Answer the following question given this paragraph: The cell cycle is controlled by a number of protein-controlled feedback processes. Two types of proteins involved in the control of the cell cycle are kinases and cyclins . Cyclins activate kinases by binding to them, specifically they activate cyclin-dependent kinases (CDK) . Cyclins comprise a group of proteins that are rapidly produced at key stages in the cell cycle. Once activated by a cyclin, CDK are enzymes that activate or inactivate other target molecules through phosphorylation. It is this precise regulation of proteins that triggers advancement through the cell cycle. Leland H. Hartwell, R. Timothy Hunt, and Paul M. Nurse won the 2001 Nobel Prize in Physiology or Medicine for their discovery of these critical proteins. Q: What are cyclins made up of? A:

Choose from: a). enzymes b). group of sugars c). steroids d). group of proteins
The answer is:
d).