2.3 DNA Conformations
DNA Structural Polymorphism
DNA can adopt different helical conformations depending on sequence, hydration, ionic conditions, and protein binding. The three main forms are A-DNA, B-DNA, and Z-DNA.
The Three Major Forms
A-DNA
- • Right-handed
- • 11 bp/turn
- • Rise: 2.6 Å
- • Wider, shorter helix
- • Dehydrated conditions
- • RNA-DNA hybrids
B-DNA
- • Right-handed
- • 10.5 bp/turn
- • Rise: 3.4 Å
- • Most common form
- • Physiological conditions
- • Watson-Crick model
Z-DNA
- • Left-handed!
- • 12 bp/turn
- • Rise: 3.7 Å
- • Zigzag backbone
- • Alternating purines/pyrimidines
- • Gene regulation role
Biological Significance
Z-DNA in Gene Regulation
Forms at transcriptionally active regions. May relieve torsional stress from RNA polymerase. Recognized by specific proteins (ADAR1, ZBP1).
A-form in RNA
Double-stranded RNA and RNA-DNA hybrids adopt A-form. Important for gene silencing (RNAi) and viral replication.