Karyotyping is a technique used to provide a snapshot of the chromosomes in an organism.
Upon isolation, cells are first incubated with a drug that arrests them in metaphase of the cell cycle. The preparation is then treated with a hypotonic solution of potassium chloride to induce swelling and proper spreading, and the swollen cells are preserved with fixative.
Following resuspension, the sample is carefully placed on a slide, treated with trypsin—an enzyme that denatures specific regions of DNA—and washed. Afterwards, a dye, such as Giemsa stain, can be applied.
After rinsing…drying…and coverslipping, the slide can be viewed using a light microscope to reveal specific features, such as densely packed gene-poor regions rich in A-T bases, or a reverse Giemsa stain that highlights bands rich in G-C bases.
In the lab, scientists first remove cells from a culture flask and place them on a glass slide. Next, slides are dipped in trypsin and then washed. Banding dyes, such as Giemsa, are now applied.
The chromosomes are then observed under a fluorescence microscope to visualize the number and structure of chromosomes.