CHROMOSOMAL ORGANIZATION Chapter 16, Section 3

Chromatin: DNA at its associated proteins. DNA, the Double Helix ■Sugar-phosphate backbones on the exterior (negative charge). ■Nitrogenous bases in the interior (hydrophobic interactions). ■Diameter: 2nm Histones ■100 amino acids ■Four types: H2A, H2B, H3, and H4 –Two copies of each form an octamer core particle. –Very similar in eukaryotes. ■Many histones are positively charged. DNA wrapped around histones (1.7 times per octamer). Histone octamer. DNA replication bubble.

Nucleosome: “beads on a string” ■Unfolded chromatin is 10 nm in diameter. ■Each “bead” is a nucleosome, the basic unit of DNA packing; the “string” between the DNA is called linker DNA. ■The N-terminus of each histone extends outward from the nucleosome. Nucleosome TEM (10nm in diameter) DNA wrapped around histones (1.7 times per octamer).

30nm Fiber ■Results from interactions between the histone tails of one nucleosome and the linker DNA and nucleosomes on either side. ■Histone H1 binds to the linker DNA and to the nucleosome core.

Looped Domains ■The 30nm fiber forms loops (looped domains) attached to a chromosome scaffold made of proteins. ■Scaffold is rich is one type of topoisomerase and H1. ■Looped domains may continue to coil and fold to further compact the chromatin to produce the characteristic chromosomes (700nm)