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~2 h
Undifferentiated
G2
M
S
G1
Differentiated
G2
M
G1
S
Undifferentiated
G2
M
G1
S
Differentiated
G2
M
G1
S
G2
M
G1
S
(Becker et al., 2006)
G2
M
S
G1
(Singh & Dalton, 2009)
Figure 7.1 Cell-cycle structure of undifferentiated and differentiated ESCs and iPSCs.
Cell-cycle structure of undifferentiated and differentiated ESCs from different species
is shown, with boxes representing the length of different cell cycle stages. S phase
length of differentiated and undifferentiated ESCs are estimated to be similar.
Undifferentiated ESCs have short gap phases with majority of the cell cycle spent
in S phase. ( Becker et al., 2006; Fluckiger et al., 2006; Ohtsuka & Dalton, 2008; Stead
et al., 2002; White & Dalton, 2005 ).
The mechanisms that normally facilitate somatic cells to pause the cell
cycle to deal with damage (i.e., checkpoint mechanisms) also seem to act
negatively in maintaining ESCs. For example, somatic cells use a cell-cycle
checkpoint to induce apoptosis or pause cell cycle to repair DNA damage
when necessary ( Aladjem et al., 1998; Zhao & Xu, 2010 ), and this is medi-
ated by p53. However, murine ESCs lack a G1/S cell-cycle checkpoint;
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