U01.01.019 Cell cycle phases

The cell cycle is a tightly regulated process where a cell grows, replicates its DNA, and divides into two daughter cells. Here’s a breakdown of the key players and their roles, presented in a table format for better organization:

Component	Description
Checkpoints	Control mechanisms positioned throughout the cell cycle; ensure critical events (DNA replication, chromosome segregation) occur correctly before proceeding.
Cyclin-Dependent Kinases (CDKs)	The combination of a cyclin and its corresponding CDK; phosphorylates other proteins, coordinating cell cycle events.
Cyclins	Regulatory proteins specific to each cell cycle phase; bind to CDKs, activating them for specific tasks.
Cyclin-CDK Complexes	Cyclin-dependent kinases (CDKs)
Tumor Suppressors (e.g., p53, Rb)	Guardian proteins that ensure the cell cycle doesn’t progress if there are problems (DNA damage); halt the cycle at specific checkpoints.

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Cell Cycle Phases:

The cell cycle can be divided into two main parts: the interphase and the mitotic phase (M phase). Here’s a table outlining the interphase stages:

Interphase Phase	Description
G1 Phase (Growth)	The cell grows, synthesizes proteins and organelles, and prepares for DNA replication (variable length).
S Phase (Synthesis)	Cell grows, synthesizes proteins and organelles, and prepares for DNA replication (variable length).
G2 Phase (Gap 2)	DNA replication occurs, and each chromosome is duplicated.

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The mitotic phase (M phase) consists of several subphases, followed by cytokinesis. Here’s a breakdown of these stages:

Mitotic Phase (M Phase)	Description
Prophase	Spindle fibers (responsible for chromosome separation) attach to chromosomes. The nuclear envelope disappears completely.
Prometaphase	Spindle fibers (responsible for chromosome separation) attach to chromosomes. Nuclear envelope disappears completely.
Metaphase	Chromosomes align at the center of the cell (metaphase plate) for separation.
Anaphase	Sister chromatids are pulled apart by the spindle fibers towards opposite poles of the cell.
Telophase	Nuclear envelopes reform around the separated chromosomes at each pole. Chromosomes decondense.
Cytokinesis	Cytoplasm and organelles physically divide, creating two daughter cells.

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G0 Phase (Resting Phase):

Some cells, like mature nerve cells, may exit the cycle and enter a resting phase (G0). They remain here unless stimulated to re-enter for specific purposes.

Cell Types and Cell Cycle Progression:

Different cell types have varying abilities to proliferate. Here’s a table summarizing these categories:

Cell Type	Description
Permanent Cells	Never re-enter the cell cycle, remain in G0 (e.g., neurons, skeletal muscle). Regenerated from stem cells.
Stable (Quiescent) Cells	Re-enter the cycle from G0 when stimulated (e.g., liver cells, some immune cells).
Labile Cells	Divide rapidly with a short G1 phase (e.g., skin cells, bone marrow cells). Highly affected by chemotherapy.

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Regulation by Growth Factors:

Growth factors are signaling molecules that can stimulate the transition from G1 to S phase, promoting cell division. They bind to specific receptors on the cell surface, triggering a cascade of events that activate cyclin-CDK complexes.

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U01.01.019 Cell cycle phases

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