Question

what regulates the timing of the cell cycle in eukaryotic cells

Answer 1

This really depends on the time scale under consideration.

For example, in a broad sense, the timing of the cell cycle in eukaryotic cells can be regulated by many things - in some cases by hormones or other signalling molecules from other cells, by circadian patterns, by nutrient availability and many other things acting in combination.

In an increasingly less broad sense, it is controlled by a series of signalling pathways - usually consisting of proteins and small intracellular signalling molecules - that take these environmental conditions and translate them into a cellular response.

And in the finest and most immediate sense, it is controlled by cell cycle specific proteins on which these signalling pathways act.

Answer 2

this sorta helps but the question im trying to answer is
"Proteins called ____ regulates the timing of the cell cycle in eukaryotic cells

Answer 3

The class of proteins you are probably looking for are cyclins.