4 Most SAQ’s of Cell Cycle and Cell Division Chapter in Inter 1st Year Botany (TS/AP)

4 Marks

SAQ-1 : Which division is necessary to maintain constant chromosome number in all body cells of multicellular organism and why?

Introduction

In multicellular organisms, the cells that make up the body need to maintain a constant chromosome number to ensure proper growth, development, and functioning. One process that plays a crucial role in achieving this constant chromosome number is mitosis. Mitosis is a type of cell division that produces identical daughter cells with the same number of chromosomes as the parent cell.

1. Mitosis Maintains Constant Chromosome Number: Mitosis is essential to maintain a consistent chromosome number in all body cells of multicellular organisms. During mitosis, a single cell divides into two identical daughter cells, each with the same ploidy (chromosome number) as the parent cell. This process ensures that the genetic content of the daughter cells remains nearly identical to that of the original cell.
2. Cell Growth, Wound Healing, and Regeneration: Mitosis plays critical roles in the growth and repair of multicellular organisms. During growth, cells undergo mitosis to increase their numbers, allowing the organism to develop and increase in size. Additionally, when an organism is injured and needs to heal, mitosis helps in the regeneration of damaged tissues, enabling the body to recover and repair itself.
3. Restoring Surface to Volume Ratio: As organisms grow, the volume of cells increases more rapidly than their surface area. This can lead to an imbalance in the surface to volume ratio of cells, affecting their efficiency in exchanging nutrients and waste. Mitosis helps in cell division, restoring a balanced surface to volume ratio, ensuring optimal cell functioning.

Summary

Mitosis is essential for maintaining a constant chromosome number in all body cells of multicellular organisms. It ensures that daughter cells have the same genetic content as the parent cell. Mitosis also plays a key role in cell growth, wound healing, and regeneration, contributing to the proper development and functioning of multicellular organisms. Additionally, mitosis helps restore the surface to volume ratio of cells, enabling efficient nutrient exchange and waste removal.

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SAQ-2 : Though redundantly described as a resting phase, interphase does not really involve rest. Comment.

Introduction

Interphase, often mistakenly referred to as a resting phase, is actually a period of intense activity and preparation within the cell cycle. It is during interphase that a cell undergoes growth and DNA replication, preparing it for subsequent cell division.

Misconception of “Resting Phase”

1. The term “resting phase” is a misnomer when applied to interphase. This term inaccurately suggests that the cell is inactive or dormant during this period.
2. In reality, interphase is a phase of high metabolic activity where the cell grows and undertakes critical functions.

Key Activities During Interphase

1. G1 Phase (First Gap Phase):
   • During the G1 phase, the cell undergoes significant growth and biosynthesis.
   • This phase involves the synthesis of various enzymes and proteins necessary for DNA replication and cell growth.
2. S Phase (Synthesis Phase):
   • The S phase is marked by the replication of DNA.
   • Each chromosome is duplicated, ensuring that each new cell will have a complete set of genetic instructions following cell division.
3. G2 Phase (Second Gap Phase):
   • The G2 phase involves further cell growth and preparation for mitosis.
   • The cell synthesizes proteins and organelles, and makes necessary repairs.

Summary

Interphase is a critical phase in the cell cycle, characterized by cell growth, DNA replication, and preparation for cell division. Far from being a resting phase, it is a period of significant cellular activity and metabolism. Understanding the processes that occur during interphase is essential for comprehending how cells grow, replicate, and divide.

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SAQ-3 : Explain prophase I of meiosis

Introduction

Prophase I is a crucial stage in Meiosis I, and it is more complex and lengthier compared to the prophase of mitosis. It is further divided into five distinct phases, each characterized by specific chromosomal behaviors.

Phases of Prophase I

1. Leptotene:
   • In this phase, the nucleus increases in size by absorbing water from the cytoplasm.
   • The chromatin material organizes itself into a specific number of chromosomes, and these chromosomes become visible under the microscope.
2. Zygotene:
   • During the Zygotene phase, the chromosomes undergo further changes.
   • They become shorter and thicker, and they start to pair up with their homologous counterparts. This pairing of homologous chromosomes is known as synapsis, and the paired chromosomes are called bivalents.
3. Pachytene:
   • At the Pachytene stage, the bivalent chromosomes become clearly visible as tetrads.
   • Recombination nodules appear, and crossing over occurs, where the genetic material between the paired chromosomes is exchanged. This process of genetic exchange enhances genetic diversity.
4. Diplotene:
   • In the Diplotene phase, the synaptonemal complex that held the homologous chromosomes together starts to dissolve.
   • The paired chromosomes separate from each other, except at the sites where crossing over occurred. These sites of attachment are known as chiasmata.
5. Diakinesis:
   • Diakinesis is the final stage of prophase I of Meiosis I. In this stage, the chiasmata reach their maximum point of condensation and terminalization.
   • The nucleolus, a structure within the nucleus, disappears, and the nuclear envelope breaks down.

Summary

Prophase I of Meiosis I is a highly intricate process involving multiple phases, each contributing to the successful division of homologous chromosomes and the generation of genetic diversity through crossing over. It sets the stage for the subsequent stages of Meiosis I, leading to the formation of haploid daughter cells with unique genetic characteristics.

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SAQ-4 : How does cytokinesis in plant cells differ from that in animal cells?

Introduction

Cytokinesis is the process of cell division that follows the end of mitosis or meiosis. It involves the division of the cytoplasm to form two separate daughter cells. Cytokinesis occurs differently in plant cells compared to animal cells due to their distinct structural features.

Cytokinesis in Animal Cells

1. Formation of Cleavage Furrow:
   • In animal cells, cytokinesis begins with the formation of a cleavage furrow.
   • This furrow is formed by the contraction of actin and myosin filaments that pinch the cell membrane inward.
2. Constriction of Cell Membrane:
   • The cleavage furrow gradually deepens and constricts the cell membrane.
   • This constriction eventually leads to the physical separation of the cell into two daughter cells.

Cytokinesis in Plant Cells

1. Cell Plate Formation:
   • Plant cells undergo cytokinesis through the formation of a cell plate.
   • The cell plate forms in the center of the cell and grows outward toward the cell walls.
2. Involvement of Golgi Apparatus:
   • The cell plate is constructed with materials (like cellulose and pectin) delivered by vesicles from the Golgi apparatus.
   • These vesicles coalesce at the middle of the cell, forming the new cell wall.
3. Development of New Cell Wall:
   • Once the cell plate reaches the existing cell walls, it fuses with them.
   • This process results in the formation of a new cell wall that divides the original cell into two separate daughter cells.

Summary

Cytokinesis differs significantly in plant and animal cells. In animal cells, it involves the formation of a cleavage furrow that constricts the cell into two. In plant cells, cytokinesis occurs through the formation of a cell plate that develops into a new cell wall, dividing the cell in two. These differences arise from the structural variations between plant and animal cells, especially the presence of a rigid cell wall in plant cells.