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2.1Unit 2 — Multicellular Organisms

Producing New Cells and Mitosis — SQA National 5 Biology Revision Notes & Topical Questions

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Part 1 of 2The process and stages of mitosis

The Four Stages of Mitosis
Cell division by mitosis in five labelled steps
THE PROCESS OF CELL DIVISION BY MITOSIS — INTERACTIVE SIMULATION
Mitosis and the cell cycle
1
Purpose of mitosis
Provides new cells for growth, repair of damaged tissues, and replacement of dead cells.
2
Chromosome complement
  • Most animal and plant cells are diploid (2n): they possess two matching sets of chromosomes.
  • Mitosis maintains the diploid chromosome complement, ensuring each daughter cell receives an identical, complete copy of genetic information necessary for normal cellular functioning.
3
Stages of mitosis in sequence
    • 1. DNA ReplicationPrior to division, chromosomes replicate into two identical sister chromatids joined at a centromere.
    • Chromosomes condense and become visible under a microscope.
    • 2. Nuclear Membrane DisintegrationThe nuclear membrane breaks down.
    • Spindle fibres form and attach to the centromere of each replicated chromosome.
  3. 3. Equator AlignmentChromosomes line up single-file along the equator (middle) of the dividing cell.
  4. 4. Chromatid SeparationSpindle fibres contract, pulling sister chromatids apart to opposite poles of the cell (now referred to as daughter chromosomes).
    • 5. Nuclear Reformation & CytokinesisNuclear membranes reform around each set of separated chromosomes.
    • The cytoplasm divides, producing two genetically identical diploid daughter cells.

Part 2 of 2Stem cells and cellular differentiation

r087-cell-differentiation
Cell Differentiation
Cell Differentiation
Red blood cells, white blood cells and platelets
Stem cells
  • Unspecialised animal cells that have the capacity to divide repeatedly by mitosis while remaining unspecialised, and can differentiate into specialised cells of one or more types.
  • Involved in growth and tissue repair.
Types of stem cells
  • Embryonic stem cells
    • Obtained from very early embryos (blastocysts);
      • capable of differentiating into all cell types in the human body (pluripotent).
  • Tissue (adult) stem cells
    • Found in specific tissues throughout life (e.g. bone marrow, skin, brain);
      • more restricted and can only differentiate into cell types from their tissue of origin (e.g. bone marrow stem cells differentiate into red blood cells, white blood cells, and platelets).
Levels of cellular organisation

Cells → Tissues → Organs → Systems → Organism

Key terms

Mitosis: Producing Two Genetically Identical Diploid Daughter Cells
TermMeaning
MitosisNuclear cell division producing two genetically identical diploid daughter cells.
DiploidHaving two complete matching sets of homologous chromosomes (2n).
ChromatidOne of the two identical replicated copies of a chromosome joined by a centromere.
Stem cellAn unspecialised animal cell with the capacity to self-renew and differentiate into specialised cell types.

Avoid these mistakes

Avoid these mistakes
Confusing chromatids and chromosomes — before separation they are called sister chromatids; once pulled to opposite poles they are individual daughter chromosomes.
Stating adult stem cells can turn into any cell type — adult tissue stem cells can only differentiate into cell types of their specific source tissue.

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Cambridge 9700 AS Biology Core Formula

Magnification Triangle & Unit Conversion

Formula: M = I / A  ⇔  A = I / M  ⇔  I = M × A

Units: 1 mm = 1,000 μm = 1,000,000 nm.
Rule: Convert Image size (ruler mm) to μm by ×1,000 before dividing by Actual size.