WEEK 13 CELL CYCLE

The Cell Cycle

The cell cycle consists of two main stages:

• Interphase

• Cell Division

Interphase

• This is the stage where the cell grows and carries out its normal functions, preparing for cell division. It is a critical period for cell growth, DNA replication, and overall preparation for mitosis or meiosis.

  • "G1" - first gap phase: The cell performs its regular activities and grows. This is a period of active gene transcription and protein synthesis.

    • The DNA within the nucleus is organized into chromosomes, appearing as single, thin structures. Each chromosome consists of a single DNA molecule.

  • "S" – synthesis phase: DNA replication occurs with high fidelity. This ensures that each daughter cell receives an identical copy of the genetic material.

    • Each chromosome duplicates, resulting in two identical chromatids attached at the centromere. It is still considered a single chromosome at this stage, but it now contains two identical DNA molecules.

  • "G2" - second gap phase: The cell prepares for division, including the duplication of organelles and synthesis of proteins required for cell division. Quality control mechanisms ensure that DNA replication is complete and any DNA damage is repaired before the cell enters mitosis.

Cell Division

Cell division includes:

1. Mitosis – division of the nucleus

• Prophase: Chromatin condenses into mitotic chromosomes, and centrioles divide and migrate to opposite poles of the cytoplasm. The nuclear envelope starts to break down.

• Prometaphase: Spindle fibers extend from the centrioles towards the centromeres; the nucleoli and nuclear envelope disappear. Chromosomes attach to the spindle fibers via their kinetochores.

• Metaphase: Chromosomes align along the center of the cell, forming the metaphase plate. This alignment is crucial for ensuring each daughter cell receives the correct number of chromosomes.

• Anaphase: Chromatids separate and are moved towards the centrioles by spindle fibers. Each chromatid is now considered an individual chromosome.

• Telophase: Nuclear envelopes and nucleoli reform around the chromosomes, and the chromosomes uncoil. The cell begins to elongate and prepare for cytokinesis.

2. Cytokinesis – division of the cytoplasm

• Begins in anaphase, continues through telophase, and concludes with complete separation into two distinct cells. In animal cells, a cleavage furrow forms, while in plant cells, a cell plate forms to divide the cytoplasm.

Apoptosis

Apoptosis = Programmed cell death

• Crucial for regulating:-

  • Cell number in tissues.

  • Removal of dangerous cells, such as those infected with viruses or with damaged DNA.

  • Elimination of damaged cells: Prevents the proliferation of cells with mutations that could lead to cancer.

• Gene-coded proteins enter cells and initiate the process of apoptosis, activating a cascade of events that lead to cell shrinkage, DNA fragmentation, and finally, cell death. Caspases, a family of proteases, play a central role in executing apoptosis.