Inside a cell, there is a complex interplay of molecular events of which DNA repair occupies a significant importance. If proteins or RNA molecules are damaged, they can be immediately replaced using the information encoded in the DNA. However, if a DNA is damaged, it is irreplaceable and repairing the damages of the DNA, therefore, becomes imperative for the sustainability of the cell. A DNA can be damaged by a variety of processes—some are spontaneous, some are catalysed by the environmental agents and some are caused during the process of replication. The chemistry of DNA damage is diverse and complex. The damaged DNA causes alteration in the structure and the properties of proteins or enzymes. This affects the trait or phenotype.
Every day, thousands of damages or DNA lesions occur inside the cell. The sources of damage can be either ‘endogenous’ or ‘exogenous’. The endogenous sources of the DNA damage include the by-products of normal physiological processes, including DNA mismatches, reactive oxygen species from oxygen metabolism and other lesions that block DNA replication and transcription. The exogenous sources of the DNA damage include ultraviolet light, which creates single-strand breaks in the DNA, ionizing radiation, which causes double-strand breaks, and environmental chemicals, which cause cancers (Figure 8.1). A variety of genetically inherited disorders such as xeroderma pigmentosum, Cockayne’s syndrome, trichothiodystophy and hereditary non-polyposis colon cancer (HNPCC) are linked with DNA damages. DNA damages not only cause diseases but are also implicated in aging.
Figure 8.1 Mutagens and DNA repair systems
