Cell Division: Mitosis

Mitosis is nuclear division producing two genetically identical daughter nuclei, each containing the same number of chromosomes as the parent cell. This process is fundamental to growth, repair, and asexual reproduction in eukaryotic organisms.

Key Terminology:

Chromatin → loosely arranged DNA-protein complex in interphase Chromosome → condensed, visible DNA structure during division Chromatid → one of two identical copies of a replicated chromosome, joined at the centromere Centromere → region where sister chromatids attach; also the kinetochore attachment point for spindle fibres Spindle fibres → microtubule structures that move chromosomes Cytokinesis → division of cytoplasm following nuclear division

The Stages of Mitosis (PMAT mnemonic):

Prophase: Chromatin condenses into visible chromosomes (each comprising two sister chromatids). Centrioles move to opposite poles. Nuclear envelope breaks down. Spindle apparatus forms.

Metaphase: Chromosomes align at the cell equator (metaphase plate). Spindle fibres attach to centromeres from opposite poles.

Anaphase: Sister chromatids separate when centromeres divide. Spindle fibres shorten, pulling chromatids to opposite poles. Each chromatid is now an independent chromosome.

Telophase: Chromosomes decondense into chromatin. Nuclear envelopes reform around each set of chromosomes. Spindle fibres disintegrate.

Cambridge Definition: Mitosis is the division of a nucleus into two genetically identical daughter nuclei.

Outcome: Two diploid daughter cells (2n) from one diploid parent cell (2n), maintaining chromosome number. Formula: Parent cell (2n) → 2 daughter cells (2n)

Why Mitosis Matters in Living Systems:

Mitosis enables growth in multicellular organisms—a human develops from a single zygote into trillions of cells through repeated mitotic divisions. In your skin, approximately 30,000-40,000 dead cells shed every minute, replaced continuously by mitotic division in the basal layer. Wound healing demonstrates mitosis beautifully: when you cut yourself, cells surrounding the injury undergo rapid mitosis to regenerate damaged tissue.

Analogy: The Photocopier Model

Think of mitosis as a high-precision photocopier creating exact duplicates. The parent cell is the original document (with 46 pages in humans). Before copying (during S phase of interphase), the cell creates a duplicate set—like having two copies of each page stapled together (sister chromatids). During mitosis, these copies separate, ensuring each daughter cell receives one complete set of 46 pages.

Real-World Applications:

Cancer Biology: Uncontrolled mitosis characterizes cancer. Oncogenes and tumour suppressor genes normally regulate the cell cycle; mutations disrupting these checkpoints cause excessive division. Chemotherapy drugs like vincristine target spindle fibre formation, preventing cancer cell division.

Plant Propagation: Gardeners exploit mitosis in stem cuttings—all cells are genetically identical clones of the parent plant, preserving desirable characteristics.

Stem Cell Research: Embryonic stem cells undergo mitosis while maintaining pluripotency, offering therapeutic potential for regenerative medicine.

Asexual Reproduction: Hydra and planarians reproduce through budding/fragmentation, using mitosis to create offspring genetically identical to the parent—advantageous in stable environments but limiting genetic diversity.

Question 1: A cell with diploid number 12 undergoes mitosis. Describe the chromosome number at metaphase and in each daughter cell. [3 marks]

Solution: Examiner approach: Address each stage specifically.

• At metaphase: 12 chromosomes visible at the equator [1 mark]
• Each chromosome consists of two sister chromatids joined at the centromere [1 mark]
• Each daughter cell contains 12 chromosomes (diploid number maintained) [1 mark]

Examiner note: Students often confuse chromatid number with chromosome number. At metaphase, count chromosomes (12), not chromatids (24).

Question 2: Explain why mitosis is important in organisms. [4 marks]

Solution: Command word 'explain' requires reasons/mechanisms.

• Produces genetically identical cells, maintaining genetic consistency [1 mark]
• Essential for growth by increasing cell number in multicellular organisms [1 mark]
• Enables tissue repair/replacement of damaged or dead cells [1 mark]
• Allows asexual reproduction, producing offspring genetically identical to parent [1 mark]

Question 3: The diagram shows a cell in anaphase. The diploid number is 6. State how many chromatids are moving to each pole. [1 mark]

Solution:

• 6 chromatids (now independent chromosomes) move to each pole [1 mark]

Examiner note: Once centromeres divide in anaphase, each chromatid becomes a chromosome. Precise terminology matters.

Key Exam Tip: Always specify whether counting chromosomes or chromatids—Cambridge mark schemes penalize ambiguity.