2 2 2
H . F I R K E T
T A B L E
II
M e a n duration o f the cell cycle phases (in hours)
Types o f cultures
S
G 2
Total
cycle
References
Chick, fibroblasts
4 - 7
4
Firket (1958a)
Chick, fibroblasts
5
3
Baeckeland, Chevremont
and Lennes (1962)
Mouse, L-strain fibroblasts 9-11
6-7
3-4
20 Stanners and Till (1960)
Whitmore et al (1961)
Hamster, fibroblasts
5-6
6
3
Taylor (1960)
H u m a n , U-12 fibroblast
strain
16
9
4
30 Harrington (1960)
H u m a n , H e L a
± 1 4
5-6
2-8
Painter and Drew (1959)
(a) The G x period. It is the most variable in length and probably
responsible for the variations in the duration of the whole cycle
mentioned earlier, the other periods being more constant (Fig. 8)
(Firket, 1958a; Sisken and Kinosita, 1961b).
The beginning of G x is undoubtedly a recovery stage after division.
Increase of the chondriome and a substantial production of protein (as
shown by the overall mass increase) accompanied by water absorption
are its main known characteristics. It is not yet established whether the
R N A metabolism in vertebrate cultures shows any difference at this
moment by comparison with later stages in the cell life, but experiments
on plant cells suggest a brief outburst of very active synthesis in the
nucleus (Woodard, Rasch and Swift, 1961). For most ofthe rest of G ±
the cells are more stable. Non-growing tissue, where the idea of a cycle
seems meaningless, may be considered as remaining physiologically at
this stage. (See Chapter 12.)
(b) The period of DNA synthesis (S). At the end of G 1? some unknown
mechanism causes the accumulation of deoxyribosides and nucleotides
(Schneider and Brownell, 1957; Foster and Stern, 1959). In plant cells,
at least, phosphorylation of nucleosides and deoxyribonuclease activity
are also considerably increased (Stern, 1961; Hotta and Stern, 1961)
just before the stage of D N A synthesis (S) sets in.
The incorporation of tritiated thymidine often seems to begin very
near the nucleolus (Firket, 1958a; Harris, 1959, 1961). Cytophotometric
measurements have shown the synthesis of histones to be practically
parallel to that of D N A (Bloch and Godman, 1955). D N A synthesis
H . F I R K E T
T A B L E
II
M e a n duration o f the cell cycle phases (in hours)
Types o f cultures
S
G 2
Total
cycle
References
Chick, fibroblasts
4 - 7
4
Firket (1958a)
Chick, fibroblasts
5
3
Baeckeland, Chevremont
and Lennes (1962)
Mouse, L-strain fibroblasts 9-11
6-7
3-4
20 Stanners and Till (1960)
Whitmore et al (1961)
Hamster, fibroblasts
5-6
6
3
Taylor (1960)
H u m a n , U-12 fibroblast
strain
16
9
4
30 Harrington (1960)
H u m a n , H e L a
± 1 4
5-6
2-8
Painter and Drew (1959)
(a) The G x period. It is the most variable in length and probably
responsible for the variations in the duration of the whole cycle
mentioned earlier, the other periods being more constant (Fig. 8)
(Firket, 1958a; Sisken and Kinosita, 1961b).
The beginning of G x is undoubtedly a recovery stage after division.
Increase of the chondriome and a substantial production of protein (as
shown by the overall mass increase) accompanied by water absorption
are its main known characteristics. It is not yet established whether the
R N A metabolism in vertebrate cultures shows any difference at this
moment by comparison with later stages in the cell life, but experiments
on plant cells suggest a brief outburst of very active synthesis in the
nucleus (Woodard, Rasch and Swift, 1961). For most ofthe rest of G ±
the cells are more stable. Non-growing tissue, where the idea of a cycle
seems meaningless, may be considered as remaining physiologically at
this stage. (See Chapter 12.)
(b) The period of DNA synthesis (S). At the end of G 1? some unknown
mechanism causes the accumulation of deoxyribosides and nucleotides
(Schneider and Brownell, 1957; Foster and Stern, 1959). In plant cells,
at least, phosphorylation of nucleosides and deoxyribonuclease activity
are also considerably increased (Stern, 1961; Hotta and Stern, 1961)
just before the stage of D N A synthesis (S) sets in.
The incorporation of tritiated thymidine often seems to begin very
near the nucleolus (Firket, 1958a; Harris, 1959, 1961). Cytophotometric
measurements have shown the synthesis of histones to be practically
parallel to that of D N A (Bloch and Godman, 1955). D N A synthesis