ISSN 10227954, Russian Journal of Genetics, 2011, Vol. 47, No. 10, pp. 1151–1160. © Pleiades Publishing, Inc., 2011.
Original Russian Text © L.A. Shakina, V.Yu. Strashnyuk, 2011, published in Genetika, 2011, Vol. 47, No.10, pp. 1297–1306.
Polytene chromosomes are structural modifica
tions of actively functioning interphase chromosomes
. They were first described by Balbiani in 1881 
and are at present the most convenient objects for a
visual analysis of gene activity, spatial and structural
genome organization [3–8].
Polyteny is widely spread in animals and plants,
especially in metabolically active nuclei [5, 6, 9]. The
basis of this phenomenon is endoreduplication, i. e., a
mechanism providing the occurrence in nuclei of mul
tiple uniform DNA copies in the absence of chromatin
condensation, chromosome segregation, and cytoki
nesis. This saves the time and cell resources and pro
motes a more rapid growth of tissues and organs .
The metabolic activity of a highly polyploid cell
can be functionally equivalent to that of many diploid
cells [5, 10]. Endoreduplication occurs most fre
quently in gland, reserve, and nutritive cells and pro
vides higher than normal levels of gene expression
when the constraints of time and space require it.
According to Nagl , endoreduplication and simi
lar events, such as cell polyploidy and endomitosis, are
Endoreduplication is often described as a novel cell
cycle, because chromosomes replicate, but cells do not
divide. However, endoreduplication is so common in
plants and animals that it would be more appropriate
to describe it as an alternative cell cycle or as a cell
cycle of terminal differentiation .
Until recently, the molecular mechanisms of a
shortened cell cycle remained unknown, and polyteny
was considered to be a consequence of mutations in
genes controlling several aspects of the cell cycle, divi
sion spindle defects, or incomplete readiness of cells
for mitosis because of the competitive processes of
proliferation and differentiation . Only in the mid
1990s, owing to the achievements in the study of the
molecular genetic aspects of cell cycle regulation, sys
tematic investigations of the mechanisms underlying
differentiation of polytene cells began.
The aims of this work are analysis of the data on the
molecular genetic mechaisms controlling the transi
tion from mitotic divisions to endoreduplication and
regulating endoreduplication in the course of cell dif
ferentiation as well as the description of genes and
their products involved in these processes.
GENETIC AND MOLECULAR MECHANISMS
In modern approaches, an important role in the
process of transition from the mitotic cell cycle to the
endoreduplication cycles is assigned to the alteration
of the content and activity of cell cycle regulatory pro
teins, such as cyclins and cyclindependent kinases
(CDKs), and to modulators of their activity (for their
detailed description see ).
Normally, chromosomes are replicated only once
per cell cycle, and progression through the M phase is
required to “license” the initiation of replication for
the next round of chromosome duplication. Oscilla
tions in the level of cyclindependent kinase activity
allow multiple cycles of licensing and DNA replica
tion without intervening mitosis .
Cyclins and cyclindependent kinases are unstable
proteins, and the sequence of normal cell cycle events
depends on their coordinated work . By their
structure and phases of action cyclins may be arbi
trarily subdivided into two subfamilies:
are maximally active in
(groups C, D, E, J) and
mitotic cyclins whose content and activity peak in the
transition period (groups A, B). Cyclindepen
dent kinases consist of the serine/threonine protein
kinase and a cyclin regulatory subunit. The temporal
activity of CDKs is controlled mainly by changes in
the relative content of the cyclin components as well as
Genetic, Molecular, and Humoral EndocycleRegulating Mechanisms
L. A. Shakina and V. Yu. Strashnyuk
Karazin National University of Khar’kov, Khar’kov, 61022 Ukraine;
Received April 15, 2011
—The review presents data on the molecular genetic mechanisms controlling endoreduplication.
The issues concerning the activity of the main cycle cell regulators, such as cyclins, cyclindependent kinases,
and their inhibitors, under conditions of a modified cell cycle of polytene cells are considered. Specific fea
tures of regulation at the replication origin points and the role of hormones and phytohormones in the onto
genetic control of endoreduplication are analyzed.
REVIEWS AND THEORETICAL ARTICLES