Complex sentence: The possible letters are A, C, G, and T, representing the four nucleotide subunits of a DNA strand (adenine, cytosine, guanine, thymine), and typically these are printed abutting one another without gaps, as in the sequence AAAGTCTGAC.
Split into simpler sentences: The possible letters are A ",' C", "G", and "T", representing the four nucleotide subunits of a DNA strand-adenine, cytosine, guanine, thymine. Typically these are printed abutting one another without gaps, as in the sequence AAAGTCTGAC.