Mitosis has six subphases:  interphase, prophase,
prometaphase, metaphase, anaphase, and telophase.

During mitosis the DNA is separated
into daughter cells and the cytoplasm is divided.

Interphase has three subunits:
gap one (G1), synthesis, and gap two (G2). 

Interphase is the period between nuclear divisions.
A eukaryotic cell is in this "condition" for most of its life.
During interphase, DNA and centrosomes
are replicated by the nucleus

Cyclin D is present only in the G1 phase. 
It is made at the start of G1.
Cyclin D binds to CDK4 and changes its structure.
Thus, it activates CDK4
Cyclin D is broken down at the
end of G1 and herein inactivates CDK4.

CDK4 stands for cyclin dependent kinase.

CDK4 is present in all steps of the cell cycle and is
responsible for adding phosphate groups to other proteins. 
In doing this, CDK4 changes the biochemistry
of proteins by changing their shape.
This protein moves the cell
from the G1 phase to the S phase.
CDK4 is inactive unless it is bound to Cyclin D.