When the guanine tracts are oriented in opposite directions, the edgewise or diagonal loops link two antiparallel strands
to form an antiparallel G-quadruplex .
This regularity along two long antiparallel strands
leads to the famous, repeating "double helix" structure of DNA familiar to everyone.
The second a-sheet is composed of S5 (Arg161-Tyr163) and S6 (Glu167-Thr169) antiparallel strands
. The last [beta]-sheet of the domain I is built by S8 (Glu222-Arg229) and S9 (Thr23 1-Ile237) antiparallel strands