C-banding is a specialized Giemsa technique that primarily stains chromosomes at the centromeres, which have large amounts of AT-rich satellite DNA. Yet another method is C-banding (Figure 1d), which can be used to specifically stain constitutive heterochromatin, or genetically inactive DNA, but it is rarely used for diagnostic purposes these days. R-banding is often used to provide critical details about gene-rich regions that are located near the telomeres. The heat treatment is thought to preferentially melt the DNA helix in the AT-rich regions that usually bind Giemsa stain most strongly, leaving only the comparatively GC-rich regions to take up the stain. In R-banding (Figure 1c), the chromosomes are heated before Giemsa stain is applied. R-banding, which is used in parts of Europe, also involves Giemsa stain, but the procedure generates the reverse pattern from G-banding. G-banding is not the only technique used to stain chromosomes, however. Trypsin partially digests some of the chromosomal proteins, thereby relaxing the chromatin structure and allowing the Giemsa dye access to the DNA. In G-banding, the variant of Giemsa staining most commonly used in North America, metaphase chromosomes are first treated briefly with trypsin, an enzyme that degrades proteins, before the chromosomes are stained with Giemsa. The molecular causes for staining differences along the length of a chromosome are complex and include the base composition of the DNA and local differences in chromatin structure. Today, most karyotypes are stained with Giemsa dye, which offers better resolution of individual bands, produces a more stable preparation, and can be analyzed with ordinary bright-field microscopy. Since then, researchers have developed a variety of other chromosome banding techniques that have largely supplanted Q-banding in clinical cytogenetics. demonstrated that quinacrine produced characteristic and reproducible banding patterns for individual chromosomes. Q-banding involves use of the fluorescent dye quinacrine, which alkylates DNA and is subject to quenching over time. This changed in 1970, when Torbjorn Caspersson and his colleagues described the first banding technique, known as Q-banding. Prior to the development of these banding techniques, distinguishing chromosomes from one another proved very difficult, and chromosomes were simply grouped according to their size and the placement of their centromeres. Thus, to make analysis more effective and efficient, cytologists have developed stains that bind with DNA and generate characteristic banding patterns for different chromosomes. Without any treatment, structural details of chromosomes are difficult to detect under a light microscope. The nuclei are then treated with a chemical fixative, dropped on a glass slide, and treated with various stains that reveal structural features of the chromosomes. The cells are next treated with a hypotonic solution that causes their nuclei to swell and the cells to burst. After a period of cell growth and multiplication, dividing cells are arrested in metaphase by addition of colchicine, which poisons the mitotic spindle. The process of generating a karyotype begins with the short-term culture of cells derived from a specimen. For prenatal diagnosis, amniotic fluid or chorionic villus specimens are used as the source of cells.
#ARTIFACT MEANING BIOLOGY SKIN#
For other diagnoses, karyotypes are often generated from peripheral blood specimens or a skin biopsy. For cancer diagnoses, typical specimens include tumor biopsies or bone marrow samples. A variety of tissue types can be used as a source of these cells. Karyotypes are prepared from mitotic cells that have been arrested in the metaphase or prometaphase portion of the cell cycle, when chromosomes assume their most condensed conformations.
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