Cell Cycle: Phases and Cell Types

• The cell cycle involves a cell dividing into two cells.

S Phase

• DNA replication occurs.
• The amount of DNA doubles from 2n (46 chromosomes) to 4n (92 chromosomes).

Pre-synthetic Phase (G1)

• Also known as growth phase 1.
• The cell prepares for DNA replication.

G2 Phase

• Post-synthetic but pre-mitotic phase.
• The cell prepares to divide replicated DNA into daughter cells.

M Phase (Mitosis)

• Separates chromatids.

G0 Phase

• A resting phase that some cells enter after division instead of re-entering G1.
• Cells in G0 can potentially re-enter the cell cycle later.

Cell Types and Cell Cycle Activity

• Labile cells constantly multiply, such as skin cells and cells of the gastrointestinal tract (GIT).
• Stable cells (e.g., hepatocytes, kidney tubules, pancreatic cells) enter G0.
• Permanent cells (e.g., neurons, skeletal and myocardial muscle cells) are permanently in G0.

Mitosis

• Division of nuclear material.
• Separates 92 chromosomes into 46 for each daughter cell.

Interphase

• Consists of G1 phase, S phase, and G2 phase.

Stages of Mitosis

• Prophase: Chromatin condenses, chromosomes become visible, the nuclear membrane dissolves as limenine proteins are phosphorylated, and centrioles move to opposite poles.
• Metaphase: Microtubules form the mitotic spindle, chromosomes align on the metaphase plate, and microtubules attach to chromosome kinetochores.
• Anaphase: Chromosomes separate into single structures and are pulled to opposite poles.
• Telophase: A nuclear membrane reforms around each group of chromosomes at the poles, and chromatin loosens.

Cytokinesis

• Organelles are replicated.
• The cell membrane pinches at the equatorial plane, splitting the cytoplasm nearly equally between daughter cells.

Cell Cycle Checkpoints

• G1-S checkpoint: Ensures no DNA mutations before S phase.
• G2-M checkpoint: Checks DNA again before mitosis.
• Mutations found at checkpoints halt the cell cycle until corrected.
• M-checkpoint (in metaphase): Some believe it exists to ensure proper alignment before division.

Control of Cell Cycle Progression

• Green genes are activated to move the cell from G1 to S phase.
• Red genes stimulate replication of the green genes.
• Blue genes inhibit the green genes.

Red Gene Components

• Growth factor (GF): Produced and released by cells, then attaches to the GF receptor on other cells.
• Growth factor receptor (GF receptor): Located on the cell membrane.
• Responder: Produces a signal transducer.
• Transcription factor (TF) gene: Creates a TF.
• Cyclin gene: Produces cyclin.
• Cyclin-dependent kinase (CDK) gene.

Mechanism of Cell Cycle Progression

• The growth factor binds to the GF receptor, which releases a signal to the signal transducer.
• The signal transducer goes to the TF gene and creates a TF.
• The TF activates the cyclin gene to produce cyclin.
• Cyclin binds to and activates inactive cyclin-dependent kinases (CDKs), forming a complex.
• The cyclin-CDK complex phosphorylates a protein that holds a transcription factor (TF).
• Phosphorylation releases the TF, which then transcribes green genes.
• Expression of green genes allows the cell to enter the S phase.