Chapter 10: Cell Cycle and Cell Division – Long Answer Type Questions
CBSE Class 11 Biology Long Answer Questions – Cell Cycle and Cell Division (Chapter 10 | NCERT)
Course & Examination Details (Systematic Format)
Course: CBSE Class 11 Biology
Unit: III – Cell Structure and Function
Chapter: 10 – Cell Cycle and Cell Division
Based on: NCERT Textbook
Examination: CBSE Board Examination (Class XI)
Assessment Focus: Flowcharts, stage-wise comparison, conceptual understanding
Section A: Cell Cycle and Its Phases (Q1–Q7)
Q1. Explain the concept of the cell cycle and its importance in living organisms.
Answer:
The cell cycle is a series of well-coordinated events by which a cell grows, replicates its genetic material, and divides into two daughter cells. It consists of interphase and M phase. The cell cycle is essential for growth, development, tissue repair, and replacement of damaged or dead cells. It maintains genetic continuity by ensuring accurate DNA replication and equal distribution of chromosomes. In unicellular organisms, the cell cycle enables reproduction, while in multicellular organisms it supports growth and maintenance. Proper regulation of the cell cycle is crucial, as uncontrolled cell division can lead to abnormal growth and diseases such as cancer.
Q2. Describe the different phases of interphase in detail.
Answer:
Interphase is the longest phase of the cell cycle and is divided into G₁, S, and G₂ phases. During the G₁ phase, the cell grows in size, synthesizes RNA and proteins, and duplicates organelles. The S phase is marked by DNA replication, where each chromosome duplicates to form sister chromatids, ensuring genetic stability. In the G₂ phase, the cell undergoes further growth and prepares for mitosis by synthesizing spindle proteins and repairing DNA if required. Interphase is metabolically very active and prepares the cell for successful division.
Q3. Why is interphase not considered a resting phase?
Answer:
Although no visible cell division occurs during interphase, it is not a resting phase. The cell is highly metabolically active and involved in growth, synthesis of proteins and RNA, duplication of organelles, and replication of DNA. These activities are essential for preparing the cell for division. Without interphase, the cell would not have sufficient energy, materials, or genetic duplication to divide successfully. Hence, interphase is a crucial preparatory stage of the cell cycle.
Q4. Explain the significance of DNA replication during the S phase.
Answer:
DNA replication during the S phase ensures that each daughter cell receives an identical and complete set of genetic information. Each chromosome duplicates to form two sister chromatids, but the chromosome number remains unchanged. This process maintains genetic continuity across cell generations. Errors in DNA replication can lead to mutations, highlighting the importance of accuracy during this phase. The S phase is therefore vital for stability, growth, and proper functioning of cells.
Q5. Describe the role of checkpoints in the cell cycle.
Answer:
Cell cycle checkpoints are control mechanisms that ensure proper progression of the cell cycle. Major checkpoints occur at G₁, G₂, and metaphase. The G₁ checkpoint assesses cell size and environmental conditions before DNA replication. The G₂ checkpoint verifies completion and accuracy of DNA replication. The metaphase checkpoint ensures proper alignment of chromosomes before separation. These checkpoints prevent errors, maintain genetic stability, and reduce the risk of abnormal cell division.
Q6. What is cytokinesis? Explain its process in plant and animal cells.
Answer:
Cytokinesis is the division of cytoplasm following nuclear division, resulting in two separate daughter cells. In animal cells, cytokinesis occurs through the formation of a cleavage furrow that deepens until the cell splits. In plant cells, due to the presence of a rigid cell wall, cytokinesis occurs by the formation of a cell plate at the center, which later develops into a new cell wall. Cytokinesis ensures complete separation of daughter cells.
Q7. Explain the significance of the G₀ phase.
Answer:
The G₀ phase is a quiescent stage where cells exit the cell cycle and stop dividing. Cells in this phase remain metabolically active but do not proliferate. Some cells, such as nerve cells, remain permanently in G₀, while others may re-enter the cell cycle when required. The G₀ phase helps regulate cell division and prevents unnecessary or uncontrolled proliferation.
Section B: Mitosis (Q8–Q15)
Q8. Define mitosis and explain its significance.
