Chapter 4: Cell Cycle

Select the correct answer:

1. A bivalent consists of:

A) Two chromatids and one centromere

B) Four chromatids and two centromeres

C) Four chromatids and four centromeres

D) Two chromatids and two centromeres

Answer: B) Four chromatids and two centromeres

Explanation: A bivalent (tetrad) consists of a pair of homologous chromosomes, each with two sister chromatids. Thus, it has four chromatids and two centromeres.

2. During cell division spindle fibres attach a chromosome at:

A) Telomere

B) Centromere

C) upper arm of chromosome

D) lower arm of chromosome

Answer: B) Centromere

Explanation: Spindle fibers attach to the centromere of a chromosome at a specialized protein structure called the kinetochore.

3. Some students of SSC observed a thin cross section from root tip of onion plant under the microscope. They found a cell at late prophase and counted 28 chromosomes in it. The number of chromosomes in daughter cells should be:

A) 14

B) 28

C) 56

D) 07

Answer: B) 28

Explanation: Mitosis produces two daughter cells with the same number of chromosomes as the parent cell. Since the cell had 28 chromosomes, the daughter cells will also have 28 chromosomes.

4. Crossing over results in genetic recombination. It occurs between:

A) Two chromatids of the same chromosome.

B) Two chromatids of any two homologous chromosomes

C) Two chromatids of opposite sister chromatids

D) Two chromatids of homologous chromosomes

Answer: D) Two chromatids of homologous chromosomes

Explanation: Crossing over occurs between non-sister chromatids of homologous chromosomes during prophase I of meiosis.

5. The spindle apparatus of plants differs from that of animals in not having:

A) microtubules

B) equator of spindle

C) centrioles

D) centromere

Answer: C) centrioles

Explanation: Plant cells lack centrioles, but they still form a spindle apparatus. Animal cells have centrioles that organize the spindle.

6. Substance and energy required for the replication of DNA is accumulated in the cell during:

A) G₁

B) G₂

C) S phase

D) M phase

Answer: B) G₂

Explanation: The G₂ phase is the second gap phase where the cell synthesizes proteins and accumulates energy and materials needed for mitosis, including those required for DNA replication (which occurred in S phase).

7. All of the following events takes place both in mitosis and meiosis except:

A) Condensation of chromatin to form chromosomes

B) Formation of spindle apparatus

C) Nuclear envelope and nucleolus disappear

D) Chromosomes pair for crossing over

Answer: D) Chromosomes pair for crossing over

Explanation: Crossing over occurs only in meiosis (prophase I), not in mitosis.

8. The cell shown in this diagram is passing through: (Diagram of a cell with homologous chromosomes separated)

A) Prophase I of meiosis

B) prophase of mitosis

C) telophase of meiosis I

D) anaphase of meiosis II

Answer: D) anaphase of meiosis II

Explanation: The diagram shows sister chromatids separating and moving to opposite poles, which occurs in anaphase of meiosis II (or mitosis). Since the question implies meiosis, it is anaphase II.

9. The longest phase of meiosis is:

A) Interphase I

B) Prophase I

C) Interphase II

D) prophase II

Answer: B) Prophase I

Explanation: Prophase I of meiosis is the longest phase because it includes several substages: leptotene, zygotene, pachytene, diplotene, and diakinesis. Crossing over occurs during this phase.

10. What causes the number of chromosomes to reduce to half when a cell divides by meiosis?

A) replication of DNA during interphase I

B) separation of homologous chromosomes during meiosis I

C) separation of sister chromatids of all the chromosomes during meiosis I

D) crossing over during meiosis I

Answer: B) separation of homologous chromosomes during meiosis I

Explanation: The reduction in chromosome number occurs during meiosis I when homologous chromosomes separate, resulting in two haploid cells.

11. Which of the following cell is at Metaphase I stage?

(Diagrams A, B, C, D)

Answer: B

Explanation: Metaphase I is characterized by homologous chromosome pairs (bivalents) aligned at the metaphase plate.

12. This diagram is showing different stages of crossing over. Which stage contains chiasma? (Diagrams A, B, C, D)

Answer: C

Explanation: Chiasma (the point of contact between non-sister chromatids) is visible in the stage where crossing over is occurring.

13. Which processes involve meiosis? (Venn Diagram with Gamete formation, Crossing over, Regeneration)

Answer: B (Gamete formation and Crossing over)

Explanation: Meiosis is involved in gamete formation and crossing over. Regeneration involves mitosis.

14. Diagram shows replication of chromosomes. (Diagram of chromosome replication) What is the total number of chromosomes in this diagram?

A) 2

B) 3

C) 4

D) 6

Answer: A) 2

Explanation: The diagram shows one chromosome before replication and two sister chromatids after replication. There are two chromosomes shown (one pair of homologous chromosomes).

