Question

How is DNA protected from spontaneous and environmental damage, and how is damage, once inflicted, reversed?

Answer 1

Each cell has a proofreading mechanism by which it detects the damaged or mismatched base pair in the DNA strand. This proofreading process is done by DNA polymerase. In this process, the DNA polymerase reads the newly added base pairs and if a mismatch base pair is present it removes the incorrect base pair by making a cut at the phosphodiester bond. This removal process is done by the exonuclease action of DNA polymerase. Once the incorrect nucleotide has been removed, a new one will be added again.

Besides the inbuilt system, natural resources are also considered in protection against DNA damage as they are a rich source of bioactive components. These bioactive compounds contain antioxidant and DNA damage protection properties. one example is of Naringenin which is present in citrus fruits and protects plasmid DNA from UV-induced DNA damage.

There are various methods to reverse or repair the damaged DNA. Here I am gonna discuss the most important of them.

1. SOS repair system: In this type of repair system bacterial system induce the expression of certain genes that allows DNA replication to occur even in the absence of mismatch pair. Although this type of repair system is not good enough as it allows the insertion of mismatch and incorrect nucleotides.

2. Photoreactive repair: This type of repair system is used by the body to reverse the effect of the damage done by UV light. In this repair, system bacteria produces a protein called photoreactivation enzyme (PRE). This enzyme cleaves the bonds between thymine dimers (formed by UV light), thus directly reversing the effect of UV radiation on DNA.

3. Base excision repair (BER): In this system first the altered base is recognized by DNA glycosylase enzyme. This enzyme cuts the glycosidic bond between the bases and leaves an apurinic site. This site is then recognized by AP endonuclease. This endonuclease then makes a cut in the phosphodiester backbone. This enzyme then removes the deoxyribose sugar, and the gap is filled by DNA polymerase and DNA ligase.

4. Nucleotide excision repair( NER): This process is almost the same as the BEN. The only difference is that it can repair long and bulky lesions in the DNA strand.

5. Homologous recombination system: This type of repair system works when there is a double-strand break occurs in DNA strands. This type of system uses both DNA strands for replication. The first one strand will use the other strand as a template and replicate itself. After one stand is completely repaired it will serve as the template strand for another strand.