Mitosis

The cell life cycle details the life span of an average cell

Interphase

• G₀ phase - normal cell functions
• G₁ phase - prepares for cell division, duplicates organelles, centriole replication
• S phase - DNA replication and chromosome duplication
• G₂ phase - protein synthesis, centriole replication completed

Mitosis

• M phase - cellular division & cytokinesis

Activities during M phase

• Chromosomes separate into two nuclei
• Divide organelles and cytosol into two cells by cytokinesis
• Offspring cells are genetically identical to each other and to parental cell

Prophase

1. Chromatin shortens & thickens, condenses into chromosomes, two chromatids (copies) are attached by centromere (surrounded by protein kinetochore)
2. Nucleolus disappears
3. Centriole pairs move to opposite poles & spindle forms, astral rays
4. Nuclear membrane disappears
5. Chromatids attach to spindle fibers by kinetochores

Metaphase

1. Chromosomes align at metaphase plate of mitotic spindle

Anaphase

1. Kinetochores split & chromatids separate
2. Separated chromosomes move to opposite poles along spindle fibers

Telophase

1. Spindle disappears
2. Nuclear membrane reforms
3. Chromosomes uncoil
4. Nucleolus reforms

Cytokinesis

1. Cytoplasmic division - cleavage furrow forms in anaphase or telophase & constricts original cell into two smaller cells
2. Each cell has a copy of hereditary information in its chromosomes

How often does cell division occur?

1. Varies, generally inversely proportional to cell life span
2. Some cells remain in G₀ and seldom divide, e.g. neurons & skeletal muscle
3. Rapidly dividing cells go from M to G₁, bypassing G₀, e.g. stem cells
4. Hormones, growth factors activate genes that promote cell division
5. Repressor genes inhibit cell division, abnormal repressor genes in many cancers
6. Mutated oncogenes cause excessive production of growth factors of receptors, stimulates mitosis