Cell Nucleus Biogenesis, Structure and Function
407
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
(i)
Fig. 6
The S-phase program. During S-phase, different classes of
DNA elements are replicated at speciFc times. Chromatin with the
majority of transcribed genes is replicated over the Frst
4hofthe
S-phase. During this period active sites of DNA synthesis are in
discrete foci dispersed throughout the nuclear interior (a–c). These
are designated early or type 1 replication patterns. During mid
S-phase (d–f), replication switches to peripheral inactive chromatin.
These mid S-phase patterns are also referred to as type 2 (d–e) and
type 3 (f) patterns. During late S-phase (g–i) the replication of
extended blocks of constitutive heterochromatin occurs throughout
the nuclear interior. These late S-phase patterns are also referred to
as type 4 (g–h) and type 5 (i) patterns. Encapsulated HeLa cells were
synchronized at the onset of S-phase and replication sites labeled at
1 h intervals for 10 h. Images shown are replication sites labeled in
permeabilized cells – using biotin-dUTP. (Reproduced from Hozak,
P., Jackson, D.A., Cook, P.R. (1994) Replication factories and nuclear
bodies: the ultrastructural characterization of replication sites during
the cell cycle,
J. Cell Sci.
107
, 2191–2202. With permission of the
Company of Biologists. The bar is 5
µ
m.)
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