Phases of the cell cycle Knowledge-Based Study

Study notes

1. What is the Cell Cycle?

   The cell cycle is an ordered series of events that leads to cell growth, duplication of genetic material, and division into two genetically identical daughter cells. In eukaryotic cells, the cycle is broadly divided into two main stages:

   • Interphase: The preparatory phase during which the cell grows, replicates its DNA, and synthesizes necessary proteins.
   • M Phase (Mitotic Phase): The division phase that includes mitosis (nuclear division) and cytokinesis (cytoplasmic division).

   Cells that are not actively preparing to divide may exit the active cycle and enter a specialized resting state designated as G0 phase.

   Stages of Interphase

   Interphase accounts for approximately 90% of a typical cell cycle. It consists of three sequential subphases:

   • G1 Phase (First Gap): The cell experiences rapid metabolic activity and growth, synthesizing enzymes, structural proteins, and organelles. Chromosomes exist as single, un-replicated chromatids.
   • S Phase (Synthesis): The cell duplicates its nuclear DNA. At the end of S phase, each chromosome consists of two identical sister chromatids joined at the centromere. Centrosomes are also duplicated during this phase.
   • G2 Phase (Second Gap): The cell continues to grow and synthesizes specific proteins required for spindle apparatus assembly, such as tubulin, while finalizing preparations for nuclear division.

   Mitosis and Cytokinesis

   Once interphase is complete, the cell enters the M Phase, which involves two linked processes:

   1. Mitosis: Division of the replicated genetic material into two nuclei. It progresses through four distinct stages: prophase, metaphase, anaphase, and telophase.
   2. Cytokinesis: Division of the cytoplasm to form two separate cells. In animal cells, this occurs through the formation of a cleavage furrow driven by an actin-myosin contractile ring. In plant cells, a rigid cell plate forms along the metaphase plane from fused Golgi vesicles, developing into a new cell wall.

   Regulation of the Cell Cycle

   Progress through the cell cycle is strictly regulated by internal surveillance mechanisms called checkpoints. These ensure that critical processes are completed correctly before the next phase begins:

   • G1 Checkpoint (Restriction Point): Evaluates cell size, nutrient availability, DNA integrity, and external growth factor signals. Passing this checkpoint commits the cell to division.
   • G2 Checkpoint: Confirms successful, accurate DNA replication and checks for DNA damage before triggering entry into mitosis.
   • M Checkpoint (Spindle Assembly Checkpoint): Operates during metaphase to verify that all sister chromatids are properly attached to kinetochore microtubules before anaphase separation begins.

   Key regulatory molecules, including cyclins and cyclin-dependent kinases (CDKs), coordinate transitions past these critical checkpoints.