Oxygen poses a threat to cell structures because a particular form of oxygen, known as a free radical, is very reactive and can break down cellular molecules, according to Ohio State University. These free radicals are oxygen atoms with unpaired electrons, which are produced during metabolic processes in the mitochondria.
Oxygen is used by the cell's mitochondria at the end of a process known as the electron transport chain. The mitochondria use this process to create chemical energy usable by the cell. Usually, this process shifts electrons from one chemical to another, finally ending with oxygen. Oxygen atoms normally have all their electrons arranged in pairs, but occasionally this process is not completed successfully, leaving the oxygen with an unpaired electron.
The primary site of free radical damage to the cell is in the mitochondrial DNA. The mitochondria are organelles that have DNA separate from the DNA in the nucleus of the cell. This DNA gives chemical instructions to the mitochondria that enable them to function. There are many mechanisms to repair damage to nuclear DNA, but the mitochondria do not possess these mechanisms. Damage done to their DNA is generally permanent, and builds up until the mitochondria no longer function.