Chapter 10: Molecular Structure of Chromosomes and Transposable Elements

In prokaryotes, DNA exists as a circular molecule within the nucleoid region, compacted through supercoiling and organized into topologically independent loop domains. DNA gyrase, a type II topoisomerase, introduces negative supercoils that reduce DNA volume and regulate transcriptional activity by controlling DNA accessibility. Topoisomerases I and II play critical roles in managing DNA topology during replication and transcription by relieving tension from unwinding and rewinding of the double helix. Eukaryotic chromosomes employ a more sophisticated packaging strategy built on the nucleosome as the fundamental repeating unit, in which approximately 147 base pairs wrap around an octamer containing two copies each of histone proteins H2A, H2B, H3, and H4. These nucleosomes form a 10-nanometer fiber with a beads-on-a-string appearance, which further compacts into a 30-nanometer structure through interactions with linker histone H1. Progressive levels of organization proceed through radial loop domains anchored to the nuclear matrix and scaffold proteins, ultimately generating the highly condensed metaphase chromosomes visible during cell division. The chapter distinguishes between euchromatin, transcriptionally active and loosely packed chromatin, and heterochromatin, transcriptionally silent and tightly condensed chromatin. Constitutive heterochromatin remains permanently condensed at regions such as centromeres and telomeres, while facultative heterochromatin can alternate between condensed and active states, exemplified by X chromosome inactivation in female mammals. Matrix attachment regions (MARs) and scaffold attachment regions (SARs) anchor chromatin loops and stabilize three-dimensional chromosome architecture within the nucleus. Chromosome territories organize the interphase nucleus, with positioning influencing gene expression patterns and genomic regulation. The balance between protection and accessibility is fundamental to chromosome function, and dysregulation of chromatin remodeling contributes to developmental abnormalities and malignant transformation in cancer cells.