The Cell Cycle, Interphase, Mitosis, and Cytokinesis

Overview of the Cell Cycle and Interphase

• Definition of the Cell Cycle: The cell cycle is an ordered set of events that results in cellular growth and division into two daughter cells.

• Phases of the Cell Cycle: During each cycle, cells progress through a sequence of four primary phases:

  • $G_1$ phase
  • $S$ phase
  • $G_2$ phase
  • $M$ phase (Mitosis / Cell Division)

• Interphase Overview:

  • Interphase consists of the $G_1$, $S$, and $G_2$ phases.
  • It represents the period during which DNA replication occurs and the cell actively prepares for division by duplicating its DNA and manufacturing necessary proteins.

• The $G_0$ Subphase:

  • $G_0$ is a specific subphase of the $G_1$ phase.
  • Cells in $G_1$ that are not ready to progress further in the cell cycle exit the main path and enter $G_0$.
  • Cells residing in $G_0$ remain there indefinitely until they are prepared to proceed.
  • Cellular Behavior in $G_0$:
    • Certain cell types may remain in the $G_0$ phase indefinitely and never leave it.
    • Other cell types, such as bone marrow cells, skip the $G_0$ phase entirely.

• Detailed Stages of Interphase:

  • $G_1$ Phase (Gap 1):
    • The cell prepares itself specifically for DNA duplication.
    • Synthesis of RNA and proteins actively takes place.
  • Transition from $G_1$ to $S$ Phase:
    • The initiation of DNA replication marks the precise transition from the $G_1$ phase to the $S$ phase.
  • $S$ Phase (Synthesis):
    • The total genetic material / DNA content is duplicated into two exact copies.
  • $G_2$ Phase (Gap 2):
    • RNA and proteins that are required for cell division are synthesized.
    • Proofreading mechanisms inspect the duplicated DNA.

DNA Quality Control, Apoptosis, and Mitosis

• Proofreading and Repair:

  • The DNA replicated during the $S$ phase is systematically checked for mistakes.
  • If mistakes are identified, the cell attempts to repair them.
  • If the damaged DNA cannot be repaired, the cell receives a signal to self-destruct via programmed cell death, termed apoptosis.

• The $M$ Phase (Mitosis):

  • In the $M$ phase, the cell undergoes mitosis to divide its duplicated contents into two identical daughter cells.
  • Mitosis is divided into four sequential subphases:
    1. Prophase
    2. Metaphase
    3. Anaphase
    4. Telophase

Sequential Stages of Mitotic Division and Cytokinesis

• Prophase:

  • Chromosomal Changes: Replicated chromosomes condense and split into identical halves called chromatids, which are held together at a region known as the centromere.
  • Cytoplasmic Changes:
    • Within the cytoplasm, centrioles replicate and move to opposite poles of the cell.
    • Spindle fibers form within the cytoplasm.
  • Nuclear Structure Breakdown:
    • The nuclear membrane and nucleolus disintegrate completely.
    • Spindle fibers migrate into the area where the nucleus was previously situated.

• Metaphase:

  • Attachment: Each chromosome, consisting of two chromatids, attaches to the spindle fibers via its centromere.
  • Alignment: The attached chromosomes move to the midpoint of the spindle, aligning along the equatorial plate of the cell.

• Anaphase:

  • Separation: Centromeres split, and sister chromatids are pulled apart toward opposite sides/poles of the cell.
  • Termination: Anaphase terminates precisely when the chromatids reach the opposite poles.
  • Reclassification: Upon reaching the poles, each sister chromatid is considered a new chromosome.

• Telophase:

  • Chromosomal Uncoiling: Chromatids uncoil and revert back to their interphase form, totaling $46$ chromosomes.
  • Rebuilding Structures: The nuclear envelope forms around each set of chromosomes, and nucleoli reappear.
  • Disassembly: Spindle fibers disintegrate and disappear.

• Cytokinesis:

  • Cytokinesis is defined as the cleavage and division of the cytoplasm.
  • The cytoplasm divides into equal parts, completing the creation of two identical daughter cells.

Progression to Subsequent Cell Cycles

• Completion: Following cytokinesis, two identical daughter cells are fully formed.
• Cell Cycle Renewal: These newly formed cells are immediately prepared to enter interphase and begin another round of the cell cycle.