Quadruple helix in DNA: G-tetrads

In this example, a single-stranded DNA turns over itself to form an association of 4 local strands in parallel, which while approaching form 3 guanosine tetrads. This kind of structure may be found specially in telomeres (with a repetitive sequence ttaggg).

DNA: 5′ .. .. 3′

just one tetrad of paired guanosines.

Potassium ions, K⁺, coordinated by the carbonyl groups in guanines, help stabilise the structure.

Structure of the guanosine tetrad:

  (animated scheme)

• Two Hoogsteen-type hydrogen bonds between each two guanines.
• Octaedral coordination of the monovalent ion (K⁺) with the O⁶ in guanines (4 from one tetrad and 4 from the next one)
• Four-chain structure of DNA when several consecutive tetrads are stacked.