2
Basic Cytogenetics and the Role of Genetics in Cancer Development
Interphase
V
Prophase
V
Metaphase
V
Anaphase
V
Telophase
V
Cytokinesis
Fig. 2.2
The four different phases constituting
the mitotic process
(cytokinesis being included in the telophase).
the diploid value of somatic cells and provide a complete zygotic
genome.
The Chromosome Structure
The chromosomes are composed of DNA and associated his-
tone and non-histone proteins.
This combination, called chromatin, is individualized
into visible chromosomes only during mitosis. The double
helix of DNA described by Watson and Crick is supercoiled
around protein cores in a complex structure of nucleosomes.
Compacted nucleosomes constitute chromatin segments of
approximately 30 nm in diameter observable in electron
microscopy. Further condensation makes it optically identifi-
able as heterochromatin in the interphase and as chromo-
somes at the late prophase. An animation on cell division
and chromosome structure can be found at http://www.
johnkyrk.com.3
The extremities of the chromosomes are called telom-
eres. They preserve the integrity of chromosomal extremi-
ties by allowing replication to occur without loss of coding
sequences, but undergo repetitive shortenings themselves
after each cellular division. The so-called "mitotic clock"
counts the number of cell divisions that have occurred and
pushes the cell to apoptosis before a critical telomeric short-
ening is reached. If this should occur, chromosomes would
be prone to fuse end to end, giving rise to sticky ends that
would favor mitotic aberrations and promote the accumula-
tion of subsequent genetic rearrangements, possibly leading
toward the first crucial steps in the development and progres-
sion of neoplasia.4
Methodology
The Karyotype
In the 1950s and 1960s, human chromosomes were studied with
Giemsa or Wright stains, making it possible for these chromo-
somes to be counted accurately and grouped together according
to their length and the position of the centromeric constriction.
The 22 pairs of autosomes and the sex chromosomes were thus
classified into seven groups, A to G. The largest pairs are num-
bered 1 to 3 in group A. The centromere is located in the mid-
dle of chromosomes 1 and 3 and displaced in a submetacentric
position in pair 2. Group B is composed of pairs 4 and 5, both
with a subtelomeric centromere. Group C is the largest and is
composed of medium-sized chromosomes including pairs 6 to
12 and chromosome X. Most of them are submetacentric and
roughly classified by decreasing length. Group D is composed of
chromosome pairs 13-15 and characterized by a distal acrocen-
tric centromere. Group E contains the metacentric pair 16 and
the submetacentric 17 and 18 sets. Chromosome pairs 19 and
20 are smaller metacentric chromosomes and constitute group F.
Group G is composed of small acrocentric chromosomes arbi-
trarily placed in pairs 21 and 22. The small Y chromosome is
included in group G.
Accurate individual classification of chromosomes was ren-
dered possible by the banding techniques developed first by
applying fluorescent quinacrine mustard on metaphase prepara-
tions.5 This fluorescent agent reveals transverse bright bands (Q
banding) of different intensities along the chromosome arms.
Other procedures using trypsin digestion (which removes pro-
teins from chromatin) and Giemsa staining yield dark G bands
superimposed on the bright Q bands. This led to a very pre-
cise identification of each individual chromosome (Fig. 2.3).
Techniques with heat denaturation in saline solution obtained
a reverse staining called R bands with optional enhancing of
telomeric ends in T banding.
The different banding pattern for each of the 23 different
chromosomes allows for a perfect pairing of homologues. The
number of bands can be raised up to 800 by the high-resolution
staining technique obtained on prometaphase chromosomes.
The dark G bands correspond to a compact conformation of
the chromatin while the clear bands are composed of rather
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