Detailed Notes Chapter10- Cell Cycle And Cell Division

Introduction

1. Origin:

   • All organisms, irrespective of their eventual size or complexity, begin as a single cell.

2. Cell Traits:

   • Cells, being fundamental units of life, exhibit two essential traits: growth, the increase in size or mass, and reproduction, the ability to generate offspring.

3. Reproduction Process:

   • Cells perpetuate through a process called cell division or mitosis, wherein a single cell divides into two identical daughter cells.

4. Doubling Effect:

   • With each round of division, the number of cells doubles, as each parental cell gives rise to two daughter cells.

5. Growth and Division:

   • The newly formed daughter cells inherit the capability to grow and divide, sustaining the cycle of replication.

6. Resulting Structures:

   • Through successive cycles of growth and division, a single cell and its descendants can construct complex structures comprising millions or billions of cells, representing the formation of multicellular organisms.

10.1 CELL CYCLE

1. Cell Division’s Significance:

   • Cell division is vital for all living organisms. It involves not just splitting the cell but also replicating DNA and increasing cell size.

2. Harmony of Processes:

   • For cell division to occur correctly and produce healthy new cells, DNA replication, cell growth, and cell division must happen in a synchronized manner.

3. Defining the Cell Cycle:

   • The cell cycle is the series of steps a cell goes through: copying its DNA, making other cellular components, and eventually splitting into two daughter cells.

4. Cellular Activities:

   • While cell growth is ongoing, DNA replication occurs at a specific stage of the cell cycle.

5. Chromosome Handoff:

   • After DNA replication, the copied chromosomes are shared between the two daughter cells during cell division, a process tightly controlled by genetics.

10.1.1 Phases of Cell Cycle

1. Phases of the Cell Cycle:

   • The cell cycle in eukaryotic cells is typically divided into two main phases: Interphase and M Phase (Mitosis phase).

2. Duration Variation:

   • The duration of the cell cycle varies among organisms and cell types. For example, human cells in culture divide approximately every 24 hours, while yeast cells can complete the cell cycle in about 90 minutes.

3. Interphase:

   • Interphase is the phase between two successive M phases. It is significant to note that in the 24-hour average duration of the cell cycle of a human cell, cell division proper lasts for only about an hour, while interphase accounts for over 95% of the cycle’s duration.

4. M Phase (Mitosis Phase):

   • The M Phase encompasses the actual cell division or mitosis. It begins with nuclear division (karyokinesis), involving the separation of daughter chromosomes, and usually ends with the division of the cytoplasm (cytokinesis).

5. Further Division of Interphase:

   • Interphase, though often termed the resting phase, is a time of preparation for division. It includes three sub-phases:
     • G1 Phase (Gap 1): The interval between mitosis and the initiation of DNA replication. The cell grows metabolically but does not replicate its DNA during this phase.
     • S Phase (Synthesis): The period of DNA synthesis or replication. The amount of DNA per cell doubles, but the chromosome number remains the same.
     • G2 Phase (Gap 2): The phase where proteins are synthesized in preparation for mitosis, while cell growth continues.

6. Cellular Activities in Animal Cells:

   • During the S phase in animal cells, DNA replication begins in the nucleus, and centrioles duplicate in the cytoplasm. G2 phase sees the synthesis of proteins in preparation for mitosis, while cell growth persists.

7. Quiescent Stage (G0):

   • Some cells, like those in adult animals, may exit the cell cycle during G1 phase and enter a quiescent stage (G0), where they remain metabolically active but do not proliferate unless required by the organism.

8. Mitotic Cell Division in Animals and Plants:

   • In animals, mitotic cell division typically occurs in diploid somatic cells, with few exceptions like male honey bees. In plants, mitotic divisions can occur in both haploid and diploid cells.

9. Mitosis in Haploid Cells in Plants:

   • In plants, mitotic divisions in haploid cells can be observed during specific stages of alternation of generations. For example, in mosses and ferns, mitosis occurs in the gametophyte stage, which is haploid. Similarly, in angiosperms, mitosis occurs in haploid cells during the production of pollen grains and ovules.

10.2 M PHASE

1. Significance of M Phase:

   • The M Phase is the most dynamic period of the cell cycle, marked by significant reorganization of nearly all cell components.

2. Equational Division:

   • Since the number of chromosomes in the parent and progeny cells remains the same, the M Phase is also referred to as equational division.

3. Mitosis Stages:

   • Mitosis is conveniently divided into four stages of nuclear division, known as karyokinesis. However, it’s crucial to understand that cell division is a continuous and progressive process, and distinct boundaries between stages may not always be evident.

4. Stages of Karyokinesis:

   • Karyokinesis involves the following four stages:
     • Prophase: The first stage characterized by condensation of chromosomes, breakdown of the nuclear envelope, and formation of the spindle apparatus.
     • Metaphase: Chromosomes align at the cell’s equator, forming the metaphase plate, and spindle fibers attach to the centromeres of each chromosome.
     • Anaphase: Sister chromatids separate and move towards opposite poles of the cell, pulled by spindle fibers, ensuring each daughter cell receives an identical set of chromosomes.
     • Telophase: Chromosomes reach the poles of the cell, nuclear envelopes re-form around them, and chromosomes begin to de-condense. Cytokinesis, the division of the cytoplasm, typically occurs during or after telophase, completing the process of cell division.

