis II (similar to mitosis, separating sister chromatids). Cytokinesis results in four haploid cells
• with changed genetic makeup due to crossing over. Meiosis is significant for maintaining the chromosome number
• in sexual reproduction (e.g., human gametes have 23 chromosomes; zygote has 46) and for creating genetic
• variation through crossing over and independent assortment. In plants, meiosis occurs during spore formation.

60-Second Revision

• Meiosis = 4 haploid cells.
• Diploid (2n) → Haploid (n).
• Phases: Interphase I → Meiosis I → Interphase II → Meiosis II.
• Prophase I = longest; synapsis, bivalents, chiasmata, crossing over.
• Interphase II = no DNA replication.
• Meiosis II = like mitosis.
• Significance: Chromosome maintenance + genetic variation.
• Human: 46 → 23 → 46.
• Plants: Meiosis during spore formation.
• Frequently Asked Questions (FAQs)
• Q1. What is meiosis? Answer: Meiosis is a type of cell division which gives rise to four daughter cells, each having
• half the number of chromosomes of the parent cell. It takes place only in cells involved in sexual reproduction.
• Q2. What is the difference between diploid and haploid cells? Answer: Cells which have two sets of
• chromosomes are called diploid (2n). Cells with half the number of chromosomes are called haploid (n).
• Q3. What are the main phases of meiosis? Answer: The main phases are Interphase I, Meiosis I, Interphase II, and
• Meiosis II. Meiosis I is divided into Prophase I, Metaphase I, Anaphase I, and Telophase I. Meiosis II is divided into
• Prophase II, Metaphase II, Anaphase II, and Telophase II.
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• Q4. What happens during Prophase I? Answer: Prophase I is the longest phase of meiosis. Chromosomes
• condense. Homologous chromosomes line up point to point against each other and form pairs (synapsis). Each pair
• is called a bivalent. Non-sister chromatids join at points called chiasmata, where crossing over (exchange of genetic
• material) occurs. Nucleoli and nuclear envelope disappear, and the spindle apparatus forms.
• Q5. What is crossing over? Answer: Crossing over is the exchange of segments of non-sister chromatids during
• Prophase I of meiosis I. It occurs at points called chiasmata.
• Q6. What is a bivalent? Answer: A bivalent is a pair of homologous chromosomes that have paired up during
• synapsis in Prophase I of meiosis.
• Q7. Why is there no DNA replication in Interphase II? Answer: Interphase II is a resting phase between Meiosis I
• and Meiosis II. There is no DNA replication because the cells are already haploid, and the purpose of Meiosis II is to
• separate sister chromatids, not to replicate DNA.
• Q8. What is the significance of meiosis? Answer: Meiosis is significant for:
• 1. Maintenance of chromosomes in sexual reproduction: It produces haploid gametes; their fusion
• restores the diploid chromosome number (e.g., human 46 → 23 → 46).
• 2. Genetic variations: Crossing over and independent assortment create new combinations of genes,
• providing variation necessary for survival in changing environments.
• Q9. How many chromosomes do human gametes have? Answer: Human gametes (eggs and sperms) formed by
• meiosis have 23 chromosomes. Somatic cells have 46.
• Q10. How does meiosis differ from mitosis? Answer: Mitosis occurs in somatic cells, divides once, produces two
• diploid daughter cells, has no crossing over, and produces genetically alike cells. Meiosis occurs in germ line cells,
• divides twice, produces four haploid daughter cells, involves crossing over, and produces genetically different cells.
• Multiple Choice Questions (MCQs)
• Q1. Meiosis produces: A. Two diploid cells B. Four haploid cells C. Two haploid cells D. Four diploid cells Correct
• Answer: B Explanation: Meiosis gives rise to four daughter cells, each with half the number of chromosomes
• (haploid).
• Q2. Meiosis occurs in: A. Somatic cells B. Germ line cells C. Epithelial cells D. Muscle cells Correct Answer: B
• Explanation: Meiosis occurs in cells involved in sexual reproduction (germ line cells).
• Q3. The longest phase of meiosis is: A. Prophase II B. Metaphase I C. Prophase I D. Telophase II Correct Answer: C
• Explanation: Prophase I is the longest phase of meiosis.
• Q4. The pairing of homologous chromosomes during Prophase I is called: A. Crossing over B. Synapsis C. Chiasma D.
• Bivalent Correct Answer: B Explanation: The pairing process of homologous chromosomes is called synapsis.
• Q5. The exchange of segments between non-sister chromatids is called: A. Synapsis B. Crossing over C. Chiasma D.
• Independent assortment Correct Answer: B Explanation: Crossing over is the exchange of segments of non-sister
• chromatids during Prophase I.
• Q6. During which phase does no DNA replication occur? A. Interphase I B. Interphase II C. Prophase I D. S phase
• Correct Answer: B Explanation: There is no further DNA replication during Interphase II.
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• Q7. How many chromosomes do human gametes have? A. 46 B. 23 C. 92 D. 12 Correct Answer: B Explanation:
• Human gametes formed by meiosis have 23 chromosomes.
• Q8. Which of the following is a source of genetic variation in meiosis? A. DNA replication B. Crossing over C.
• Cytokinesis D. Interphase Correct Answer: B Explanation: Crossing over and independent assortment are sources of
• genetic variation in meiosis.
• Q9. In plants, meiosis takes place during: A. Vegetative propagation B. Spore formation C. Budding D. Grafting
• Correct Answer: B Explanation: In plants, meiosis takes place during spore formation.
• Q10. Which of the following is true about Meiosis II? A. It is a reduction division B. It involves crossing over C. It is
• similar to mitosis D. It produces diploid cells Correct Answer: C Explanation: Meiosis II is similar to mitosis and
• separates sister chromatids.
• Subtopic 4.4 complete. Reply "next" to approve and I will produce Subtopic 5: 4.5 Comparison of Mitosis and
• Meiosis and Significance (Summary Table included in the text).