15. Chromosome number of Fruit fly is 8. The gametes of fruit fly contain:

A) 2 chromosomes

B) 4 chromosomes

C) 8 chromosomes

D) 16 chromosomes

Answer: B) 4 chromosomes

Explanation: Gametes are haploid. If the diploid number is 8, the haploid number is 4.

1. Explain spindle apparatus in detail.

Answer: The spindle apparatus is a structure composed of microtubules that forms during cell division (mitosis and meiosis). It originates from the centrosomes (in animal cells) or from the nuclear envelope (in plant cells). The spindle fibers attach to the kinetochores on the centromeres of chromosomes. They are responsible for aligning chromosomes at the metaphase plate and separating sister chromatids (in mitosis and meiosis II) or homologous chromosomes (in meiosis I) to opposite poles of the cell. The spindle apparatus ensures accurate distribution of genetic material to daughter cells.

2. What is the significance of Crossing over?

Answer: Crossing over is the exchange of genetic material between non-sister chromatids of homologous chromosomes during prophase I of meiosis. Its significance includes:

Genetic Variation: It creates new combinations of alleles on a chromosome, increasing genetic diversity in the population. Evolution: Genetic variation is the raw material for natural selection and evolution. Accurate Segregation: It helps ensure proper segregation of homologous chromosomes during meiosis I. Genetic Mapping: The frequency of crossing over is used to map the distance between genes on a chromosome.

3. Enlist the events taking place during G₁ phase.

Answer: The G₁ phase (first gap phase) is the first stage of interphase. Events include:

Cell Growth: The cell increases in size. Protein Synthesis: Synthesis of proteins and enzymes needed for DNA replication. Organelle Duplication: Duplication of organelles (e.g., mitochondria, ribosomes). Metabolic Activity: High metabolic activity; the cell carries out its normal functions. Checkpoint: The cell checks if conditions are favorable for DNA synthesis. If not, it may enter G₀ (quiescent) phase.

4. Cell cycle below shows the formation of two daughter cells, cell A and cell B. Cell A continues in the cell cycle while cell B exits in G₀. What will be the difference in materials they synthesize after the production of cell A and cell B?

Answer: Cell A continues in the cell cycle, so it will synthesize materials needed for the next round of division: DNA (in S phase), proteins, and organelles (in G₁ and G₂). Cell B exits the cell cycle into G₀ (quiescent phase). In G₀, the cell is metabolically active but does not prepare for division. It will synthesize only the materials needed for its normal cellular functions, not for DNA replication or division. Thus, Cell A synthesizes division-related materials, while Cell B synthesizes maintenance-related materials.

5. Why do epithelial cells of skin divide continuously?

Answer: Epithelial cells of the skin divide continuously to replace the cells that are constantly shed from the surface. The skin is a protective barrier that is exposed to wear and tear, so it needs constant renewal. Stem cells in the basal layer of the epidermis divide by mitosis to produce new cells that migrate to the surface and eventually slough off. This continuous division is essential for maintaining the skin's integrity and function.

6. Give differences between meiosis I and meiosis II.

Answer:

7. In rapidly dividing cells which phase of cell cycle is reduced. Explain.

Answer: In rapidly dividing cells, the G₁ phase is often reduced. The G₁ phase is the first gap phase where the cell grows and prepares for DNA synthesis. In rapidly dividing cells (e.g., embryonic cells, cancer cells), the cell cycle is accelerated, and the G₁ phase is shortened or bypassed. These cells prioritize rapid division over growth, so they quickly move from mitosis to S phase (DNA synthesis) to replicate their DNA and divide again. The G₂ phase may also be reduced.

8. What is the difference between cytokinesis of a plant cell and an animal cell?

Answer:

Animal Cell: Cytokinesis occurs by the formation of a cleavage furrow. A contractile ring of actin and myosin filaments forms around the equator of the cell and pinches the cell into two daughter cells. Plant Cell: Cytokinesis occurs by the formation of a cell plate. Vesicles from the Golgi apparatus line up along the equator and fuse to form a cell plate, which eventually develops into a new cell wall. The cell plate grows outward to divide the cell into two.

9. Both skin cells and cancerous cells divide rapidly. Why cancerous cells are harmful but skin cells are not?

Answer: Skin cells divide rapidly in a controlled manner to replace shed cells, maintaining tissue homeostasis. Cancerous cells divide rapidly but lose the regulatory mechanisms that control cell division. They divide uncontrollably, forming tumors, invading nearby tissues, and spreading to other parts of the body (metastasis). Cancerous cells also have abnormal shapes, lack contact inhibition, and can survive in unfavorable conditions. Thus, uncontrolled division makes cancer harmful, while controlled division in skin cells is normal and beneficial.

