9th Class Biology Chapter # 4 Exercise Solutions – Punjab Board
Cell Cycle
9th Class Biology Chapter 4 Solution (MCQs)
1. In which phase of cell cycle, maximum growth occurs in cell?
Options:
a) M phase
✅ c) G1 phase
b) S phase
d) G2 phase
Explanation:
During the G1 phase, the cell grows rapidly, synthesizes proteins and organelles — it’s the longest growth phase.
2. In which phase of cell cycle, the chromosomes duplicate?
Options:
a) Mitosis
b) G1 phase
✅ d) S phase
c) G2 phase
Explanation:
In the S phase (Synthesis phase), DNA is replicated and chromosomes are duplicated.
3. Which of the following is NOT a characteristic of mitosis?
Options:
a) It occurs in somatic cells.
b) It results in genetically identical daughter cells.
✅ c) The chromosome number is halved in daughter cells.
d) It results in the formation of two daughter cells.
Explanation:
Mitosis maintains the same chromosome number; halving occurs in meiosis, not mitosis.
4. At which stage of mitosis chromosomes line up in the centre?
Options:
a) Prophase
✅ b) Metaphase
c) Anaphase
d) Telophase
Explanation:
During metaphase, chromosomes align at the equator (middle) of the cell.
5. If you observe a cell in which nuclear membrane is reforming around two sets of chromosomes, what stage of cell cycle is this?
Options:
a) Anaphase
✅ b) Telophase
c) Prophase
d) Metaphase
Explanation:
In telophase, the nuclear membrane reforms around chromosomes at each pole.
6. How does the centrosome contribute to mitosis?
Options:
a) Initiates DNA replication
✅ b) Makes mitotic spindle
c) Forms the nuclear envelope
d) Duplicates organelles
Explanation:
Centrosomes help form the spindle fibers that pull chromosomes apart during mitosis.
7. Centrosome makes mitotic spindle in:
Options:
✅ a) Animal cells
b) Plant cells
c) Prokaryotic cells
d) All of these
Explanation:
Animal cells have centrosomes that produce spindle fibers. Plant cells organize spindles without centrosomes.
8. An organism has 4 pairs of chromosomes. After meiosis-I, how many chromosomes and chromatids will be present in each daughter cell?
Options:
a) 8 chromosomes and 16 chromatids
✅ b) 4 chromosomes and 8 chromatids
c) 4 chromosomes and 4 chromatids
d) 8 chromosomes and 8 chromatids
Explanation:
In meiosis-I, homologous pairs separate — so chromosome number is halved but each chromosome still has 2 chromatids.
9. Which event is unique to meiosis but not mitosis?
Options:
a) DNA replication
✅ c) Crossing over
b) Chromosome alignment
d) Nuclear division
Explanation:
Crossing over occurs only in meiosis-I, during prophase I, resulting in genetic variation.
9th Class Biology Chapter 4 Solution Mcqs Complete.
9th Class Biology Chapter 4 Solution Short Answer Questions
1. Enlist the events that occur during the G1 phase of interphase.
Cell increases in size
Synthesis of proteins and RNA
Preparation for DNA replication
Formation of organelles
2. What is the main purpose of the S phase in the cell cycle?
The S (Synthesis) phase is for DNA replication, where each chromosome duplicates to form two sister chromatids.
3. During which phase of mitosis do sister chromatids separate?
Anaphase — spindle fibers pull the sister chromatids apart to opposite poles.
4. How does crossing over contribute to genetic variation in meiosis?
During prophase-I of meiosis, homologous chromosomes exchange segments of DNA. This crossing over results in new gene combinations, increasing genetic diversity.
5. What is the role of spindle fibres in mitosis?
Spindle fibers attach to centromeres of chromosomes and help in separating sister chromatids during anaphase.
6. How is cytokinesis in animal cells different from plant cells?
In animal cells, cytokinesis occurs by cleavage furrow.
In plant cells, a cell plate forms between daughter nuclei due to the rigid cell wall.
7. What is the difference between prophase of mitosis and prophase-I of meiosis?
In mitosis, homologous chromosomes do not pair up.
In meiosis-I, homologous chromosomes pair (synapsis) and undergo crossing over.
8. How does meiosis differ from mitosis in chromosome number?
Mitosis: Daughter cells have same number of chromosomes as parent cell.
Meiosis: Daughter cells have half the number of chromosomes.
9. What are the key events of anaphase of mitosis?
Sister chromatids separate at the centromere.
Move to opposite poles by spindle fibers.
Each pole receives an identical set of chromosomes.
10. What is the function of the centrosome during cell division?
Centrosome helps form the mitotic spindle, which organizes and moves chromosomes during cell division.
11. What are sister chromatids, and do they separate in meiosis?
Sister chromatids are two identical copies of a chromosome joined at the centromere.
They do not separate in meiosis-I.
They do separate in meiosis-II.
12. How is mitosis related to the process of regeneration?
Mitosis produces genetically identical cells, helping in regeneration of lost tissues like skin, limbs (in some animals), and wound healing.
9th Class Biology Chapter 4 Solution Short Question Answer Complete.
9th Class Biology Chapter 4 Solution in Detail
1. Describe the events that occur during the phases of mitosis.
Mitosis is the process of nuclear division that produces two identical daughter cells. It occurs in five main stages:
a) Prophase:
Chromatin condenses into visible chromosomes.
Each chromosome appears as two sister chromatids joined at the centromere.
The nuclear envelope (membrane) starts to disintegrate.
