The Cell Cycle

$G_1$ Phase (First Gap): The cell undergoes intense metabolic activity and growth. It synthesizes proteins and produces new organelles to prepare for DNA replication.

S Phase (Synthesis): This is the most critical sub-phase where DNA replication occurs. Each chromosome is duplicated to form two identical sister chromatids joined at a centromere.

$G_2$ Phase (Second Gap): The cell continues to grow and completes final preparations for division, such as synthesizing the proteins required for the mitotic spindle.

During Interphase, the DNA exists in a loosely packed form called chromatin, which allows the cellular machinery to access the genetic code for transcription and replication.

The cell cycle is controlled by a molecular signaling system that acts like a 'control clock' with specific checkpoints to ensure the process is error-free.

$G_1$ Checkpoint: Often called the 'restriction point'; if the cell receives a go-ahead signal here, it usually completes the entire cycle. If not, it exits to $G_0$.

$G_2$ Checkpoint: Ensures that DNA replication in the S phase was successful and that the DNA is undamaged before entering mitosis.

M Checkpoint (Spindle Checkpoint): Occurs during metaphase to verify that all chromosomes are properly attached to spindle microtubules before anaphase begins.

Regulation is mediated by proteins called Cyclins and Cyclin-Dependent Kinases (CDKs), which fluctuate in concentration and activity to trigger phase transitions.

Centrosome vs. Centromere: The centrosome is the organelle that organizes microtubules, while the centromere is the specific DNA region where sister chromatids are joined.

Animal vs. Plant Cytokinesis: Animal cells divide via a cleavage furrow (contractile ring of microfilaments), whereas plant cells form a cell plate due to their rigid cell walls.

Chromosome Counting: Remember that the number of chromosomes is determined by the number of centromeres. During S phase, the DNA amount doubles, but the chromosome count remains the same until anaphase.

Sequence Recognition: Always look for the order: $G_1 \rightarrow S \rightarrow G_2 \rightarrow M$. Questions often swap $G_1$ and $G_2$ to confuse students.

Checkpoint Logic: If a question mentions a drug that inhibits spindle formation, the cell will likely be arrested at the M checkpoint.

DNA Content Graphs: Be prepared to interpret graphs showing DNA mass over time. The mass stays at $x$ in $G_1$, rises to $2x$ during $S$, stays at $2x$ through $G_2$ and early $M$, and drops back to $x$ after cytokinesis.