Spontaneous chemical changes

DNA can undergo spontaneous changes in its chemistry that result in both
deletions and substitutions. DNA naturally loses purine bases at times in a
process called apurination. Most often, a purine’s lost when the bond
between adenine and the sugar, deoxyribose, is broken. (See Chapter 6 for a
reminder of what a nucleotide looks like.) When a purine is lost, replication
treats the spot occupied by the orphaned sugar as if it never contained a
base at all, resulting in a deletion.