Phases of the Cell Cycle to Know for Cell Biology

The cell cycle is a series of stages that cells go through to grow and divide. It includes interphase, mitosis, and cytokinesis, ensuring proper DNA replication and distribution to daughter cells, which is crucial for life and growth in all organisms.

1. Interphase (G1, S, G2)

   • G1 phase: Cell grows, synthesizes proteins, and produces organelles.
   • S phase: DNA replication occurs, resulting in two sister chromatids for each chromosome.
   • G2 phase: Cell continues to grow and prepares for mitosis, checking for DNA damage.

2. Mitosis (Prophase, Metaphase, Anaphase, Telophase)

   • Prophase: Chromatin condenses into visible chromosomes; the mitotic spindle begins to form.
   • Metaphase: Chromosomes align at the cell's equatorial plane, attached to spindle fibers.
   • Anaphase: Sister chromatids are pulled apart to opposite poles of the cell.
   • Telophase: Chromatids reach the poles, nuclear envelopes reform, and chromosomes de-condense.

3. Cytokinesis

   • The process that divides the cytoplasm of a parental cell into two daughter cells.
   • In animal cells, a contractile ring forms, pinching the cell membrane.
   • In plant cells, a cell plate forms, leading to the creation of a new cell wall.

4. G0 phase (quiescent state)

   • A resting phase where cells exit the cell cycle and do not actively divide.
   • Cells may remain metabolically active but are not preparing for division.
   • Can be temporary or permanent, depending on cell type and environmental conditions.

5. Cell cycle checkpoints

   • Critical control points that assess the cell's readiness to proceed to the next phase.
   • G1 checkpoint: Checks for DNA damage and cell size before DNA synthesis.
   • G2 checkpoint: Ensures DNA replication is complete and checks for damage before mitosis.
   • M checkpoint: Confirms that all chromosomes are properly attached to the spindle before anaphase.

6. Cyclins and cyclin-dependent kinases (CDKs)

   • Cyclins are proteins that regulate the cell cycle by activating CDKs.
   • CDKs are enzymes that, when activated by cyclins, phosphorylate target proteins to drive the cell cycle forward.
   • The levels of cyclins fluctuate throughout the cell cycle, ensuring proper timing of events.

7. DNA replication during S phase

   • The process of duplicating the cell's DNA to ensure each daughter cell receives an identical set of chromosomes.
   • Involves the unwinding of the double helix and synthesis of new complementary strands.
   • Key enzymes include DNA polymerase, which adds nucleotides to form new DNA strands.

8. Chromosome condensation

   • The process by which chromatin fibers coil and fold to form distinct, visible chromosomes.
   • Occurs during prophase, allowing for easier segregation of chromosomes during mitosis.
   • Ensures that DNA is compacted and protected during cell division.

9. Nuclear envelope breakdown

   • The disintegration of the nuclear envelope at the onset of prophase.
   • Allows spindle fibers to access chromosomes for proper alignment and separation.
   • Essential for the transition from interphase to mitosis.

10. Spindle formation

    • The assembly of microtubules into a spindle apparatus that orchestrates chromosome movement.
    • Spindle fibers attach to kinetochores on chromosomes during metaphase.
    • Ensures accurate segregation of chromosomes to daughter cells during anaphase.