DNA accessibility refers to how easily the genetic material in a cell can be accessed by proteins and other molecules that regulate gene expression. DNA is tightly packed in a structure called chromatin, which can be either open or closed. When chromatin is open, genes can be easily read and expressed; when it is closed, access is restricted, preventing gene expression. Factors such as histone modifications and DNA methylation play crucial roles in determining DNA accessibility. These chemical changes can either promote or inhibit the opening of chromatin, influencing how genes are turned on or off in response to various signals and environmental conditions.