Polarised light micrograph of crystalline cytosine, one of the four organic bases of DNA. Pairs of bases, connected by hydrogen bonds, join the two strands of nucleotides which comprise the DNA double-helix molecule. Of the four bases, adenine (A) is always paired with thymine (T) and cytosine (C) with guanine (G). It is a precise sequence of bases along a section of DNA that represents the genetic coding for a specific protein. Sections of DNA with such identifiable attributes are called genes. Magnification x40 at 4x5" size, x23 at 6x7mm size."