Sangam Age Notes (ICSE IX)

1. The Cell Cycle
  • Definition: The series of events that take place in a cell leading to its division and duplication.
  • Phases: Divided into two main phases:
    • Interphase: Growth and DNA replication.
    • M-Phase (Mitotic Phase): Cell division (Mitosis and Cytokinesis).
2. Interphase
  • The longest phase of the cell cycle, during which the cell grows, replicates its DNA, and prepares for division. It consists of three sub-phases:
    • G1 Phase (First Gap): Cell grows and generally carries out its normal metabolic functions. Organelles duplicate.
    • S Phase (Synthesis): DNA replication occurs. Each chromosome is duplicated to form two identical sister chromatids attached at the centromere.
    • G2 Phase (Second Gap): Cell continues to grow and synthesizes proteins and organelles in preparation for mitosis. Checks for DNA integrity.
3. M-Phase: Mitosis and Cytokinesis
  • Mitosis: The process of nuclear division, resulting in two identical nuclei. It is divided into four main stages:
    • #### 3.1. Prophase
      • Chromatin condenses into visible chromosomes (each consisting of two sister chromatids).
      • The nuclear envelope begins to break down.
      • The mitotic spindle (composed of microtubules) begins to form from the centrosomes, which move towards opposite poles of the cell.
      • Nucleolus disappears.
    • #### 3.2. Metaphase
      • Chromosomes align along the metaphase plate (equatorial plate), an imaginary plane equidistant from the two poles.
      • Each sister chromatid is tethered to a spindle fiber from opposite poles, ensuring accurate separation.
      • This alignment is crucial for ensuring that each new daughter cell receives a complete set of chromosomes.
    • #### 3.3. Anaphase
      • Sister chromatids separate at the centromere, becoming individual chromosomes.
      • These newly separated chromosomes are pulled towards opposite poles by the shortening of the spindle microtubules.
      • The cell elongates as non-kinetochore microtubules lengthen.
    • #### 3.4. Telophase
      • Chromosomes arrive at opposite poles and begin to decondense (unravel).
      • New nuclear envelopes form around the two sets of chromosomes.
      • The mitotic spindle disappears.
      • The nucleoli reappear within the newly formed nuclei.
  • Cytokinesis: The division of the cytoplasm, typically overlapping with Telophase.
    • In Animal Cells: A cleavage furrow forms and deepens, eventually pinching the cell into two daughter cells.
    • In Plant Cells: A cell plate