Centrosomes move to opposite poles, forming the spindle apparatus made of microtubules.
b) Metaphase:
Chromosomes align at the metaphase plate (equator) of the cell.
Spindle fibers attach to the centromere of each chromosome from both poles.
c) Anaphase:
Centromeres split, and sister chromatids are pulled apart toward opposite poles.
The separated chromatids are now called daughter chromosomes.
d) Telophase:
Chromosomes reach the poles and begin to decondense into chromatin.
Nuclear membranes reform around each set of chromosomes.
The spindle apparatus disappears.
Two distinct nuclei are visible.
e) Cytokinesis:
Division of cytoplasm follows nuclear division.
In animal cells, it occurs by the formation of a cleavage furrow.
In plant cells, a cell plate forms which later becomes the new cell wall.
2. Describe cytokinesis in animal and plant cells.
Cytokinesis is the final step in cell division where the cytoplasm divides, forming two daughter cells.
In Animal Cells:
A cleavage furrow forms around the center of the cell.
This furrow is created by a ring of contractile proteins (actin and myosin) that constrict the membrane.
Eventually, the cell splits into two identical daughter cells.
In Plant Cells:
Due to the rigid cell wall, cleavage cannot occur.
Instead, a cell plate forms in the center of the cell.
Vesicles from the Golgi apparatus gather in the middle and fuse to form the plate.
This cell plate becomes a new cell wall separating the two daughter cells.
3. Describe the significance of mitosis.
Mitosis plays a vital role in the life of multicellular organisms:
Growth: Helps in the growth of tissues, organs, and the whole organism.
Tissue Repair: Replaces damaged or dead cells (e.g., wound healing).
Regeneration: Enables organisms to regrow lost parts (e.g., starfish arm).
Asexual Reproduction: Unicellular organisms (e.g., amoeba) reproduce through mitosis.
Genetic Stability: Ensures that daughter cells have the same number of chromosomes and identical genetic content as the parent cell.
4. Describe the events that occur during the phases of meiosis-I.
Meiosis-I is the first division in meiosis and is also called reductional division because it halves the chromosome number.
a) Prophase-I:
Longest and most complex stage.
Homologous chromosomes pair up (synapsis) to form bivalents or tetrads.
Crossing over occurs: Exchange of genetic material between non-sister chromatids at points called chiasmata, increasing genetic variation.
Nuclear envelope disappears and spindle fibers begin to form.
b) Metaphase-I:
Paired homologous chromosomes align at the metaphase plate.
Orientation is random, contributing to independent assortment.
c) Anaphase-I:
Homologous chromosomes are pulled apart to opposite poles.
Sister chromatids remain attached at this stage.
d) Telophase-I and Cytokinesis:
Chromosomes reach opposite poles.
Cytoplasm divides, forming two haploid daughter cells, each with half the number of chromosomes.
5. Describe the significance of meiosis.
Meiosis is crucial for sexual reproduction and plays multiple roles:
Production of Gametes: Produces haploid sperm and egg cells in animals and pollen and ovules in plants.
Maintains Chromosome Number: Prevents chromosome doubling in every generation by halving the number in gametes.
Genetic Variation: Through crossing over and independent assortment, meiosis introduces genetic differences among offspring.
Evolution: These variations are essential for evolution and natural selection.
- 9th Class Biology Chapter 4 Solution Details Complete.
9th Class Biology Chapter 4 Solution Inquisitive Questions
1. What role might mistakes in the cell cycle checkpoints play in the emergence of cancer?
The cell cycle checkpoints are control mechanisms that ensure each phase of the cell cycle is completed accurately before moving to the next stage. The main checkpoints are:
G1 checkpoint: Verifies if the cell has sufficient size, nutrients, and checks for DNA damage.
G2 checkpoint: Ensures that DNA replication has been completed correctly and without errors.
M (metaphase) checkpoint: Ensures that all chromosomes are properly aligned and attached to the spindle apparatus before the cell proceeds with division.
If these checkpoints fail:
Damaged or mutated DNA is not repaired and still gets passed on to daughter cells.
Cells with genetic errors continue to divide unchecked.
This leads to uncontrolled cell proliferation, which is the fundamental characteristic of cancer.
The accumulation of mutations over multiple cycles can convert normal cells into cancerous (malignant) cells.
Real-world example:
The p53 gene is a key tumor suppressor gene. It plays a major role in the G1 checkpoint by stopping the cycle if DNA is damaged.
If p53 is mutated, this checkpoint is lost. Cells with damaged DNA continue to divide, greatly increasing the risk of tumor formation.
2. Why do skin cells divide continuously, but nerve and muscle cells permanently exit the cell cycle?
Skin Cells:
Constantly exposed to friction, injury, UV rays, and environmental damage.
To maintain the protective outer layer, damaged or dead skin cells must be quickly replaced.
Thus, skin cells remain in the active phases of the cell cycle:
G1 → S → G2 → M, undergoing frequent mitosis.This rapid turnover ensures tissue regeneration and wound healing.
Nerve and Muscle Cells:
These cells are highly specialized and complex in function and structure.
Once fully developed (especially in adults), they exit the active cycle and enter a permanent resting phase called G0.
In this phase:
They do not divide or replicate.
They focus on maintaining specific cellular functions like transmitting nerve impulses (neurons) or contracting (muscle fibers).
This lack of regeneration explains why injuries to the brain, spinal cord, or heart muscles are difficult to heal.
However, in some parts of the brain, limited neurogenesis (new neuron formation) may occur under special conditions.
- 9th Class Biology Chapter 4 Solution Inquisitive Question Complete.
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