adenine
Every one of your ancestors
    cytosine
Survived to successfully reproduce
      guanine
The genes they passed to their descendants
        thymine

  guanine
Every one of your ancestors
    adenine
Bore the marks of many mutations in
      thymine
The genes they passed to their descendants
        cytosine

  thymine
Every one of your ancestors
    guanine
Bequeathed you a present most precious:
      cytosine
The genes they passed to their descendants
        adenine

  cytosine
Every one of your ancestors
    thymine
Existed and, even now, endures in
      adenine
The genes they passed to their descendants
        guanine

  adenine
Every one of your ancestors
    cytosine
Survived to successfully reproduce
      guanine
The genes they passed to their descendants
        thymine