Unit 4 Cell Communication and Cell Signal - Cell Cycle

Cell Cycle Overview

Unit 4: Cell Communication and Cell Signal

Understanding the cell cycle is crucial in both biology and medical science. The cell cycle impacts growth, development, and the propagation of diseases such as cancer. Its study provides insights into how cells behave in various conditions and how therapeutic interventions can be designed to manipulate cell functions.The cell cycle consists of several phases, including interphase, mitosis, and cytokinesis, each playing a significant role in cellular proliferation and regulation.

Overview of the Life of a Cell

Stages of Cell Life:

1. Growth: Cells increase in size and mass.

2. More Growth with DNA Synthesis: Cells prepare genetically for division — DNA is replicated.

3. More Growth: Final preparations before division are made.

4. Divide via Mitosis (or Apoptosis): Depending on internal and external signals, the cell undergoes mitosis for reproduction or apoptosis to eliminate damaged cells.

Analogy with Human Life:

• Grow: A cell grows much like a human during childhood.

• Produce Enough of Yourself to Share: Analogous to reproduction in humans — cells prepare to divide.

• Continued Growth: Represents development and maturation throughout life stages.

• Pass it Down and Finish Up: The life cycle of a cell concludes with cell death (apoptosis).

Apoptosis and Cell Death Mechanisms

Mechanisms of Cell Death:

• MC: Mechanism of cell death can vary, including apoptosis and necrosis.

• Comparison: Apoptosis is programmed cell death, while necrosis is uncontrolled and often results in inflammation.

• Inducers: Caspase-2 activation is a crucial step, impacting cellular structures and biological processes.

Apoptosis Pathways

1. Intrinsic Pathway:

   • Internal cellular signals, such as DNA damage and hypoxia, lead to apoptosis activation primarily through mitochondria.

2. Extrinsic Pathway:

   • External signals from factors such as cytokines trigger apoptosis via "Death Receptors" on the cell surface, often sent by immune cells like T lymphocytes.

Regulation by microRNAs (miRNAs)

• Various miRNAs (e.g., miR-15/16, miR-34, etc.) play essential roles in regulating different phases of the cell cycle and modulating apoptosis processes, providing layers of regulatory control that ensure cellular health.

The Cell Cycle Phases

• INTERPHASE: The cell is metabolically active and prepares for division.

  • G1 Phase: Cell growth, metabolic processes, and preparation for DNA synthesis.

  • S Phase: Synthesis phase where DNA replication occurs, resulting in two sister chromatids for each chromosome.

  • G2 Phase: Cell continues to grow and prepares for mitosis, checking for DNA errors and making necessary repairs.

• M-PHASE (Mitosis):

  • The process by which the cell divides, encompassing four phases (PMAT):

    • Prophase: Chromosomes condense, and the nuclear envelope begins to disintegrate.

    • Metaphase: Chromosomes align at the cell's equatorial plane.

    • Anaphase: Sister chromatids separate and move to opposite poles of the cell.

    • Telophase: Nuclear membranes re-form around each set of chromosomes, which then de-condense back into chromatin.

  • Cytokinesis: Marks the final split, forming two daughter cells; in animal cells, this occurs via a cleavage furrow, while in plant cells, a cell plate forms.

Cell Cycle Checkpoints

• Critical points in the cell cycle (G1, G2, and M checkpoints) ensure that all cellular components are intact and functioning before the cell progresses to the next phase, thereby preventing errors that could lead to diseases like cancer.

Variation in Cell Cycle Rates

Discussion Question:

• Do all human cells undergo the cell cycle at the same rate? Why?

  • Fact: No, different cell types exhibit varying cycles; some cells may enter a resting phase (G0) and only divide in response to specific signals (e.g., liver cells), while others (e.g., skin cells) constantly divide to replace lost cells.

Purpose of the Cell Cycle

• Transforms one cell into two genetically identical daughter cells, essential for growth, tissue repair, and organismal development.

Factors Affecting Cell Growth

• Release of Growth Factors: Adjacent cells release signals that promote cell growth and division.

• Autocrine Signaling Mechanisms: Cells respond to signals they produce themselves, affecting their own growth cycle.

• Balance Between Growth and Inhibitory Factors: Regulates cell proliferation to avoid uncontrolled growth associated with cancer development.

p53 - Tumor Suppressor Protein

• Acts as the main regulatory protein of the cell cycle, detecting DNA damage and initiating repair processes or inducing apoptosis if damage is irreparable, thus preventing tumor formation.

Impact of External Factors on Cell Cycle

• Various agents, including heat, UV radiation, and genotoxic drugs, can disrupt the normal progression of the cell cycle and contribute to DNA damage, potentially leading to cancer development.

Cyclins and CDKs

• Cyclins: Regulatory proteins that control cell cycle progression by activating cyclin-dependent kinases (CDKs).

• CDKs: Enzymes essential for cell cycle advancement, whose activity is regulated by the concentration of cyclins, ensuring accurate timing of the cell cycle events.

Regulation Mechanisms in Cells

• Cells synthesize cyclins in response to environmental cues, signaling progression through the cell cycle stages and ensuring cellular responses are aligned with external conditions.

Cell Cycle Summary

• Most of a cell’s life is spent in INTERPHASE:

  • G1 phase: Growth and production of essential enzymes.

  • If unprepared for division, cells enter G0 phase, a resting state.

  • G2 phase: Final preparations and checks before mitosis.

• M phase: The actual division process following the stages of mitosis (PMAT).

Stages of Mitosis

1. Prophase: Chromosomes condense and become observable under a microscope.

2. Metaphase: Chromosomes align at the cell's midline, facilitating equal distribution.

3. Anaphase: Sister chromatids are pulled apart to opposite ends of the cell by spindle fibers.

4. Telophase: Nuclear envelopes reform around each set of chromosomes, now at opposite poles.

5. Cytokinesis: Final division process occurs, resulting in two distinct daughter cells, completing the cycle.

Additional Cell Division Considerations

• Differences between animal and plant cell division, notably in cytokinesis (cleavage furrow formation in animal cells vs. cell plate formation in plant cells).

• Key Vocabulary: Chromosome, Telomeres, Centrosomes, Spindle fibers, Centromeres.

Conclusion

The cell cycle is a highly regulated and complex process of growth, DNA replication, and cell division, influenced by an array of internal and external factors that ensure proper cellular function and health throughout an organism's life.