10.2.1 Prophase

1. Preceding Phases:

   • Prophase follows the S (Synthesis) and G2 (Gap 2) phases of interphase. During these phases, newly formed DNA molecules are not distinct but intertwined.

2. Initiation of Chromosome Condensation:

   • Prophase is characterized by the beginning of condensation of chromosomal material. Chromatin, the tangled mass of DNA and proteins, starts to condense into compact mitotic chromosomes.

3. Centrosome Movement:

   • The centrosome, which duplicated during the S phase of interphase, begins to migrate towards opposite poles of the cell. Each centrosome emits microtubules called asters. Together with spindle fibers, these asters form the mitotic apparatus.

4. Characteristic Events of Prophase Completion:

   • The conclusion of prophase is marked by specific events:
     • Chromosomal material condenses, forming compact mitotic chromosomes. These chromosomes consist of two chromatids joined at the centromere.
     • Centrosomes move towards opposite poles of the cell, and asters radiate from each centrosome, forming the mitotic apparatus.

5. Microscopic View of Cells at the End of Prophase:

   • Cells observed under a microscope at the end of prophase do not display golgi complexes, endoplasmic reticulum, nucleolus, or the nuclear envelope. These structures become less visible or disassemble as the cell progresses through prophase.

10.2.2 Metaphase

1. Nuclear Envelope Disintegration:

   • The onset of Metaphase is marked by the complete breakdown of the nuclear envelope. Consequently, chromosomes are dispersed throughout the cell’s cytoplasm.

2. Chromosome Condensation:

   • By this stage, the condensation of chromosomes is complete, making them clearly visible under a microscope. Metaphase is thus an ideal stage for studying chromosome morphology.

3. Chromosome Structure:

   • Each metaphase chromosome consists of two sister chromatids joined together at the centromere. Small disc-shaped structures called kinetochores are present at the surface of the centromeres.

4. Role of Kinetochores:

   • Kinetochores serve as attachment sites for spindle fibers, which are formed by the mitotic apparatus. These spindle fibers connect to the chromosomes and help move them into position at the center of the cell.

5. Chromosome Alignment:

   • During Metaphase, all chromosomes align at the equator of the cell, forming what is known as the metaphase plate. Each chromatid of a chromosome is connected by its kinetochore to spindle fibers from opposite poles of the cell.

6. Key Features of Metaphase:

   • Spindle fibers attach to the kinetochores of chromosomes.
   • Chromosomes are guided to the spindle equator and aligned along the metaphase plate by spindle fibers extending from both poles of the cell.

10.2.3 Anaphase

1. Onset of Anaphase:

   • Anaphase begins as each chromosome positioned at the metaphase plate is divided simultaneously. The two resulting daughter chromatids, now called daughter chromosomes, commence their migration towards opposite poles of the cell.

2. Direction of Chromosome Movement:

   • As each chromosome moves away from the equatorial plate, the centromere of each chromosome remains oriented towards the pole. Consequently, at the leading edge, the centromere of each chromosome leads the way, while the chromosome arms trail behind.

3. Characteristics of Anaphase:

   • The anaphase stage is defined by the following key events:
     • Centromeres split, causing the chromatids to separate from each other.
     • The separated chromatids, now termed daughter chromosomes, move towards opposite poles of the cell.

10.2.4 Telophase

1. Initiation of Telophase:

   • Telophase marks the final stage of karyokinesis, the division of the cell nucleus. At the outset of telophase, the chromosomes that have migrated to their respective poles begin to decondense and lose their distinct individuality.

2. Chromosome Decondensation:

   • As telophase progresses, the individual chromosomes become less visible, and each set of chromatin material tends to aggregate at each of the two poles of the cell. Chromosomes lose their compact structure and revert to a less condensed state.

3. Key Events of Telophase:

   • Telophase is characterized by several key events:
     • Chromosomes cluster at opposite ends of the cell, near the spindle poles, and their identity as discrete entities diminishes.
     • Nuclear envelope components, such as the nuclear membrane, begin to reassemble around the chromosome clusters at each pole, forming two distinct daughter nuclei.
     • Cellular structures like the nucleolus, golgi complex, and endoplasmic reticulum (ER), which disassembled during earlier stages of mitosis, start to reform in the vicinity of the newly forming nuclei.

10.2.5 Cytokinesis

1. Definition and Purpose:

   • Cytokinesis is the final stage of cell division following karyokinesis (nuclear division). Its purpose is to physically divide the cytoplasm of the parent cell into two daughter cells, completing the process of cell division.

2. In Animal Cells:

   • In animal cells, cytokinesis is achieved by the formation of a furrow in the plasma membrane. This furrow gradually deepens and eventually pinches the cell in two, dividing the cytoplasm into two separate daughter cells.

3. In Plant Cells:

   • Plant cells, which are enclosed by a rigid cell wall, undergo cytokinesis through a different mechanism. The process begins with the formation of a cell plate in the center of the cell. This cell plate grows outward to meet the existing lateral walls, eventually forming a new cell wall that separates the two daughter cells.

4. Organelle Distribution:

   • During cytokinesis, organelles such as mitochondria and plastids are evenly distributed between the two daughter cells, ensuring both cells have the necessary cellular machinery for survival and function.

5. Multinucleate Condition:

   • In some organisms, such as certain plant tissues, karyokinesis (nuclear division) may not be followed by cytokinesis. This can result in a multinucleate condition where multiple nuclei coexist within a single cell, leading to the formation of a syncytium. An example of this is the liquid endosperm in coconut.