Which characteristic of DNA packaging allows a gene to be turned on or off?

The winding of DNA around histones is a crucial characteristic that directly influences the regulation of gene expression. DNA is wrapped around histone proteins to form a complex called chromatin. The structure of chromatin can exist in two forms: euchromatin, which is loosely packed and accessible for transcription, and heterochromatin, which is tightly packed and generally not accessible for the transcription machinery.

When DNA is tightly wound around histones, the gene regions may be inaccessible to RNA polymerase and other transcription factors, effectively turning those genes off. Conversely, when DNA is loosely associated with histones, it allows access for the transcription machinery, enabling gene expression. This dynamic regulation of the structure of chromatin, therefore, plays a vital role in determining which genes are turned on or off in different cell types and under various conditions.

Understanding this concept is essential in explaining how epigenetic modifications, such as the addition or removal of chemical groups to histones, can further influence the state of gene expression.