DNA structure is dominated by the well-known double helix formed by Watson-Crick base-pairing of C
with G and A with T. This is known as B-form DNA, and is overwhelmingly the most favorable and
common state of DNA; its highly specific and stable base-pairing is the basis of reliable genetic
information storage. DNA can sometimes occur as single strands (often needing to be stabilized by
single-strand binding proteins) or as A-form or Z-form helices, and occasionally in more complex 3D
structures such as the crossover at Holliday junctions during DNA replication.
RNA, in contrast, forms large and complex 3D tertiary structures reminiscent of proteins, as well as
the loose single strands with locally folded regions that constitute messenger RNA molecules. Those
RNA structures contain many stretches of A-form double helix, connected into definite 3D
arrangements by single-stranded loops, bulges, and junctions.