drift and lead to the evolution of what is now referred to as ageing. Free radical theory Free radicals are reactive molecules produced by cellular and environmental processes, and can damage the elements of the cell such as the cell membrane and DNA and cause irreversible damage. The free-radical theory of aging proposes that this damage cumulatively degrades the biological function of cells and impacts the process of aging. The idea that free radicals are toxic agents was first proposed by Rebeca Gerschman and colleagues in 1945, but came to prominence in 1956, when Denham Harman proposed the free-radical theory of aging and even demonstrated that free radical reactions contribute to the degradation of biological systems. Oxidative damage of many types accumulate with age, such as oxidative stress that oxygen-free radicals, because the free radical theory of aging argues that aging results from the damage generated by reactive oxygen species (ROS). ROS are small, highly reactive, oxygen-containing molecules that can damage a complex of cellular components such as fat, proteins, or from DNA; they are naturally generated in small amounts during the body's metabolic reactions. These conditions become more common as humans grow older and include diseases related to aging, such as dementia, cancer and heart disease. Amount of free radicals in the cell can be reduced with help of antioxidants. But there's a problem that some free radicals are used by organism as signal molecules, and too active general reduction of free radicals causes to organism more harm than good. Some time ago idea of slowing aging using antioxidants were very popular but now high doses of antioxidants are considered harmful. At present some scientists try to invent approaches of local suppression of free radicals only in certain places of cells. Efficiency of such approach remains to be unclear, research is ongoing. DNA damage theories DNA damage has been one of the major causes in diseases related to aging. The stability of the genome is defined by the cells machinery of repair, damage tolerance, and checkpoint pathways that counteracts DNA damage. One hypothesis proposed by physicist Gioacchino Failla in 1958 is that damage accumulation to the DNA causes aging. The hypothesis was developed soon by physicist Leó Szilárd. This theory has changed over the years as new research has discovered new types of DNA damage and mutations, and several theories of aging argue that DNA damage with or without