The Cell Cycle

• Mitosis: The process of nuclear division. It is divided into stages (Prophase, Metaphase, Anaphase, Telophase) that ensure chromosomes are aligned and separated accurately into two new nuclei.

• Cytokinesis: The physical division of the cytoplasm that follows mitosis. In animal cells, a cleavage furrow pinches the cell in two. In plant cells, a cell plate forms to build a new cell wall between the daughter cells.

• The end result of the M phase is two daughter cells that are genetically identical to each other and to the original parent cell.

• The cycle is controlled by internal chemical signals called cyclins, which fluctuate in concentration to trigger transitions between phases.

• G1 Checkpoint: Checks for cell size, nutrients, and DNA damage. If the DNA is damaged, the cycle stops for repair or the cell enters a non-dividing state ($G_0$).

• S Checkpoint: Ensures that DNA replication is complete and accurate before the cell proceeds further.

• G2 Checkpoint: A final check for DNA damage after replication. This prevents the cell from entering mitosis with mutated or incomplete genetic material.

• Metaphase (M) Checkpoint: Occurs during mitosis to verify that all chromosomes are correctly attached to the spindle fibers before they are pulled apart.

Plant vs. Animal Cytokinesis

• Animal Cells: Involve the constriction of the cell membrane via a contractile ring of actin and myosin, creating a cleavage furrow.
• Plant Cells: Involve the transport of vesicles containing cell wall materials to the center of the cell, fusing to form a cell plate that grows outward.

• DNA Mass Tracking: Always check the mass of DNA. It doubles during the S phase and is halved during cytokinesis. If a graph shows DNA mass staying at $2x$ for a period, that represents the G2 phase and the start of mitosis.

• Phase Identification: If a question asks where protein synthesis is highest, look for G1. If it asks about spindle fiber preparation, look for G2.

• Checkpoint Logic: Remember that checkpoints are 'quality control' gates. A failure at the G2 checkpoint is more likely to lead to mutations in daughter cells than a failure at G1, because the DNA has already been copied.

• Sequence Check: Ensure you can distinguish between 'nuclear division' (mitosis) and 'cell division' (the whole cycle or cytokinesis).