DNA is chemically stable; more stable than RNA22. Large genomes could not be made of RNA because they would degenerate far too quickly. Some viruses (HIV is the best-known example) have "genomes" made of RNA rather than DNA, but (a) viral "genomes" are very much smaller than organism genomes, and (b) RNA viruses are notorious for their capacity to mutate rapidly.
Despite its chemical stability, DNA remains vulnerable to damage, particularly from the products of oxygen metabolism and radiation. Cells contain chemical protection systems such as anti-oxidants that eliminate these damaging metabolites before they do too much damage to the DNA. Limited damage can be repaired.
In addition, the genome is attended by a coterie of molecular "maintenance mechanics" that scan the DNA for damage, mend it, and keep the DNA in working order. These "maintenance mechanics" are enzymes. They detect "bumps" in the double helix that result from copying errors, then they carry out surgery: they cut out the incorrect piece of the molecule and substitute the right one. If the maintenance crew fails in its duty, misprints accumulate in the library's master documents and the cell starts to malfunction.
When we age, the cell's chemical protection systems become less effective and the maintenance crews cannot keep pace with the increased work-load. Errors accumulate in the DNA, cells malfunction and sometimes die, cell-cell signalling becomes abnormal, tissues degenerate, and cancers begin. But ageing is not the only way of inflicting more damage than the maintenance crews can handle. The cumulative effect of environmental factors, such as radiation and toxic chemicals, will eventually alter our DNA, causing cancers and other disorders. Interestingly, anti-oxidants provide some protection against the effects of (for instance) radiation damage.
22 Explaining the difference involves some rather complicated organic chemistry, but essentially it involves the pentose sugar unit in the sugar-phosphate backbone of the nucleic acid. In RNA there is a hydroxyl group at the 2' position of the sugar (ribose) and this hydroxyl group can attack the sugar-phosphate bonds, especially in alkaline solution, breaking the "popper-bead necklace" chain. In DNA, the sugar (deoxyribose) lacks this hydroxyl group so there is nothing to attack the bonds. While this difference confers much greater chemical stability on DNA, it also confers chemical activity on RNA; some important enzymes consist of RNA rather than protein.
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