Free Radical Damage
What Exactly Is a Free Radical?
A free radical is an especially reactive atom or group of atoms that has one or more unpaired electrons. If you remember your high school chemistry, you will remember that electrons come in pairs. When one electron is lost from that pair, it makes the atom "highly reactive" as it looks to replace that lost electron anywhere it can. In your body, those replacement electrons come from cells in your body--destroying those cells in the process. Free radicals put your body in a state of oxidative stress in which your body is no longer able to maintain a balance between the appearance of reactive oxygen species and its ability to detoxify those free radicals or to repair the resulting damage. That's why free radicals function as cellular killers that wreak havoc by damaging DNA, altering biochemical compounds, corroding cell membranes, and destroying cells outright. 
Aside from impaired energy production, damage to the Mitochondria leads also to increased production of toxic molecules called free radicals. Compounds called antioxidants act as free radical scavengers by initiating reactions that make free radicals non-toxic to cells. Evidence indicates that damage by free radicals is a contributing factor to the pathology of neurological diseases. If free radicals overwhelm the body's ability to regulate them, a condition known as oxidative stress ensues. Free radicals thus adversely alter lipids, proteins, and DNA and trigger a number of human diseases.Free radicals attack important macromolecules leading to cell damage and homeostatic disruption. Targets of free radicals include all kinds of molecules in the body. Among them, lipids, nucleic acids, and proteins are the major targets. Hence application of external source of antioxidants can assist in coping this oxidative stress.
Free radicals and other Reactive Oxygen Species (ROS) are derived either from normal essential metabolic processes in the human body or from external sources such as exposure to X-rays, ozone, cigarette smoking, air pollutants, and industrial chemicals. ROS induce DNA damage, as the reaction of free radicals with DNA includes strand break base modification and DNA protein cross-links. Numerous investigators have proposed participation of free radicals in carcinogenesis, mutation, and transformation; it is clear that their presence in biosystem could lead to mutation, transformation, and ultimately cancer.
Free Radicals and Antioxidants
Free radicals are atoms or molecules that are highly reactive with other cellular structures because they contain unpaired electrons. Free radicals are natural by-products of ongoing biochemical reactions in the body, including ordinary metabolic processes and immune system responses. Free radical-generating substances can be found in the food we eat, the drugs and medicines we take, the air we breathe, and the water we drink. These substances include fried foods, alcohol, tobacco smoke, pesticides, air pollutants, and many more. Free radicals can cause damage to parts of cells such as proteins, DNA, and cell membranes by stealing their electrons through a process called oxidation. (This is why free radical damage is also called “oxidative damage.”) When free radicals oxidize important components of the cell, those components lose their ability to function normally, and the accumulation of such damage may cause the cell to die. Numerous studies indicate that increased production of free radicals causes or accelerates nerve cell injury and leads to disease.
Antioxidants , also known as “free radical scavengers,” are compounds that either reduce the formation of free radicals or react with and neutralize them. Antioxidants often work by donating an electron to the free radical before it can oxidize other cell components. Once the electrons of the free radical are paired, the free radical is stabilized and becomes non-toxic to cells. Therapy aimed at increasing the availability of antioxidants in cells may be effective in preventing or slowing the course of neurological diseases. 
Oxidative stress refers to a pathological state that arises from the damage caused by free radicals (collectively known of as reactive oxygen species, ROS) when they interact with molecules in the body. These free radicals can damage any component of the cell such as the DNA, the proteins and the lipids and give rise to cancer and various other disease states such as heart disease and neurodegenerative disorders.
Oxidative damage to DNA
Many experiments clearly provide evidences that DNA and RNA are susceptible to oxidative damage. It has been reported that especially in aging and cancer, DNA is considered as a major target..