Biomedical Engineering Reference
In-Depth Information
Table 7.3
Types of chromosome banding
Type
Treatment
Stain
G
Trypsin
Giemsa
R
Heat
Giemsa
Q
Fluorescence
Quinacrine
C
Alkali solution
Giemsa
High resolution
Methotrexate
Giemsa
cytogenetic anomalies.
Q and C banding
are often done to highlight variants
in the length or brightness of different heterochromatic areas in the chro-
mosomes. These variants are often useful familial markers for gene linkage
studies.
7.6.2 High resolution banding
High resolution banding
is accomplished by studying chromosomes in pro-
phase or prometaphase, when the chromosomes have not condensed to the
stage of metaphase. Whereas typical metaphase chromosomes have a level
of resolution of about 400 bands/haploid set, high resolution analysis is done
at the level of 600 bands or higher. The technical diffi culty of obtaining long
chromosomes for analysis is that it is very diffi cult to not have extensive
overlapping of the chromosomes, which can hamper proper chromosome
identifi cation.
7.6.3 Fragile site analysis and fl uorescent
in situ
hybridization (FISH) methods
Along with these banding methods two other methodologies are often used
in cytogenetic laboratories to rule out possible clinical mutations: fragile
site analysis and FISH methods.
Fragile site analysis
can be done at a chromosomal level by fi rst growing
cells in low levels of folic acid or treatment of cultures with either metho-
trexate or fl uorodeoxyuridine, which inhibit folate metabolism. This type
of culture manipulation will result in expression of a fragile site at band
Xq27.3 in patients with X-linked mental retardation. This folate-sensitive
fragile site in the chromosome coiling structure appears in metaphase chro-
mosomes as a constricted area at the end of the chromosome. Molecular
methods are now available, which are based on the identifi cation of differ-
ences in specifi c repeated triplets at the mutational site of the fragile X gene
and have replaced cytogenetic methods for determining the presence of this
mutation. Figure 7.15 shows the fragile site on the X chromosome.
