The principle of heredity is the ability of the sequence of DNA bases to be copied faithfully to new daughter DNA polymers to preserve the genetic code in the next generation. In a manner somewhat analogous to RNA transcription, the enzyme DNA polymerase separates the double strands, and initiates two daughter strands, each copied from one of the parental strands. However, during meiosis, homologous DNA strands from different polymers exchange material at specialised crossover structures termed chiasmata. This process of DNA recombination also occurs in non-germ line cells during the rearrangement of immunoglobulin genes.DNA REPAIR
DNA is not an inert molecule, since the sequence of a single strand can alter after its synthesis-for example, as a result of spontaneous chemical changes, particularly if DNA is exposed to external radiation or chemical carcinogens . It is critical that such coding errors are not transmitted to the daughter strands when the DNA is replicated. The base-pairing of DNA provides an opportunity to recognise such events, since they will result in abnormal or mismatched base-pairings. If repair mechanisms fail, or if the mispaired daughter strands are immediately separated into single strands to be replicated themselves, such errors may persist, leading to a mutation. The mutation rate in genomic DNA (which is protected by the nucleus and chromatin) is as low as 10 to the power 9 to 10 to the power 12. The most frequent single-base mutation in humans is a C to T substitution, resulting from failure correct deaminated cytosine. If mutations occur in cells contributing to the germ line, this coding error will be transmitted to the next generation. More commonly,a new mutation will arise in a somatic cell,when transmission to the progeny of the particular cell only affects a proportion of cells in a given individual. Cells have complex detection and regulatory mechanisms to optimise DNA repair prior to DNA replication.
Although some initiators of DNA damage are naturally occurring, are unavoidable or carry a potential therapeutic benefit (e.g. cancer chemotherapy), others may be minimised. For example, the international efforts to reduce depletion of the ozone layer may reduce the consequences of terrestria ultraviolet (UV) light exposure.