| As you discovered in Chromosome
Classification, metaphase chromsomes can be classified via several
methods, one of which , is by their banding patterns when stained prior
to microscopic examination.
Certain
chemical treatments of mammalian chromosomes
yield differentially
stained regions on chromosomes. The patterns obtained depend on the
treatment
used.
C-banding
stains centromeres.
R-banding
is the reverse of C-banding and
stains non-centromeric
regions in preference to centromeres.
G-banding
is obtained with Giemsa stain, but
can be produced
used other stains. It yields a series of lightly and darkly stained
bands
(See Figure 1). G-banding drawings as seen here are called IDIOGRAMS,
which
are explained in detail in How do you
read a Human Chromosome Idiogram? .
The dark
bands have been found to contain more Adenine
and Thymine bases
while the light bands have more Guanine & Cytosine bases in the
DNA.
A common misconception is that bands represent single genes, but in
fact
the thinnest bands contain over a million base pairs and potentially
hundreds
of genes. For example, the size of one small band is about equal to the
entire genetic information for one bacterium.
Q-banding
is a fluorescent pattern obtained
using quinacrine
for staining. The pattern of bands is very similar to that seen in
G-banding.
More
details on C-, R-, G- and Q- banding procedures
with examples may
be found at this U.
of Washington Department of Pathology website.
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