Cell Cycle and Cell Division

• The cell cycle is the series of events a cell undergoes to divide into two cells.

S Phase

• DNA is replicated during this phase.
• Chromosome number goes from 2n (46) to 4n (92).

Pre-Synthetic Phase/Growth Phase 1/G1

• 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/Rest Phase

• Some cells enter this phase instead of returning to G1 after division.
• Cells in G0 can re-enter the cell cycle later.

Cell Types Based on Division Activity

• Labile cells constantly multiply, e.g., skin and GIT cells.
• Stable cells enter G0, e.g., hepatocytes, kidney tubules, and pancreas cells.
• Permanent cells are permanently in G0, e.g., neurons, skeletal, and myocardial muscle cells.

Mitosis

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

Interphase

• Includes G1, S phase, and G2.

Stages of Mitosis

• Prophase: Chromatin condenses, chromosomes become visible, nuclear membrane dissolves, and centrioles move to opposite poles and nuclear membrane dissolves by phosphorylation of limenine proteins by enzymes.
• Metaphase: Microtubules form the mitotic spindle, chromosomes align on the metaphase plate, and microtubules attach to kinetochores.
• Anaphase: Chromosomes separate into single structures and move to the poles.
• Telophase: Nuclear membranes reassemble around two new groups of chromosomes and chromatin loosens.

Cytokinesis

• Organelles replicate.
• The cell membrane pinches, dividing the cytoplasm nearly equally between daughter cells.

Cell Cycle Checkpoints

• G1-S checkpoint: Checks for DNA mutations before S phase.
• G2-M checkpoint: Checks DNA again before mitosis.
• Mutations found at checkpoints prevent cell cycle progression until corrected.
• M-checkpoint: Some believe there's a checkpoint in metaphase to ensure proper alignment.

Control of Cell Cycle Progression

• Activated "green" genes facilitate the G1 to S phase transition.
• "Red" genes stimulate the replication of green genes.
• "Blue" genes inhibit green genes.

Red Gene Components and Function

• Growth factor (GF)
• Growth factor receptor (GF receptor)
• Responder
• Transcription factor (TF) gene
• Cyclin gene
• Cyclin-dependent kinase (CDK) gene
• Growth factor attaches to GF receptor, which signals the signal transducer to TF gene to create TF.
• TF activates the cyclin gene, producing cyclin.
• Cyclin binds to and activates inactive CDKs, forming a complex.
• The complex phosphorylates a protein holding a TF.
• Phosphorylation releases the TF, which transcribes green genes.
• Expression of green genes allows entry into S phase.