Variable Structure and Folding of DNA

• The three-dimensional, double-helix structure of DNA has numerous variations depending on the sequence of internal base pairs and the presence of interacting external molecules. • Methylation of DNA is a chemical modification of the cytosine and adenosine residues that results in changes in the control of gene expression and DNA imprinting. • Proteins interacting with DNA to control gene expression have characteristic structures and distort the double helix upon binding. • The folding of DNA into chromosomes involves the formation of nucleosomes, which consist of a core of four different histones with DNA wrapped around the outside. • Further folding involves the ordering of nucleosomes into fibers and chromatin domains by the formation of loops, which are attached at their bases to a nuclear scaffold. • Single chromosomes tend to form domains within the nucleus and have nuclear-membrane attachment sites.