Karyotyping and why metaphase is used
A karyotype is a chart of a person's chromosomes, made from cells halted in metaphase (the stage of mitosis when chromosomes line up in the middle of the cell).
Remember
Cells are halted in metaphase because the chromosomes are then most condensed, so each one can be seen and compared. The result shows chromosome number and structure, so large changes appear but single-gene mutations do not.
A karyotype is a chart of all of a person's chromosomes. It comes from a metaphase spread, which shows all the chromosomes from one cell.
In the laboratory, cells are stimulated to divide, and colchicine, a chemical, then halts them in metaphase. The cells are swollen so that the chromosomes spread out, and they are stained and photographed. The chromosomes are then arranged in homologous pairs (the matching maternal and paternal copies of each chromosome) by size and shape.
Metaphase is chosen because the chromosomes are most condensed at that point, which means packed down. Each one shows as a separate rod with a visible centromere (the narrow waist of the chromosome) and a banding pattern, the light and dark stripes that appear after staining.
In the other stages, the chromosomes are uncondensed, only beginning to condense or already being pulled apart. In interphase (the long period between divisions) they are loose, uncondensed threads of chromatin, so they cannot be seen as separate rods. In anaphase (when sister chromatids, the two parts of each replicated chromosome, are pulled apart towards opposite ends of the cell) each chromosome has split into its sister chromatids, so the chromosomes are no longer in the replicated form that is arranged and compared.
Because the number and structure of the chromosomes can be checked, a karyotype detects extra or missing chromosomes, such as the extra chromosome 21 of Down syndrome, and large structural changes such as translocations (pieces joined to the wrong chromosome). It misses single-gene defects, which need more specialised tests.
Key facts
- Karyotype
- a chart of all of a person's chromosomes, made from a metaphase spread.
- Stage used
- metaphase, when the chromosomes are most condensed and lined up at the equator (the middle of the cell).
- Laboratory step
- colchicine stops dividing cells in metaphase.
- What it detects
- extra or missing chromosomes and large structural changes.
- What it misses
- single-gene defects, which need more specialised testing.
- Written form
- the chromosome number, the sex chromosomes (for example XX or XY) and any structural change, separated by commas without spaces.
Exam traps
- Interphase (the long period between divisions, including the copying of DNA) is not the stage to examine, because the chromosomes are loose, uncondensed threads and cannot be seen as separate rods.
- Anaphase (when sister chromatids are pulled apart towards opposite ends of the cell) is not the stage used, because the chromatids of each chromosome have already been split, so the chromosomes cannot be arranged and compared.
- Gene expression (how active each gene is) is not what a karyotype measures, because it shows chromosomes, not gene activity.
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