4.5 Cell Cycle
- Syllabus
- 2025
- Topic
- 4.5
- Level
- —
Learning objectives
4.5A—Describe the events that occur in the cell cycleDescribe the events that occur in the cell cycle.• The cell cycle is a highly regulated series of events that controls the growth and reproduction of eukaryotic cells.- i. The cell cycle consists of sequential stages of interphase (G1, S, G2), mitosis, and cytokinesis.- ii. G1 phase: The cell is metabolically active, duplicating organelles and cytosolic components.- iii. S phase: DNA is in the form of chromatin and replicates to form two sister chromatids connected at a centromere.- iv. G2 phase: Protein synthesis occurs, ATP is produced in large quantities, and centrosomes replicate.- v. A cell can enter a stage (G0) in which it no longer divides, but it can reenter the cell cycle in response to appropriate cues.- vi. Nondividing cells may exit the cell cycle or be held at a particular stage in the cell cycle.4.5B—Explain how mitosis results in the transmission of chromosomes from one generation of cells to the nextExplain how mitosis results in the transmission of chromosomes from one generation of cells to the next.• Mitosis is a process that ensures the transfer of a complete genome from a parent cell to two genetically identical daughter cells in eukaryotes.- i. Mitosis plays a role in growth, tissue repair, and asexual reproduction.- ii. Mitosis occurs in sequential steps (prophase, metaphase, anaphase, telophase) and alternates with interphase in the cell cycle.- iii. Prophase: Sister chromatids condense, mitotic spindle begins to form, and centrosomes move to opposite poles of the cell.- iv. Metaphase: Spindle fibers align chromosomes along the equator of the cell.- v. Anaphase: Paired sister chromatids separate as spindle fibers pull chromatids toward poles.- vi. Telophase: Mitotic spindle breaks down, a new nuclear envelope develops, and then the cytoplasm divides.- vii. Cytokinesis: A cleavage furrow forms in animal cells or a cell plate forms in plant cells, resulting in two new daughter cells. 90 Cell Communication and Cell Cycle UNIT 4
The Cell Cycle Prepares, Copies, and Divides a Cell
The eukaryotic cell cycle is an ordered series of growth, DNA replication, and division events. Interphase includes G1, S, and G2; it prepares the cell for mitosis and cytokinesis.
| Stage | Main events | Why the stage matters |
|---|---|---|
| G1 | The cell is metabolically active and duplicates organelles and cytosolic components | Builds the material needed for growth and later division |
| S | DNA replicates; each chromosome forms two sister chromatids joined at a centromere | Produces a complete copied genome for separation |
| G2 | Protein synthesis continues, ATP is produced in large quantities, and centrosomes replicate | Completes preparation for chromosome movement |
| Mitosis | Replicated chromosomes are separated into two nuclei | Distributes one complete genome to each future daughter cell |
| Cytokinesis | The cytoplasm divides | Produces two separate daughter cells |
A cell may leave the active cycle and enter G0, where it no longer divides. Depending on the cell and its signals, it may later reenter the cycle; other nondividing cells remain outside the cycle or are held at a particular stage.
DNA replication occurs during S phase, not during mitosis. Mitosis separates the sister chromatids that were produced earlier.
How Mitosis Transmits a Complete Genome
Mitosis transfers a complete genome by separating the two sister chromatids of every replicated chromosome into opposite daughter nuclei. Because the DNA was copied before mitosis, each resulting nucleus receives the same chromosome set.
| Phase | Chromosome and spindle event | Contribution to genome transmission |
|---|---|---|
| Prophase | Sister chromatids condense, the spindle begins to form, and centrosomes move toward opposite poles | Prepares replicated chromosomes for controlled movement |
| Metaphase | Spindle fibers align chromosomes at the cell equator | Positions sister chromatids so they can move to opposite poles |
| Anaphase | Sister chromatids separate and spindle fibers pull them toward opposite poles | Gives each side one chromatid from every replicated chromosome |
| Telophase | The spindle breaks down and a nuclear envelope forms around each chromosome set | Establishes two nuclei with complete genomes |
Cytokinesis then divides the cytoplasm: animal cells form a cleavage furrow, whereas plant cells form a cell plate. The result is two genetically identical daughter cells, supporting growth, tissue repair, and asexual reproduction.
Mitosis is nuclear division; cytokinesis is cytoplasmic division. They are coordinated but are not the same event.