Answer:
Mitosis is an equational division in which a parent cell divides to form two genetically identical daughter cells with the same chromosome number. It is essential for growth, repair, and replacement of cells in multicellular organisms. Mitosis also plays a role in asexual reproduction in certain organisms. By maintaining chromosome number and genetic identity, mitosis ensures stability and continuity in body cells.
Q9. Describe the stages of mitosis with key events.
Answer:
Mitosis consists of four stages: prophase, metaphase, anaphase, and telophase. In prophase, chromatin condenses into chromosomes and spindle fibers form. During metaphase, chromosomes align at the equatorial plane. In anaphase, centromeres split and sister chromatids separate to opposite poles. Telophase involves decondensation of chromosomes and reformation of nuclear membranes. These stages ensure accurate chromosome segregation.
Q10. Why is metaphase considered the most suitable stage for karyotyping?
Answer:
During metaphase, chromosomes are highly condensed, clearly visible, and aligned at the equatorial plane. This makes it easy to observe their number, size, and shape. Hence, metaphase is ideal for preparing karyotypes and detecting chromosomal abnormalities.
Q11. Explain the events of anaphase in mitosis.
Answer:
In anaphase, centromeres split, and sister chromatids separate to become independent chromosomes. These chromosomes move toward opposite poles due to the shortening of spindle fibers. This ensures equal distribution of genetic material to both daughter cells.
Q12. What happens during telophase of mitosis?
Answer:
During telophase, chromosomes reach the poles and decondense into chromatin. Nuclear membrane and nucleolus reappear, and spindle fibers disappear. Two daughter nuclei are formed, marking the end of nuclear division.
Q13. Why is mitosis called equational division?
Answer:
Mitosis is called equational division because it maintains the same chromosome number in the daughter cells as in the parent cell. There is no reduction or increase in chromosome number.
Q14. How does mitosis contribute to tissue repair?
Answer:
Mitosis replaces damaged or dead cells with new genetically identical cells. This helps in wound healing, regeneration of tissues, and maintenance of organ function.
Q15. Mention any two differences between mitosis and meiosis.
Answer:
Mitosis involves one division and produces two identical diploid cells, while meiosis involves two divisions producing four haploid cells. Mitosis occurs in somatic cells, whereas meiosis occurs in germ cells.
Section C: Meiosis (Q16–Q22)
Q16. Define meiosis and explain its significance.
Answer:
Meiosis is a reduction division that produces four haploid daughter cells from a diploid parent cell. It is essential for sexual reproduction as it maintains chromosome number across generations. Meiosis also introduces genetic variation, which is vital for evolution.
Q17. Describe the stages of prophase I of meiosis.
Answer:
Prophase I includes leptotene, zygotene, pachytene, diplotene, and diakinesis. These stages involve chromosome condensation, pairing of homologous chromosomes, crossing over, and terminalization, leading to genetic recombination.
Q18. Explain the process and significance of crossing over.
Answer:
Crossing over is the exchange of genetic material between homologous chromosomes during pachytene of prophase I. It creates new gene combinations, increasing genetic variation and contributing to evolution.
Q19. Differentiate between meiosis I and meiosis II.
Answer:
Meiosis I is reductional, separating homologous chromosomes, while meiosis II is equational, separating sister chromatids. Meiosis I reduces chromosome number, whereas meiosis II maintains it.
Q20. Why is meiosis I called reductional division?
Answer:
Meiosis I reduces the chromosome number by half as homologous chromosomes separate, producing haploid cells.
Q21. How does meiosis contribute to genetic variation?
Answer:
Meiosis introduces variation through crossing over and independent assortment of chromosomes, producing genetically distinct gametes.
Q22. Explain the significance of meiosis in evolution.
Answer:
By generating genetic variation, meiosis provides raw material for natural selection, driving evolution and adaptation.
Section D: Significance of Cell Division (Q23–Q25)
Q23. Explain the overall significance of cell division.
Answer:
Cell division supports growth, development, repair, reproduction, and maintenance of genetic stability. It is fundamental to life processes in all organisms.
Q24. Why is controlled cell division necessary?
Answer:
Controlled cell division ensures normal growth and prevents abnormal cell proliferation. Loss of control can result in diseases like cancer.
Q25. Compare the roles of mitosis and meiosis in living organisms.
Answer:
Mitosis supports growth and repair by producing identical cells, while meiosis supports sexual reproduction and variation by producing haploid gametes.
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