Unit 4: Cellular Communication and Cell Cycle

Cellular Communication

• All cells must communicate to respond to environmental changes, which improves fitness.

Signal Transduction Process

• Signal transduction: The process whereby cells detect external signals and initiate internal changes.
  • Reception: Binding of a ligand to a receptor causing conformational changes in the receptor.
  • Transduction: A 'chain reaction' within the cell, termed a signal cascade, involves interactions of second messengers and relay proteins to transmit signals from the cell membrane to the nucleus.
  • Response: Physiological changes in the cell often involve gene regulation, which is controlled by transcription factors.

Feedback Mechanisms in Cells

• Feedback mechanisms: Used by cells to monitor changes in internal and external environments.
  • Negative Feedback: Maintains homeostasis by responding to stimuli to decrease the initial stimulus; prevalent in biological systems involving hormones and signal transduction.
  • Positive Feedback: Amplifies the initial stimulus, used less frequently in biological systems.

Cell Theory

• The cell theory outlines the basic principles of biology:
  • All organisms are made of one or more cells.
  • The cell is the fundamental unit of structure, function, and reproduction.
  • All cells arise from preexisting cells.

The Cell Cycle

• The cell cycle is how one cell divides to produce two identical daughter cells, consisting of several phases:
  • G1 Phase (Gap 1): Cell growth.
  • S Phase (Synthesis): DNA replication occurs.
  • G2 Phase (Gap 2): Further cell growth.
  • Interphase: Consists of G1, S, and G2 phases.
  • M Phase (Mitosis): Cell organizes its chromosomes and divides into two cells.
  • G0 Phase: A resting phase where cells exit the cycle when not needed for growth.

Mitosis

• Mitosis: The process of cell division involving a specific sequence:
  • Prophase: Chromosomes condense, nucleus disappears, spindle fibers form and attach.
  • Metaphase: Chromosomes align on the metaphase plate (cell equator).
  • Anaphase: Chromosomes separate and move towards opposite poles.
  • Telophase: Chromosomes decondense, nucleus reforms, spindle fibers disappear.
  • Cytokinesis: Final division of the cell into two daughter cells.

Regulation of the Cell Cycle

• The cell cycle is regulated by checkpoints to ensure proper progression:
  • Checkpoints: G1, G2, and M (metaphase).
  • Checkpoints assess factors such as room to grow, nutrient availability, growth factors, cell size, and DNA integrity.
  • The M phase checkpoint verifies proper alignment of chromosomes and even distribution during division.
  • Cyclins and Cyclin-Dependent Kinases (CDKs): Interact to form maturation-promoting factors that facilitate the transition into mitosis.

Cancer and Cell Cycle Regulation

• Failure to regulate the cell cycle can lead to cancer:
  • Cancer cells divide uncontrollably, disregarding checkpoints.
  • Related genes include tumor suppressor genes (prevent uncontrolled growth) and oncogenes (promote cell proliferation).