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.
An 18th Century Persian astrolabe used for determining the time at both day and night. The points of the curved spikes on the front rete plate mark the positions of the brightest stars, the name of each star being labeled at the base of each spike. The back plate, or mater, is engraved with projected coordinate lines. From the Whipple Museum of the History of Science collection.