10. How haploid organisms produce gametes?

Answer: Haploid organisms (e.g., some algae, fungi, and male bees) produce gametes by mitosis. Since the organism is already haploid (n), mitosis produces daughter cells that are also haploid (n). These cells can function as gametes. When two haploid gametes fuse during fertilization, they form a diploid zygote (2n), which then undergoes meiosis to produce haploid spores or cells, completing the life cycle.

11. Down syndrome is due to extra copy of chromosome number 21. That is why Down syndrome people have 47 chromosomes. Why their chromosome number is high?

Answer: Down syndrome is caused by trisomy 21, which means there are three copies of chromosome 21 instead of the normal two. This occurs due to nondisjunction during meiosis, where chromosomes fail to separate properly. As a result, a gamete receives an extra copy of chromosome 21. When this gamete fuses with a normal gamete during fertilization, the resulting zygote has 47

chromosomes (45 autosomes + 2 sex chromosomes + 1 extra chromosome 21 = 47) instead of the normal 46. The extra chromosome leads to the characteristic features of Down syndrome.

1. If a cell completes meiosis I but meiosis II fails to occur, what type of anomalies will appear in the two daughter cell thus formed?

Answer: If a cell completes meiosis I but meiosis II fails to occur, the resulting two daughter cells will have significant chromosomal anomalies.

Normal Meiosis:

Meiosis I separates homologous chromosomes, resulting in two haploid cells (n) with replicated chromosomes (each

chromosome has two sister chromatids). Meiosis II separates sister chromatids, resulting in four haploid cells (n) with unreplicated chromosomes.

Anomaly (Failure of Meiosis II):

If meiosis II fails, the two cells from meiosis I will not divide further. Each of the two daughter cells will contain replicated chromosomes (two sister chromatids per chromosome). The chromosome number will be haploid (n), but the DNA content will be diploid (2n) because each chromosome consists of two chromatids. These cells are genetically abnormal because they contain duplicated genetic material. If they participate in fertilization, they could produce offspring with abnormal chromosome numbers (e.g., triploidy or other aneuploidies). In males, failure of meiosis II can lead to sperm with duplicated chromosomes, causing miscarriages or genetic disorders in offspring. In females, failure of meiosis II can lead to eggs with extra chromosomes, resulting in conditions like Down syndrome (if chromosome 21 is involved) or other trisomies.

In summary, failure of meiosis II results in two diploid (2n) cells with replicated chromosomes, which can cause severe genetic abnormalities if involved in fertilization.

2. A cell is shown in this diagram. Answer following questions after observing it. (Diagram of a cell in anaphase with spindle fibers and chromosomes)

a. Give at least one finding on the basis of which you can identify it animal or plant cell.

Answer: The cell can be identified as an animal cell if it lacks a cell wall and has centrioles. If it has a cell wall and lacks centrioles, it is a plant cell. The diagram likely shows centrioles or a cleavage furrow, indicating an animal cell.

b. Identify the stage of cell division the given cell is passing through?

Answer: The cell is in anaphase of mitosis or meiosis II, as sister chromatids are separating and moving to opposite poles.

c. Enlist the reasons of your identification.

Answer: The reasons include:

Separation of Chromatids: Sister chromatids are being pulled apart. Spindle Fibers: Spindle fibers are attached to centromeres. Cell Elongation: The cell is elongating. Presence of Centrioles (if animal): Centrioles are visible at the poles.

d. Meiosis II is identical to mitosis, explain.

Answer: Meiosis II is identical to mitosis because: Both involve the separation of sister chromatids. Both have similar stages: prophase, metaphase, anaphase, telophase. Both result in cells with the same chromosome number as the parent cell (haploid in meiosis II, diploid in mitosis). The key difference is that meiosis II occurs in haploid cells and produces four haploid cells, while mitosis occurs in diploid cells and produces two diploid cells.

3. Explain the role of cell membrane in maintaining equilibrium while exchanging matter?

Answer: The cell membrane plays a crucial role in maintaining equilibrium by regulating the exchange of matter.

1. Selective Permeability:

The membrane is selectively permeable, allowing some substances to pass through while blocking others. Small, nonpolar molecules (O₂, CO₂) pass freely. Polar molecules and ions require transport proteins.

2. Passive Transport:

Diffusion: Movement of molecules from high to low concentration. Osmosis: Movement of water across a semi-permeable membrane.

Facilitated Diffusion: Movement through channel or carrier proteins.

3. Active Transport:

Movement of molecules against the concentration gradient using ATP. Example: Sodium-potassium pump.

4. Endocytosis and Exocytosis:

Bulk transport of materials into and out of the cell.

5. Maintaining Equilibrium:

The membrane maintains a stable internal environment (homeostasis) by balancing the intake of nutrients and the removal of waste. It regulates ion concentrations, pH, and water balance.

In summary, the cell membrane is essential for maintaining equilibrium by controlling the exchange of matter and energy between the cell and its environment.