Mitosis
The Process of Nuclear Division:
Before mitosis begins, the cell undergoes interphase, where the DNA is exactly replicated (copied). During mitosis itself:
- The replicated chromosomes condense and become visible.
- These copies align at the center of the nucleus.
- The copies are pulled apart to opposite poles of the cell.
- Two new nuclei form around the separated sets of chromosomes.
This process ensures that each new daughter cell receives an exact copy of the genetic material.
The result of mitosis is two genetically identical cells. This means they have the same chromosome number and the same DNA sequence as the original parent cell.
Note: Mitosis occurs in eukaryotic cells (cells with a nucleus). Prokaryotic cells (like bacteria) do not undergo mitosis; they reproduce via binary fission, which is a different process.
Stem cells are unspecialised cells that have the ability to divide by mitosis to produce daughter cells that can become specialised for specific functions.
- Unspecialised: They do not yet have a specific structure or function (e.g., they are not yet muscle cells or nerve cells).
- Differentiation: The process by which a stem cell becomes a specialised cell is called differentiation.
| Role | How Mitosis Achieves It |
|---|---|
| Growth | Mitosis increases the number of cells in an organism. As cells divide, the organism gets larger. The new cells are genetically identical to ensure consistent function throughout the body. |
| Repair of damaged tissues | When tissue is injured (e.g., a cut skin), mitosis produces new cells to replace the damaged or dead ones, healing the wound. |
| Replacement of cells | Some cells have short lifespans and die regularly. Mitosis replaces them. Examples include:
|
Mistake: Students often confuse mitosis with meiosis (cell division for sexual reproduction) or binary fission (bacterial reproduction).
Correction:
- Mitosis vs. Meiosis: Mitosis produces two genetically identical diploid cells. Meiosis produces four genetically different haploid gametes (sex cells). If the question asks about identical offspring or growth, it is mitosis.
- Mitosis vs. Binary Fission: Mitosis occurs in eukaryotes (plants, animals, fungi). Binary fission occurs in prokaryotes (bacteria). Never use 'mitosis' to describe bacterial reproduction.
Examiner Acceptance: Examiners look for specific keywords linking cell division to biological function. Use phrases like:
- 'Increases the number of cells' (for growth).
- 'Replaces dead or damaged cells' (for repair/replacement).
- 'Produces genetically identical offspring' (for asexual reproduction).
Reasoning: This directly addresses the definition of mitosis as a mechanism for maintaining genetic consistency while increasing biomass. Simply saying 'it makes new cells' is often insufficient; you must specify why those new cells are needed (e.g., 'to replace skin cells that are shed').
Examiner Acceptance: You must include two key components:
- They are unspecialised (or undifferentiated).
- They can divide by mitosis to produce specialised daughter cells.
Example phrase: 'Stem cells are unspecialised cells that divide by mitosis to produce daughter cells that can become specialised for specific functions.'
Reasoning: The markscheme specifically awards marks for 'unspecialised' and 'divide by mitosis'. Omitting 'unspecialised' is a common error because it misses the defining characteristic of stem cells compared to normal somatic cells.
- Chromosomes replicate (or copy) before mitosis. [1]
- The copies separate (or move apart) to opposite poles of the cell / nucleus. [1]
- Growth (increase in size/number of cells). [1]
- Repair of damaged tissues / Replacement of dead cells / Asexual reproduction. [1]
- An unspecialised (or undifferentiated) cell. [1]
- That can divide by mitosis to produce daughter cells that become specialised (for specific functions). [1]