Cell Function and Cycle Notes

The cell cycle consists of various phases that are essential for growth, replication, and function of cells.

Tumors
- Cells may develop into a tumor, which is an abnormal mass of tissue that can take nutrients away from healthy cells.

Interphase
- This is the phase where cells are growing, replicating DNA, and carrying out normal cell functions. Interphase includes:
- G1 Phase (Gap 1): Cell growth and preparation for DNA replication.
- S Phase (Synthesis): Cell replicates its DNA, thus the 'S' represents synthesis.
- G2 Phase (Gap 2): Continued cell growth and preparation for mitosis.
Mitosis: The phase where cells are actively dividing and encompasses the following steps:
- Prophase: Chromosomes condense and prepare for separation.
- Metaphase: Chromosomes line up along the middle of the cell; crucial for ensuring proper separation.
  - Checkpoint: The cell cycle checks to ensure chromosomes are properly aligned during this phase.
- Anaphase: Chromatids are pulled to opposite poles of the cell.
- Telophase: New nuclear membranes form around each set of chromosomes, beginning the formation of distinct daughter cells.
Cytokinesis: The division of the cytoplasm, which occurs after telophase, splitting the cell into two daughter cells. Note: Cytokinesis is not considered part of mitosis.

G1 Checkpoint: Checks if the cell is growing properly and if the DNA is damaged. Conditions must be met to proceed to the S phase.
G2 Checkpoint: Ensures that DNA was replicated correctly and that the cell is ready for mitosis.
If a cell fails a checkpoint and cannot be repaired, it will undergo apoptosis, which is programmed cell death. This is a crucial process for eliminating damaged cells.
- Example: During human development, apoptosis shapes fingers and toes by removing cells in the spaces between them.

Cyclins: Proteins that rise and fall throughout the cell cycle, determining whether the cell can proceed to the next phase.
Positive Regulators: Such as CDK (Cyclin-dependent kinase) help the cell cycle to continue progressing.
Negative Regulators: Halt progression of the cell cycle and are involved in apoptosis, such as p53. This protein plays a crucial role in monitoring DNA repair and inducing apoptosis if damage is irreparable.

Cells in the G0 phase are in a resting state and typically do not replicate.
- Examples: Neurons in the brain and spinal cord remain in the G0 phase and do not undergo regular cell division.

G1 Phase: Cell grows and prepares for DNA replication.
S Phase: DNA replication occurs.
G2 Phase: Cell continues to grow and prepares for mitosis.
M Phase (Mitosis): Cell division occurs, including cytokinesis which is the final act of splitting the cell.

Cancer: Defined by uncontrolled cell growth and can result from failure in checkpoint signaling mechanisms.
- In cancer, the regular signals that control cell division and apoptosis are disrupted, leading to tumor formation.
Chemotherapy: Utilizes chemicals to target and kill cancer cells, potentially marking them for removal by the immune system.
Radiation Therapy: Uses high-energy beams to destroy or damage cancer cells.
Immunotherapy: Engages and enhances immune response to identify and kill cancer cells.

The immune system comprises:
- Innate Immunity: Physical barriers (like skin) and white blood cells that detect and destroy foreign invaders.
- Adaptive Immunity: Specific immune response that is activated upon exposure to pathogens.

Important themes include the significance of tumor growth in cell function, the phases of the cell cycle with an emphasis on checkpoints, and treatment modalities for cancer.
Understanding apoptosis and the role of cyclins and regulators is critical to grasp how cells function and the implications of cellular damage.