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2.1Unit 2 — Multicellular Organisms

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

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Section 1 of 2The process and stages of mitosis

1Purpose of mitosis
: Provides new cells for growth, repair of damaged tissues, and replacement of dead cells.
2Chromosome complement2 points
:
  • 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.
3Stages of mitosis in sequence5 points
:
  1. 1. DNA Replication — Prior to division, chromosomes replicate into two identical sister chromatids joined at a centromere. Chromosomes condense and become visible under a microscope.
  2. 2. Nuclear Membrane Disintegration — The nuclear membrane breaks down. Spindle fibres form and attach to the centromere of each replicated chromosome.
  3. 3. Equator Alignment — Chromosomes line up single-file along the equator (middle) of the dividing cell.
  4. 4. Chromatid Separation — Spindle fibres contract, pulling sister chromatids apart to opposite poles of the cell (now referred to as daughter chromosomes).
  5. 5. Nuclear Reformation & Cytokinesis — Nuclear membranes reform around each set of separated chromosomes. The cytoplasm divides, producing two genetically identical diploid daughter cells.

Section 2 of 2Stem cells and cellular differentiation

1Stem cells2 points
:
  • 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.
2Types of stem cells2 points
:
  • Embryonic stem cellsObtained from very early embryos (blastocysts); capable of differentiating into all cell types in the human body (pluripotent).
  • Tissue (adult) stem cellsFound 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).
3Levels of cellular organisation
:
  • Cells → Tissues → Organs → Systems → Organism

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