A study (in mice!) finds that the epigenetic effects of exercise resemble those of epigenetic reprogramming for longevity — Tags and transcripts DNA is the building block of life, and it is made up of four basic units or nucleotides: adenine, cytosine, guanine, and thymine (A, C, G, and T). Sequences of these ‘DNA letters’ code for proteins, which are essential for our bodies to function properly.