Cell Division

Cells divide for three main reasons:

1. Growth — to increase the number of cells in an organism
2. Repair — to replace damaged or worn-out cells (e.g., skin cells, blood cells)
3. Reproduction — in asexual reproduction, organisms produce genetically identical offspring through mitosis

The cell cycle is the series of stages a cell goes through as it grows and divides. It consists of three main phases:

Phase 1: Interphase (Cell Growth)

This is the longest phase, where the cell:

• Grows and increases in size
• Replicates its DNA — each chromosome makes an identical copy, forming two sister chromatids joined at a centromere
• Increases the number of organelles (e.g., mitochondria, ribosomes) to prepare for division
• Carries out normal cell functions

Key fact: The DNA replication occurs during the S phase (synthesis phase) of interphase. Before division, the cell has twice the normal amount of DNA.

Phase 2: Mitosis (Nuclear Division)

The nucleus divides into two identical nuclei (see detailed stages below).

Phase 3: Cytokinesis (Cell Division)

The cytoplasm divides, and the cell splits into two identical daughter cells, each with a full set of chromosomes.

Mitosis produces two genetically identical daughter cells from one parent cell. It can be remembered using the mnemonic PMAT:

Prophase

• Chromosomes condense (shorten and thicken), becoming visible
• Each chromosome consists of two sister chromatids joined at a centromere
• The nuclear membrane begins to break down
• Spindle fibres form from the centrioles (in animal cells)

Metaphase

• Chromosomes line up along the equator (middle) of the cell
• Spindle fibres attach to the centromeres of each chromosome
• This ensures each daughter cell will receive one copy of each chromosome

Anaphase

• The centromeres split
• Sister chromatids are pulled apart to opposite poles of the cell by the spindle fibres
• The cell begins to elongate

Telophase

• Two sets of chromosomes arrive at opposite poles
• A nuclear membrane forms around each set of chromosomes
• Chromosomes begin to decondense (uncoil)
• The spindle fibres break down

Cytokinesis

• The cytoplasm divides
• In animal cells: the membrane pinches inward (cleavage furrow)
• In plant cells: a cell plate forms across the middle, becoming a new cell wall
• Result: two identical daughter cells, each with a complete copy of DNA

Human cells contain 46 chromosomes (23 pairs). These are called diploid cells ($2n = 46$).

• After mitosis, each daughter cell has 46 chromosomes — the same as the parent cell
• The daughter cells are genetically identical to each other and to the parent cell

Note: Meiosis (studied in the Inheritance topic) produces cells with half the chromosome number (23 chromosomes — haploid cells). Mitosis maintains the full set.

Stem cells are undifferentiated cells that can divide to produce more stem cells or differentiate into specialised cells.

Types of Stem Cells

Uses of Stem Cells

• Treating diseases: Bone marrow transplants for leukaemia; potential treatments for paralysis, diabetes, Alzheimer's
• Research: Understanding how cells develop and how diseases progress
• Growing new tissues/organs: Potential for growing replacement organs

Ethical Issues

• Embryonic stem cell research involves destroying embryos, which some people believe is morally wrong
• Others argue the potential medical benefits justify the research
• Adult stem cells avoid this controversy but are more limited in their ability to differentiate

Differentiation is the process by which a cell becomes specialised for a particular function.

• In animals, most cells differentiate early in development and then lose the ability to differentiate
• In plants, cells can differentiate throughout the plant's life (from meristems)

Examples of Differentiated Cells

• Red blood cells: No nucleus, biconcave shape → carries oxygen efficiently
• Nerve cells: Long axons, many dendrites → transmits electrical impulses over distances
• Sperm cells: Tail for movement, many mitochondria → reaches and fertilises the egg
• Xylem cells: Dead, hollow tubes, lignin walls → transports water up the plant
• Root hair cells: Long extensions → absorbs water and minerals from soil

Normally, cells divide in a controlled way. Cancer occurs when cells divide uncontrollably, forming a mass of cells called a tumour.

Types of Tumour

• Benign tumours: Grow slowly, contained in a membrane, do not spread (non-cancerous)
• Malignant tumours: Grow rapidly, invade surrounding tissues, can metastasise (spread to other parts of the body through the blood or lymph)

Causes

• Genetic mutations in genes that control cell division
• Risk factors: UV radiation, smoking, certain chemicals (carcinogens), some viruses

Question: A cell starts with 46 chromosomes. It undergoes mitosis. How many chromosomes will each daughter cell have? How many daughter cells are produced?

Solution:

Mitosis produces 2 daughter cells, each genetically identical to the parent.

Each daughter cell will have 46 chromosomes (the same as the parent cell).

Mitosis maintains the chromosome number — it does not halve it (that's meiosis).

Question: A bacterial cell divides by binary fission every 20 minutes. Starting with one cell, how many cells will there be after 2 hours?

Solution:

2 hours = 120 minutes

Number of divisions $= \frac{120}{20} = 6$

Number of cells $= 2^6 = 64$ cells

• PMAT — Prophase, Metaphase, Anaphase, Telophase. Learn this order.
• Remember: Mitosis = 2 identical cells. Meiosis = 4 different cells (half the chromosomes).
• When describing mitosis, always mention that DNA replication happens during interphase, NOT during mitosis itself.
• In 6-mark questions about stem cells, include both the scientific benefits AND the ethical arguments.

• The cell cycle consists of interphase (growth and DNA replication), mitosis (nuclear division), and cytokinesis (cytoplasmic division).
• Mitosis produces two genetically identical daughter cells and follows the stages: Prophase → Metaphase → Anaphase → Telophase.
• Stem cells are undifferentiated cells that can develop into specialised cells.
• Differentiation is when cells become specialised for particular functions.
• Cancer results from uncontrolled cell division caused by mutations in cell cycle control genes.