DNA, mitosis and transport in cells (B1.2)

This topic spans two interlinked areas: how genetic information is stored and copied, and how substances move in and out of cells. Both appear on every Gateway A paper — usually a mitosis diagram question and a required-practical osmosis graph.

DNA and genetic information

DNA (deoxyribonucleic acid) is a double helix polymer made from four nucleotide bases: adenine A, thymine (T), cytosine C and guanine (G).

Base-pairing rules: A always pairs with T; C always pairs with G.

A gene is a short section of DNA that codes for a specific protein (or polypeptide). The sequence of bases determines the sequence of amino acids assembled by ribosomes.

Humans have approximately 25,000 genes packaged into 23 pairs of chromosomes in the nucleus. The full set is called the genome.

Mitosis — cell division for growth and repair

Mitosis is division that produces two genetically identical daughter cells (clones). It is used for:

• Growth of multicellular organisms
• Repair of damaged or worn-out tissues
• Asexual reproduction in some organisms

The cell cycle

The cell cycle has three phases:

1. Interphase (G1, S, G2) — the cell grows and copies its DNA (each of the 46 chromosomes is duplicated). The cell also grows its organelles.
2. Mitosis (PMAT) — the duplicated chromosomes separate.
3. Cytokinesis — the cytoplasm and cell membrane divide; two daughter cells are formed.

Key stages of mitosis (needed for diagrams)

Stage	What happens
Prophase	Chromosomes condense and become visible; nuclear envelope breaks down
Metaphase	Chromosomes line up along the equator / centre of the cell
Anaphase	Sister chromatids are pulled to opposite poles by spindle fibres
Telophase	Two nuclei form; cell starts to divide

⚠ Common error: claiming mitosis produces four cells. It produces exactly two identical daughter cells. (Meiosis produces four non-identical cells — for reproduction only.)

Stem cells

Embryonic stem cells (from early embryos) are totipotent — able to differentiate into any cell type.

Adult stem cells (e.g. in bone marrow) are multipotent — able to differentiate into a limited range of cell types. Bone-marrow stem cells produce all blood cell types.

Therapeutic cloning: creating an embryo genetically identical to a patient, taking stem cells from it → no immune rejection.

Transport across cell membranes

1. Diffusion

Movement of particles from high concentration to low concentration (down a concentration gradient) via random particle motion. Requires no energy.

Factors that increase diffusion rate:

• Steeper concentration gradient
• Larger surface area
• Shorter diffusion distance
• Higher temperature

Examples: oxygen into red blood cells in the lungs; CO₂ out of respiring cells.

2. Osmosis

Osmosis is the movement of water molecules from a dilute (high water potential) solution to a concentrated (low water potential) solution across a selectively permeable membrane.

Think of it as: water moves to where there is less water (more solute).

• Cell in dilute solution (pure water) → water moves IN → cell swells. In animal cells this causes lysis (bursting). In plant cells the cell becomes turgid.
• Cell in concentrated solution → water moves OUT → cell shrinks (animal: crenation; plant: plasmolysis).

Required practical: osmosis in potato cylinders

1. Cut potato cylinders of the same length and diameter; record initial mass.
2. Place each in a different concentration of sugar / salt solution (e.g. 0.0, 0.25, 0.5, 0.75, 1.0 mol/dm³).
3. Leave for 20 minutes; remove, blot dry, re-weigh.
4. Calculate percentage change in mass (not absolute change — cylinders never have identical initial masses).
5. Plot % change in mass vs solution concentration; the x-intercept is the concentration at which the potato has the SAME water potential as the solution.

3. Active transport

Movement of particles from low to high concentration (against the gradient) using energy from respiration (ATP) via carrier proteins in the membrane.

Examples:

• Mineral ions (e.g. nitrate, magnesium) from soil into root hair cells.
• Glucose from the gut into blood across epithelial cells.

⚠ Common error: confusing diffusion and active transport. If movement is AGAINST the gradient, it MUST be active transport and requires energy.

Common Gateway-paper mistakes

1. Saying mitosis produces four cells — it produces TWO.
2. Confusing meiosis (for gametes) with mitosis (for growth/repair).
3. Forgetting that osmosis only applies to water; other particles use diffusion or active transport.
4. Expressing osmosis in mass-change results as absolute difference — always use PERCENTAGE CHANGE.
5. Forgetting that active transport requires ATP from respiration.