refers to a collection of processes by which a cell
identifies and corrects damage to the DNA
molecules that encode its genome
. In human cells, both normal metabolic
activities and environmental factors such as UV
light can cause DNA damage, resulting in as many as 1 million individual molecular lesions
per cell per day. Many of these lesions cause structural damage to the DNA molecule and can alter or eliminate the cell's ability to transcribe
that the affected DNA encodes. Other lesions induce potentially harmful mutations
in the cell's genome, which affect the survival of its daughter cells after it undergoes mitosis
. Consequently, the DNA repair process must be constantly active so it can respond rapidly to any damage in the DNA structure.
The rate of DNA repair is dependent on many factors, including the cell type, the age of the cell, and the extracellular environment. A cell that has accumulated a large amount of DNA damage, or one that no longer effectively repairs damage incurred by its DNA, can enter one of three possible states: an irreversible state of dormancy, known as senescence; cell suicide, also known as apoptosis or programmed cell death; or unregulated cell division, which can lead to the formation of a tumor that is cancerous.
Rust is the chemical substance formed when iron compounds corrode in the presence of oxygen and water. It is a mixture of iron oxides and hydroxides. Rusting is a common term for corrosion, and usually corrosion of steel. Iron is found naturally in the ore haematite as iron oxide, and metallic iron tends to return to a similar state when exposed to air, (hydrogen, oxygen, nitrogen, etc.) and water. This corrosion is due to the oxidation reaction when iron metal returns to an energetically favourable state. Energy is given off when rust forms. The process of rusting can be summarized as three basic stages: The formation of iron(II) ions from the metal; the formation of hydroxide ions; and their reaction together, with the addition of oxygen, to create rust.