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Natural shield protects certain DNA parts from radiation

Date: 10 November 2019 Tags: Miscellaneous

Issue

A study by researchers at the Indian Institute of Science (IISc) Bengaluru have shown that regions of the genome rich in four-stranded DNA made of guanine nucleotide base , G-quadruplexes (G4-DNA) , are more resistant to irradiation.

 

Background

It is well known that ionizing radiation can break the double-stranded DNA in one or both the strands. The research showed that there are fewer DNA breaks seen in G-quadruplexes when exposed to radiation.

 

Details

  • The lower sensitivity to radiation was seen in studies carried out in vitro and inside cells.

  • The researchers observed that contrary to the general notion that radiation-induced DNA breaks are random in nature and can occur throughout the genome, the breaks are sequence-dependent.

  • Certain regions of the genome were found to be resistant to radiation with fewer strand breaks in the DNA, and these regions are rich in G-quadruplexes.

  • G-quadruplexes typically consist of three-guanine nucleotide base found together and repeated four times.

  • When a guanine nucleotide gets repeated it tends to fold itself into a four-stranded DNA.

  • There are 3.5 to 7 lakh G-quadruplexes in the human genome, and these are found in certain regions of the genome such as the telomeres that act as caps on either end of the chromosomes.

  • During study, when single DNA strands made entirely of one of the four nucleotides , adenine, cytosine, guanine, or thymine , were exposed to gamma radiation, all except the strand made of guanine were sensitive to radiation.

  • In the case of a single DNA strand containing only thymine in one half and guanine in the other half, the guanine half alone showed better resistance to radiation.

  • Guanine loses its resistance when paired into double-strands and exposed to radiation. This showed that guanine was resistant to radiation when present in a single strand but becoming sensitive to radiation when present in a double-strand form.

  • When the researchers tested the radiation resistance of G-quadruplex inside cells, they found that there were fewer DNA breaks in the G-quadruplex present in telomeres compared with centromere [another part of the chromosome]. This suggests that G-quadruplex offers radioprotection inside the cell too.