ISSN 1062-3604, Russian Journal of Developmental Biology, 2006, Vol. 37, No. 6, pp. 337–349. © Pleiades Publishing, Inc., 2006.
Original Russian Text © E.V. Sheval, V.Yu. Polyakov, 2006, published in Ontogenez, 2006, Vol. 37, No. 6, pp. 405–418.
A special protein structure, scaffold, is a key ele-
ment of the radial-loop model of mitotic chromosomes.
It is postulated within the framework of this model that
the chromosome scaffold (CS) provides for structural
integrity of chromosomes throughout the cell cycle by
means of interaction with speciﬁc DNA sites (Laemmli
et al., 1978). The sites of DNA attachment to CS deter-
mined the boundaries of loop domains, elementary
structural-functional units of the genome (Razin,
Although the radial-loop model is now very popular,
many morphological observations can hardly be
explained within the framework of this model.
Firstly, this refers to longitudinal differentiation of
chromosomes, which is expressed after some pre- and
postﬁxation treatments of the material. It is now possi-
ble to reproduce this phenomenon at the optical and
ultrastructural levels. The most widespread is so-called
G-banding, which allows visualization in the mitotic
chromosomes of alternating dark and light-stained
blocks, G- and R-segments, respectively. It was shown
that G- and R-segmentation reﬂects differences in the
genetic status of different chromosomes regions.
Secondly, another problem not to be ignored while
discussing the radial-loop model is the presence of a
hierarchy of successive levels of DNA compactization
in the mitotic chromosomes: nucleosome ﬁbrils, 30 nm
ﬁbrils, 100–130 nm ﬁbrils or elementary chromone-
mata, and 200–250 nm ﬁbrils (Chentsov and Burakov,
2005). If visualization of CS and loop domains requires
chromatin loosening or extraction, “elementary”
chromonemata in the mitotic chromosomes or inter-
phase nuclei can be observed under the conditions,
maximally close to the native conditions. Although the
principle of structural organization of chromonemata
remains unknown, some models are developed with an
account of their existence (Kireeva et al., 2004).
In this review, we discuss the data on CS structure
and molecular composition with special reference to
the levels of compactization and the longitudinal differ-
entiation of mitotic chromosomes into G- and R-seg-
ments. This approach allowed us to consider different
models as several views on the same problem from dif-
ferent sides, rather than competing paradigms.
The ﬁrst experimental data on the existence of axial
structures in the mitotic chromosomes were obtained
by Stubbleﬁeld and Wray (1971). The concept of spe-
ciﬁc structural “skeleton” as a basis of chromosome
organization was then developed by Laemmli et al.
(1978). It was shown that extraction of almost all his-
tones and a major part of nonhistone protein from iso-
lated chromosomes by a mix of dextran sulfate and hep-
arin did not lead to their full degradation, since DNA
did not pass to solution, but remained in a complex with
nonextractable nonhistone proteins. It was also shown
using electron microscopy that nonextracted (residual)
components represented a structure resembling a swol-
Chromosome Scaffold and Structural
Integrity of Mitotic Chromosomes
E. V. Sheval and V. Yu. Polyakov
Belozersky Institute of Physicochemical Biology, Moscow State University, Vorob’evy gory, Moscow, 119992 Russia
Received October 1, 2005; in ﬁnal form, February 20, 2006
—Chromosome scaffold represents a continuous protein substructure revealed in isolated metaphase
chromosomes after harsh extraction. According to postulates of the widespread radial loop model the scaffold
plays an important role in the formation and maintenance of structural integrity of the mitotic chromosomes.
Here, the data concerning the structure and major components of the chromosome scaffold are presented. The
experiments suggesting that the scaffold represents a system of discrete linker proteins and the data about high
mobility of scaffolding proteins are discussed. Furthermore, the data about higher-level chromatin structures
(elementary chromonema and 200–250 nm ﬁbers) and behavior of scaffolding proteins are compared. The
results presented agree with the idea that at the present stage it is possible to discriminate chromatin complexes,
whose structural integrity is not maintained by the chromosome scaffold.
: chromosome, chromosome scaffold, topoisomerase II, condensins, chromonema.