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Answer all the listed questions. Provided reasoning for the correct answer. Provide reasoning for incorrect answers as well.

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19) Which of the following is not generally associated with oxidative DNA damage?

a) build-up of wastes such as reactive oxygen species.

b) DNA alkylation of bases

c) breakage if phosphodiester bonds

d) deletions and translocations

Answer 1

Question 19: The correct answer is b) DNA alkylation of bases.

Explanation: DNA damage leads to mutation. Some DNA damage is caused by environmental factors, such as radiation. Mutagens are chemical agents that increase the mutation frequency and it also damages DNA. DNA is damaged by oxidation, alkylation, and radiation.

The build-up of reactive oxygen species (e.g. O₂^- , H₂O₂ and OH $\small \bullet$ ) can cause oxidative DNA damage. Ionizing radiation and chemical agents generate these potent oxidizing agents. These oxidizing agents generate free radicals. Oxidation of guanine generates 7,8-dihydro-8-oxoguanine, or oxoG. oxoG can base pairs with adenine as well as with cytosine. So, during replication, if oxoG base pairs adenine, it can give rise to a G:C to T:A transversion mutation. Therefore, the build-up of wastes such as reactive oxygen species is associated with oxidative DNA damage.

DNA is also subject to damage from alkylation. In alkylation, methyl or ethyl groups are transferred to reactive sites on the bases and to phosphates in the backbone of DNA. So, DNA alkylation of bases is not associated with oxidative DNA damage.

Ionizing radiation can exert its effect indirectly by generating reactive oxygen species, which, in turn, react with the DNA bases. These can lead to breakage of phosphodiester bonds and that causes breaks in DNA. These breaks in DNA, if unrepaired, can result in deletions and translocations. Therefore, breakage of phosphodiester bonds, as well as deletions and translocations, are associated with oxidative DNA damage.