Cell Cycle and Cancer Biology

Overview of the Cell Cycle and Cancer

Introduction to Cell Cycle

  • The cell cycle is structured into specific phases, particularly in eukaryotic cells, as opposed to prokaryotic.
  • The primary phases include:
    • G1 (Gap 1)
    • S (Synthesis)
    • G2 (Gap 2)
    • M (Mitosis)
    • G0 (Gap 0)

Phases of Cell Cycle

  • G1 Phase:

    • The cell is newly formed, often referred to as a "baby cell."
    • Primary function: Synthesis of proteins needed for DNA replication.
    • Diagram Elements:
    • Nucleus
    • Cell membrane
    • Paired chromosomes.

  • S Phase:

    • DNA is replicated during this phase.
    • Chromosomes appear in an X pattern (mitotic chromosomes).
    • Transition from one double-stranded DNA molecule to two identical copies.

  • G2 Phase:

    • The cell prepares for division.
    • Growth continues, making necessary proteins and organelles required for mitosis.

  • Mitosis (M Phase):

    • Division occurs to create two new cells.
    • The X-shaped mitotic chromosomes revert to their original G1 shape.

G0 Phase (Gap 0)

  • Cells that have ceased to divide enter G0, a state of dormancy.

  • Examples:

    • Liver cells stop growing at a certain size but remain viable in G0.
    • Brain and spinal cord neurons may remain permanently in G0.

  • Significance of G0:

    • Cells that do not follow typical cell cycle rules; they maintain function but do not participate in division.
    • Cancer cells often lose the ability to undergo the G0 phase, leading to uncontrollable division.

Cancer and Cell Function

  • Cancer cells arise from normal cells (e.g., bone marrow or liver cells) that lose the regulatory mechanisms of the cell cycle, particularly the transition into G0.
  • Characteristics of cancer cells include:
    • Uncontrollable division
    • Loss of function; become non-specialized (e.g., a "blob").
    • Potential to secrete growth hormones, diverting nutrients from healthy cells.

Epigenetics

  • Defined as heritable phenotypic changes that occur without alterations to DNA sequences.
  • Not mutations but regulation by histone proteins which either block or promote gene expression.
  • The focus on epigenetics implies changes can still result in significant cellular impact, including cancer development.

Cell Growth Mechanism

  • Multicellular organisms grow primarily through cell division rather than existing cells enlarging.
  • Growth defined as the process where new cells are produced from mitosis.
  • Controls on cell division can be disrupted leading to cancer due to factors like genetics, toxins, and UV exposure.

Apoptosis vs. Necrosis

  • Apoptosis:

    • Programmed cell death, a regulated process where damaged cells die to prevent propagation of errors.
    • Initiated through mitochondrial signaling.

  • Necrosis:

    • Uncontrolled cell death due to injury or other external causes, not regulated as apoptosis is.

Cell Cycle Checkpoints

  • Critical points in the cell cycle where the cell evaluates conditions before allowing progression into the next phase:
    1. G1 Checkpoint: Evaluates cell size, DNA integrity, and available resources.
    2. G2 Checkpoint: Checks if DNA is replicated correctly and the cell is prepared for mitosis.
    3. M Checkpoint: Ensures chromosomes are aligned correctly before separation.

Role of Proteins and Regulators

  • Various proteins function in regulating the cell cycle:
    • Positive Regulators: Allow progression through the cell cycle.
    • Negative Regulators: Halt progression if conditions are not met, such as protein p53, critical in apoptosis initiation.

The Impact of Mutations

  • Mutations can occur due to environmental factors (UV exposure) or genetic predisposition.
  • Mutations can lead to:
    • Irregularization of oncogenes, involved in cancer formation.
    • The mechanism by which sunlight, or UV light, causes adjacent thymine bases in DNA to bond (known as thymine dimers).

Cancer Treatment Strategies

  • Treatment includes:
    • Chemotherapy: Targets rapidly dividing cells, causing side effects such as hair loss due to targeting hair follicle cells.
    • Radiation: Destroys cancer cells more specifically and is combined with drugs like fluorouracil (5-FU) that inhibit DNA replication in S phase cells.

Genetic Aspects of Cancer

  • Presence of certain oncogenes can increase cancer risk (e.g., BRCA genes and breast cancer).
  • These genes can be hereditary and predispose individuals to higher cancer risks, leading to proactive medical procedures such as mastectomies.

Conclusion and Future Research

  • Understanding the cell cycle is crucial for grasping cancer's nature and progression.
  • Continued research is vital for the development of targeted therapies and potential rejuvenation techniques for cells in permanently dormant states (like neurons) to facilitate recovery from injuries.