Chromosomes, the cell cycle, mitosis and stem cells
Body (somatic) cells contain pairs of chromosomes — long molecules of DNA that hold genes. Humans have 23 pairs (46 in total). Sex cells (gametes) are different — see B6.1.
The cell cycle
Each body cell goes through three repeating stages:
- G₁ / S / G₂ (interphase) — the cell grows, makes more sub-cellular structures (ribosomes, mitochondria), and DNA replicates so each chromosome becomes two identical copies (sister chromatids) joined at the centromere. This is the longest stage.
- Mitosis — the duplicated chromosomes line up at the equator and the sister chromatids are pulled to opposite poles by spindle fibres. Two new nuclei form.
- Cytokinesis — cytoplasm divides, producing two genetically identical daughter cells.
The whole cycle in a typical mammalian body cell takes ~24 hours; mitosis itself is only about 1 hour.
Why mitosis matters
Three biological roles:
- Growth of multicellular organisms
- Repair of damaged tissues
- Replacement of cells worn out (e.g. skin, gut lining)
- Asexual reproduction in some plants and single-celled organisms
Identical daughter cells are essential — every new skin cell must do skin-cell jobs.
Stem cells
A stem cell is an undifferentiated cell that can divide to produce more cells of the same type AND give rise to specialised cell types.
- Embryonic stem cells (from early human embryos) are pluripotent — can become almost any cell type.
- Adult stem cells (e.g. in bone marrow) are multipotent — limited range, mainly blood cells.
- Plant meristem cells (in shoot/root tips) act as a lifelong supply of stem cells, producing any type of plant cell.
Therapeutic uses
- Bone-marrow transplants for leukaemia — replace cancerous blood-cell precursors.
- Therapeutic cloning — produce embryonic stem cells with the patient's genome, avoiding rejection.
- Diabetes / paralysis research — replacing damaged β-cells or nerve cells.
Risks and ethical issues
- Possible transfer of viral infections during stem-cell therapy.
- Some object on religious or moral grounds to using human embryos.
- Cost and slow research progress.
Plant tissue culture (extension)
Meristem cells can be used to clone rare or commercially valuable plants quickly. The clones are genetically identical and disease-free.
⚠Common mistakes— Common mistakes / exam traps
- Saying "the cell divides into two new chromosomes" — confusing chromosomes with cells; it's the chromosomes that are copied and then the cell that divides.
- Calling mitosis "cell division" when asked specifically about mitosis vs cytokinesis. Mitosis = nuclear division; cytokinesis = cytoplasm division.
- "Stem cells make any cell type" — only embryonic stem cells. Adult stem cells are limited.
- Mixing up mitosis (identical, body cells) and meiosis (variable, gametes) — see B6.1.
Links
Connects to B1.1 (specialised cells), B6.1 (meiosis comparison) and B6 (DNA, inheritance).
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