We employ a large scale molecular simulation based on bond-order ReaxFF to simulate the chemical reaction and study the damage to a large fragment of DNA molecule in the solution by ionizing radiation. We illustrate that the randomly distributed clusters of diatomic OH radicals that are primary products of megavoltage ionizing radiation in water-based systems are the main source of hydrogen abstraction as well as formation of carbonyl and hydroxyl groups in the sugar moiety that create holes in the sugar rings. These holes grow up slowly between DNA bases and DNA backbone, and the damage collectively propagates to a DNA single and double strand break.