What chemicals regulate the cell cycle how do they work?

Cyclins are the proteins that regulate the cell cycle through cyclin depedent kinases (Cdks).

Explanation:

Cyclins are the proteins that control the activity of cyclin dependent kinases. The levels of various cyclins fluctuates throughout the cell cycle. The active cyclin-dependent kinases (Cdks) regulate the cell cycle by phosphorylating the enzymes and other proteins that are needed during specific phase of the cell cycle.  

Example: M-cdks are the Cdks of M phase and activated cyclin--M-Cdk complex phosphorylate the protein to make the cell proceed through mitosis.

estuaries are the partially enclosed coastal bodies of brackish water formed at the mouth of sea where river meets an ocean. it is a transition zone between river and sea water so estuaries are influenced to both marine environment such as tides, waves and to riverine such as flow of fresh water and sediments. the mixing of sea and fresh water provide high level of nutrients, making estuaries a high product habitats.

many estuaries suffers degradation from a variety of human activities like erosion from deforestation, overgrazing, overfishing, drainage and eutrophication.

so from the above statement answers will be

1. false as estuaries are formed due to both brackish and freshwater.

2.true, due to high productivity estuaries are breeding ground for many marine species.

3. direct and indirect activities of man that contribute to pollution and deposition of sediments.