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Cdk1 initiates mitosis.

^a Mitogenic signals stimulate the synthesis of cyclin D.

kinase

CCdkp

What does phosphorylation do to a protein? Recall from Chapter 3 that proteins have both hydrophilic regions (which tend to interact with water on the outside of the macromolecule) and hydrophobic regions (which tend to interact with one another on the inside of the macromolecule). These regions are important in giving a protein its three-dimensional shape. Phosphate groups are charged, so an amino acid with such a group tends to be on the outside of the protein. In this way, phosphorylation changes the shape and function of a protein. By catalyzing the phosphorylation of certain target proteins, Cdk's play important roles in initiating the steps of the cell cycle.

Cdk1 initiates mitosis.

^a Mitogenic signals stimulate the synthesis of cyclin D.

activity.

Cyclin D and E and Cdk4 and 2 phosphorylate RB, inactivating it.

(JR ("restriction point") must be passed for the cell cycle to proceed.

^Tumor suppressors can block cyclin D

activity.

Cyclin D and E and Cdk4 and 2 phosphorylate RB, inactivating it.

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Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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