How DNA is packaged inside the eukaryotic cell

DNA packaging is a critical process that enables the lengthy eukaryotic DNA to fit inside the microscopic confines of the cell nucleus. In humans, each cell contains approximately 2 meters of DNA that must be efficiently condensed to fit within a nucleus of just 5-10 µm in diameter. This is achieved through multiple levels of folding, beginning with the formation of nucleosomes—the fundamental unit of chromatin—composed of DNA wrapped around histone proteins. Subsequent higher-order structures, such as 30 nm chromatin fibers and chromatids, contribute to the further compaction of DNA, ultimately forming chromosomes. This chapter explores the mechanisms of DNA packaging, highlighting the role of supercoiling, histone proteins, and non-histone proteins. Additionally, the models of chromatin fiber organization, including the solenoid and zigzag models, are discussed. The complexity of DNA packaging not only facilitates chromosome formation but also plays an essential role in regulating gene expression, replication, and transcription.