<Li> Industrial chemicals such as vinyl chloride and hydrogen peroxide, and environmental chemicals such as polycyclic aromatic hydrocarbons found in smoke, soot and tar create a huge diversity of DNA adducts - ethenobases, oxidized bases, alkylated phosphotriesters and crosslinking of DNA, just to name a few . </Li> <P> UV damage, alkylation / methylation, X-ray damage and oxidative damage are examples of induced damage . Spontaneous damage can include the loss of a base, deamination, sugar ring puckering and tautomeric shift . </P> <P> In human cells, and eukaryotic cells in general, DNA is found in two cellular locations--inside the nucleus and inside the mitochondria . Nuclear DNA (nDNA) exists as chromatin during non-replicative stages of the cell cycle and is condensed into aggregate structures known as chromosomes during cell division . In either state the DNA is highly compacted and wound up around bead - like proteins called histones . Whenever a cell needs to express the genetic information encoded in its nDNA the required chromosomal region is unravelled, genes located therein are expressed, and then the region is condensed back to its resting conformation . Mitochondrial DNA (mtDNA) is located inside mitochondria organelles, exists in multiple copies, and is also tightly associated with a number of proteins to form a complex known as the nucleoid . Inside mitochondria, reactive oxygen species (ROS), or free radicals, byproducts of the constant production of adenosine triphosphate (ATP) via oxidative phosphorylation, create a highly oxidative environment that is known to damage mtDNA . A critical enzyme in counteracting the toxicity of these species is superoxide dismutase, which is present in both the mitochondria and cytoplasm of eukaryotic cells . </P> <P> Senescence, an irreversible process in which the cell no longer divides, is a protective response to the shortening of the chromosome ends . The telomeres are long regions of repetitive noncoding DNA that cap chromosomes and undergo partial degradation each time a cell undergoes division (see Hayflick limit). In contrast, quiescence is a reversible state of cellular dormancy that is unrelated to genome damage (see cell cycle). Senescence in cells may serve as a functional alternative to apoptosis in cases where the physical presence of a cell for spatial reasons is required by the organism, which serves as a "last resort" mechanism to prevent a cell with damaged DNA from replicating inappropriately in the absence of pro-growth cellular signaling . Unregulated cell division can lead to the formation of a tumor (see cancer), which is potentially lethal to an organism . Therefore, the induction of senescence and apoptosis is considered to be part of a strategy of protection against cancer . </P>

Which of the following causes mutations by creating thymine dimers