* Proteins around which DNA is wrapped to form repeating units called nucleosomes

* Eight histones form a core histone octamer (two each of H2A, H2B, H3, and H4)

Nucleosomes

* Consist of a core octamer of histones with 200 bp DNA wrapped in 1.67 left-handed superhelical turns

* Histone tails extend from core and can interact with one another and with other molecules

Linker DNA

* DNA segments between adjacent nucleosomes

30-nm Fiber

* Histones and nucleosomes further condense to form 30-nm fiber

* Nucleosomes organized in a zig-zag pattern (solenoid model)

Loops and Domains

* Further levels of organization occur as 30-nm fibers fold into loops and domains

* Loops and domains may be attached to nuclear matrix (scaffold) at specific points called scaffold-attachment regions (SARs)

Chromosomes

* Most condensed level of organization

* Each chromosome represents single, highly packed DNA molecule containing many genes