DNA Damage: Sources and Types
The first source of damage is classified as endogenous damage. Endogenous damage occurs from reactions by reactive oxygen species, spurring from normal metabolic byproducts or spontaneous mutation, usually within the process of oxidative deamination. In other terms, the damage comes solely from within the origin of the cell. The reactive oxygen species are responsible for oxidative stress, which plays a critical role in damaging cell structures. Oxidative damage also has been linked to being a major contributor to the functional decline, also known as aging. These endogenous damages result in altering the DNA replication mechanism therefore creating a plethora of replication errors.
The second source of DNA damage is done through exogenous damage. Exogenous damage occurs through anomalies outside of the cell, which can affect the balance of the DNA. The disturbance comes from environmental factors such as UV radiation, x-rays and gamma rays, hydrolysis, and mutagenic chemicals. The UV radiation can be split into two classes, UV-A and UV-B. UV-A creates free radicals, considered indirect DNA damage. However, UV-B radiation creates pyrimidine dimers, directly damaging the DNA. Adjacent cytosine and thymine bases crosslink with one another to form these dimers. Hydrolysis damages the DNA by, the hydrolysis of bases. Deamination and depurination are categorized under hydrolysis of bases. Deamination is the removal of an amine group from a molecule while the latter is the removal of the purine base from the deoxyribose sugar. Synonymously, thermal disruption increases the rate of hydrolysis notably in depurination and single strand breaks. Dependent on the chemical, a plethora of DNA damages can be seen when exposed to harmful substances.
DNA damages are tangible deformities that can still be recognized by enzymes therefore certain repair measures can be taken. Excess, undamaged sequences in the compliment strands or in homologous chromosomes can be used to copy and rectify the damages. If the damage is still continued, ultimately translation/ replication will be prohibited and the cell will undergo apoptosis. DNA damages that are constant and unrepaired over generations, will eventually lead to replication errors and then mutation will ensue in which repair methods are obsolete.