The Principles of Clinical Cytogenetics - Extra Materials - Springer

74 Martha Keagle and Steven Gersen
Fig. 5. C-Banding. This technique stains the constitutive heterochromatin found in each chromosome (hence
the term C-banding) and is useful for clarification of polymorphisms. Note the large heterochromatic regions in
some of the chromosomes. (Image provided by Alma Ganezer.)
NOR Staining (Silver Staining for Nucleolar Organizer Regions)
This technique selectively stains the nucleolar organizer regions (NORs) located on the satellite
stalks of the acrocentric chromosomes. These regions contain the genes for ribosomal RNA and can
be stained with silver nitrate. Theoretically, there are 10 NORs per cell, 1 for each acrocentric chromosome.
However, not all will usually stain at any one time because the silver stains the activity, not
presence, of rRNA genes. NOR staining is useful for the identification of marker chromosomes and
rearrangements or polymorphisms involving the acrocentric chromosomes. (See Fig. 6.)
DAPI/DA Staining (4,6-Diamino-2-Phenole-Indole/Distamycin A)
This stain combines DAPI, a fluorescent dye, with distamycin A, a nonfluorescent antibiotic. Both
form stable bonds preferentially to similar, but not identical, A-T-rich, double-stranded regions of
DNA. Used together, DAPI/DA fluoresces certain A-T-rich areas of constitutive heterochromatin in
the C-band regions of chromosomes 1, 9, and 16 the distal Yq, and the short arm of chromosome 15.
Prior to the development of fluorescence in situ hybridization techniques, this was the stain that
differentiated between satellite regions of any of the acrocentric chromosomes.
DAPI/DA is used to identify rearrangements of chromosome 15, to confirm variations in the polymorphic
regions of chromosomes 1, 9, and 16 and distal Yq, and to study marker chromosomes with satellites.
Fluorescence In Situ Hybridization (FISH)
The development of fluorescence in situ hybridization technology represents an important
advancement in cytogenetics. FISH is a marriage of classical cytogenetics and molecular technologies
and has a large number of applications. Chapter 17 deals with this topic in depth.