The Cell Cycle (AQA A-Level Biology): Revision Notes
The Cell Cycle
What is the cell cycle?
Not all cells in multicellular organisms continue to divide throughout their existence. Many cells follow a cell cycle - a regular pattern of division separated by periods of growth. This cyclical process consists of three main stages that cells progress through in sequence.
The duration of a complete cell cycle varies significantly between different organisms. In mammals, cells typically require approximately 24 hours to complete one full cycle, with interphase accounting for roughly 90% of this time.
The cell cycle duration can vary dramatically across species - from as little as 8 minutes in some bacteria to several days in certain plant cells, demonstrating the remarkable adaptability of cellular division processes.
The three stages of the cell cycle
- Interphase
- Nuclear division
- Cytokinesis
Interphase
Interphase represents the longest phase of the cell cycle, occupying most of the cycle's duration. This stage is often referred to as the "resting phase" since no visible division occurs during this period. However, this description is misleading because interphase involves intense biochemical activity.
Despite being called the "resting phase," interphase is actually the most metabolically active period of the cell cycle. Cells are constantly growing, synthesising proteins, and preparing for division during this time.
During interphase, cells undergo significant growth and prepare for division. The cell synthesises new proteins, organelles, and other cellular components. Most importantly, DNA replication occurs during this phase, ensuring that genetic material is duplicated before division begins.
Nuclear division
Nuclear division involves the division of the nucleus to form either two or four new nuclei. This process occurs through two different mechanisms:
- Mitosis - produces two genetically identical diploid nuclei
- Meiosis - produces four genetically different haploid nuclei
The type of nuclear division depends on the cell's purpose and whether it will form body cells or reproductive cells.
Key Distinction: Mitosis is used for growth and repair of body tissues, while meiosis is specifically used to produce gametes (sex cells) for reproduction. This fundamental difference explains why mitosis maintains chromosome number while meiosis reduces it by half.
Division of the cytoplasm (cytokinesis)
Cytokinesis follows nuclear division and completes the cell division process. During this stage, the cytoplasm divides to create separate cells. After mitosis, cytokinesis produces two new cells, while after meiosis, it results in four new cells.
Understanding cell cycle patterns
The cell cycle can be visualised as changes in cell mass and DNA content over time. During interphase, both the cell's mass and DNA content increase. The DNA content doubles during DNA replication, while cell mass increases steadily through growth. During division phases, cell mass decreases as the cell divides, and DNA content returns to its original level after nuclear division.
Understanding these patterns of mass and DNA content changes is crucial for identifying which stage of the cell cycle a cell is currently in when observed under laboratory conditions.
Cancer and the control of mitosis
Cancer development
Cancer encompasses approximately 200 different diseases characterised by uncontrolled cell growth. These conditions result from damage to genes responsible for regulating mitosis and the cell cycle. When these regulatory mechanisms fail, cells begin dividing without proper controls.
The rate of mitosis in healthy cells is carefully controlled by the cellular environment and growth factors. Additionally, two categories of genes help regulate cell division. When mutations occur in these regulatory genes, cells may lose their ability to control division properly.
Cancer is fundamentally a disease of cell cycle regulation. Understanding normal cell cycle controls is essential for comprehending how cancer develops and why certain treatments are effective.
Tumour formation
When cells divide uncontrollably, they form abnormal masses called tumours. These growths can develop in any body organ, though they occur most frequently in the lungs, prostate gland (males), breast and ovaries (females), large intestine, stomach, oesophagus, and pancreas.
Tumours are classified as either:
- Benign tumours - grow slowly, remain compact, and pose minimal life threat
- Malignant tumours - grow rapidly, are less compact, and present greater danger to health
Most abnormal cells die naturally. However, surviving mutated cells can continue dividing to form clones of themselves, leading to tumour development.
Tumour Classification: The distinction between benign and malignant tumours is critical for treatment decisions. Benign tumours rarely spread to other parts of the body, while malignant tumours can metastasise (spread) through blood and lymphatic systems.
Treatment approaches
Cancer treatment typically focuses on disrupting the cell cycle to prevent further division. Chemotherapy drugs achieve this through several mechanisms:
- Preventing DNA replication
- Interfering with spindle formation during the metaphase stage of mitosis
These treatments effectively disrupt the cell cycle, stopping cell division and halting cancer growth.
Treatment Strategy Example:
Target: Cancer cells dividing every 12 hours vs. normal cells dividing every 24-48 hours
Mechanism: Chemotherapy drug blocks DNA synthesis during S phase
- Cancer cells attempt division twice as often
- Higher probability of encountering the drug during vulnerable S phase
- Normal cells have longer intervals between divisions, reducing exposure risk
Result: Preferential targeting of rapidly dividing cancer cells
Treatment challenges
Chemotherapy drugs affect both cancerous and healthy cells, though they impact rapidly dividing cells more severely. Since cancer cells divide faster than most normal cells, they suffer greater damage from treatment. However, normal body cells that divide frequently (such as hair-producing cells) also experience significant effects, explaining common side effects like hair loss in cancer patients.
Side Effect Explanation:
Chemotherapy's side effects occur because the treatment cannot distinguish between rapidly dividing cancer cells and rapidly dividing healthy cells. This is why patients experience hair loss, digestive issues, and immune system suppression - all from damage to fast-dividing healthy tissues.
Treatment effectiveness can be monitored by tracking changes in healthy and cancerous cell numbers over time. Multiple treatment doses are typically required, with intervals allowing some recovery of healthy cells between treatments.
Key Points to Remember:
- The cell cycle consists of three stages: interphase (longest phase involving growth and DNA replication), nuclear division (mitosis or meiosis), and cytokinesis (cytoplasm division)
- Interphase occupies approximately 90% of the cell cycle duration, despite being called the "resting phase" - it involves intense cellular activity
- Cancer results from damage to genes controlling the cell cycle, leading to uncontrolled cell division and tumour formation
- Chemotherapy targets rapidly dividing cells by disrupting DNA replication and spindle formation, affecting cancer cells more than normal cells
- Understanding the cell cycle is essential for comprehending both normal development and disease processes like cancer