Direct Control: Completion of previous steps is essential to produce substrate for subsequent steps.

Indirect Control: Early steps release inhibitors of late steps and late steps feed back control to early steps. "Feedback" and "Feedforward"

G₁: the restriction point (RP) is the beginning of a build up of nucleotides, histones, and enzymes for DNA synthesis (S-phase).
Once begun, cell cannot revert back to G₁ and death of cell results.
If resources and timing are appropriate passage through S, G₂ and M produces two functional daughter cells.
In G₁ cyklin-dependent kinases act as regulatory molecules; when in low concentration G₀ results, which is a quiescent phase. Others control other phases, as certain cyclins increase, and others decrease.

Timing: G₁ until Restriction Point, then S which continues until DNA is repaired (DNA repair arrest) and completely replicated (DNA replication arrest), then G₂ moving into early Mitosis until spindle complete (M arrest), then Anaphase completing Mitosis and beginning G₁.

G₁ until Restriction Point, then ...

S phase - 10 hours mature Drosophila
< 4 minutes in embryonic cells of Drosophila ,
passing DNA repair arrest and DNA replication arrest, ...

G₂:

Maturation promoting factor (MPF) vs. cyclin (frog embryo)
too much or too little of one upsets the balance, and both decline
Production of proteins for kinetochores,
enzymes to disassemble nuclear membrane, and vice versa,
energy for spindle formation and movement,
enzymes to assemble spindle,
histones to coil chromosomes,

M-phase:

Note: relationships of microtubules, centrioles (or organizing region), kinetochores, centromeres
Centrioles divide, migrate, form spindle fibers
Centromere organizes kinetochore
Membrane begins to break down
Spindle forms, attachments occur with kinetochore
Tension forms passing M-arrest, the ...

Control point to initiate movement (kinetochore divides)
Poles move apart, fibers shorten
Chromosomes meet at pole
Membranes begin to reform, spindle broken down, ER reforms
Chromosomes uncoil in part
Gene activity resumes in "normal" way for G₁ phase.