Mitosis and The Cell Cycle
This topic covers mitosis, the process by which most body cells divide to create genetically identical copies for growth, repair, and replacement. Understanding the stages of the cell cycle and the consequences of its failure, such as cancer, is key.
Part of the ESAT Biology syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- The cell cycle consists of two main phases: Interphase, a long period of growth and DNA replication, followed by Mitosis, the actual division.
- Mitosis is a single cell division that produces two daughter cells which are genetically identical to each other and to the original parent cell.
- The primary roles of mitosis are organism growth, repairing damaged tissues, replacing worn-out cells, and enabling asexual reproduction.
- Cancer is caused by mutations that disrupt the normal regulation of the cell cycle, leading to rapid and uncontrolled cell division and tumour formation.
Formulae
Number of cells = 2n Use this to calculate the total number of cells produced from a single starting cell after 'n' complete rounds of mitosis, assuming no cells die.
Definitions
- Mitosis
- A type of cell division where a single parent cell divides to produce two genetically identical daughter cells. It is essential for growth and repair in multicellular organisms.
- Cell Cycle
- The complete sequence of events a cell undergoes from its formation to its own division, encompassing interphase and mitosis.
- Interphase
- The longest stage of the cell cycle, during which the cell grows, carries out its normal functions, and critically, replicates its DNA in preparation for mitosis.
Worked example
A sample of tissue starts with a single cell. This cell and its descendants divide by mitosis once every 6 hours. However, after 12 hours, a mutation occurs in one of the cells, causing it and all its future descendants to divide every 3 hours. How many cells are in the tissue sample 24 hours after the start?
- 1
First 12 hours:
The single cell divides every 6 hours.
Number of divisions = 12 / 6 = 2.
Total cells = 22 = 4 cells - 2
At 12 hours:
One of these 4 cells becomes cancerous.
So we now have 3 normal cells and 1 cancerous cell.
- 3
Next 12 hours (from hour 12 to 24):
The 3 normal cells continue to divide every 6 hours.
Number of divisions = 12 / 6 = 2.
Each normal cell produces 22 = 4 cells.
Total normal cells = 3 × 4 = 12.
- 4
Next 12 hours (from hour 12 to 24):
The 1 cancerous cell divides every 3 hours.
Number of divisions = 12 / 3 = 4.
This single cancerous cell produces 24 = 16 cells.
- 5
Final count:
Total cells at 24 hours = (cells from normal group) + (cells from cancerous group) = 12 + 16 = 28.
Answer: 28
Common mistakes
- ×Miscalculating the number of cell division cycles within a given timeframe. Always carefully divide the total time by the duration of a single cycle.
- ×Confusing mitosis with meiosis. Remember mitosis produces two identical body cells, while meiosis produces four genetically varied gametes.
- ×Forgetting the starting conditions. A common mistake is to be off by a factor of 2 by miscounting the number of divisions or starting cells.
- ×Making arithmetic errors with powers of two. Under time pressure, it's easy to mistake 25 for 16 instead of 32, which significantly changes the result.
No-calculator tips
- ✓Memorise the powers of 2 up to 210 (1024). This is essential for quickly solving cell division problems.
- ✓To calculate a larger power, use exponent rules. For example, 27 = 25 × 22 = 32 × 4 = 128.
- ✓In problems comparing two cell populations, focus on the difference in the number of divisions. Each additional division doubles the final population size, leading to rapid divergence.