Karyotype
A systemized arrangement of the chromosomes of a single cell prepared either by drawing or by photography, with the extension in meaning that the chromosomes of a single cell can serve as a typical example of the chromosomes of an individual or even a species.
Chromosomes in metaphase can be identified using certain staining techniques, so called banding. Cells are cultured and then stopped in metaphase. This is one of the phases of cell division when the chromosome aligns along the center of the cell. During metaphase chromosomes are highly condensed which makes it easier to maximize the number of suitable cells to identify chromosomal anomalies. They are then spread on a slide, stained with a suitable dye and visualized in the microscope. Most conventional cytogenetic analyses depend on the karyotype of banded metaphase chromosomes.
A band is defined as that part of a chromosome which is clearly distinguishable from its adjacent segments by appearing darker or brighter with one or more banding techniques. The chromosomes are visualized as consisting of a continuous series of bright and dark bands.
The banding techniques fall into two principal groups: 1) those resulting in bands distributed along the length of the whole chromosome, such as G-, Q- and R-bands and 2) those that stain a restricted number of specific bands or structures. These latter include methods which reveal centromeric bands, C-bands, and nucleolus organizer regions, NOR's (at terminal regions of acrocentric chromosomes). C-banding methods do not permit identification of every chromosome in the somatic cell complement, but can be used to identify specific chromosomes.
G- and R- bands can be bright field or fluorescent.
These G-bands are most commonly used. They take their name from the Giemsa dye, but can be produced with other dyes. In G-bands, the dark regions tend to be, late-replicating and AT rich. The bright regions tend to be, early-replicating and GC rich.