PRIMER DNA is not free in the nucleus; it is bound by the proteins that package it. This fact is important because DNA must be accommodated in a small place. Also, as we will see in Chapter 6, the packaging provides a level of transcriptional control. In this chapter, the goal is to become familiar with the proteins that are involved in packaging and their effects on DNA. The first part of the chapter examines the 3-D structure of the nucleosome and its components, the histones. This structure is the highest order of DNA packaging. Other proteins (nonhistone), however, have the ability to bend DNA and, therefore, provide another degree of organization. Such architectural proteins are important because linear DNA is not very efficient when it comes to regulation. Regulatory elements are often separated by long distances, and they must come close for interaction. Also, bending DNA makes it more accessible for interactions with proteins. These nonhistone proteins possess different structural motifs that mediate interaction with DNA. Their structural characteristics and effects on DNA are presented. The importance of these architectural proteins is also stressed in later chapters.