During the hour or so between DNA replication and the beginning of mitosis, there is a period of very active repair and "Proof reading" of DNA stands. That is, wherever in appropriate DNA nucleotides have been matched up with the nucleotides of the origin template strand, special enzymes cut out the defective areas and replace these with the appropriate complimentary nucleotides. This is activated by the same DNA polymerizes and DNA ligase that are used in the process of replication. This repair process is referred to as DNA proof reading. Because of repair and proof reading, the transcription process almost never makes a mistake. But when a mistake is made, this is called MUTATION, it in turn will cause the formation of some abnormal protein in the cell, often leading to abnormal cellular function and some times even to death.[32]
A variety of agents in the cells environment, both chemical and physical, can damage DNA. Organisms have developed a variety of mechanisms for repairing copying errors produced by damaged DNA, usually by enzymatically excising them. The enzymes DNA polymerase then catalyzes the replacement of the excised segment with the correct nucleotides, using the undamaged DNA strand as a template. Eucaryotic cells have a greater variety of DNA repair mechanisms than to bacteria. Malfunctioning of the repair mechanisms can lead to genetic disease, abnormal function, or cancer, xeroderma pigmentosum, a lethal human disease that is recessively inherited, involves several defective repair mechanisms.[33]