DNA structure: implications for chromatin structure and function

Abstract

Abstract DNA has a simple yet intricate structure that serves as the template for the assembly of a chromosome. Each chromosome has one DNA molecule, a long unbranched double helix consisting of two antiparallel polynucleotide chains. The structure is stabilized by base-stacking interactions and hydrogen bonding between the bases: G (guanine) being paired to C (cytosine) and A (adenine) to T (thymine). Physical studies using X-ray diffraction indicate that, under conditions of physiological ionic strength, DNA is a somewhat regular helix with a diameter of 2 nm, making a complete turn every 3.0 to 3.4 nm. This particular DNA structure (B form) has approximately 10.5 bp/tum of the helix, with a variation due to base sequence of 9.9 to 11.1 bp/tum. Every base pair is rotated approximately 34° around the axis of the helix relative to the next base pair, resulting in a twisting of the two polynucleotide strands around each other. A double helix is formed that has a minor groove (very approximately 1.2 nm across) and a major groove (very approximately 2.2 nm across). The geometry of the major and minor grooves of DNA is crucial in determining the interactions of proteins with the DNA backbone sugars and phosphates. The double helix is usually right handed.