Bond View
Tap Coding / Non-Template Strand
10.5bp / turn
34 Åpitch
3.4 Årise / bp
5′
3′
3′
5′
5′→3′ Backbone
3′→5′ Backbone
DNA: A – T
(2 H-bonds)
│
mRNA: A – U
(2 H-bonds)
DNA: G – C
(3 H-bonds)
│
mRNA: G – C
(3 H-bonds)
ℹStructure
🧬 Double Helix
DNA is a right-handed double helix — two antiparallel
strands wound around a central axis like a twisted ladder. Discovered by
Watson & Crick in 1953 using Franklin's X-ray data.
↕ Antiparallel Strands
One strand runs 5′ → 3′ and the complementary strand
runs 3′ → 5′. They always run in opposite directions —
this is essential for DNA replication and transcription.
🔗 Base Pairing Rules
Only specific bases can pair (complementary base pairing):
Adenine (A)
⟷ Thymine (T)
·· 2 H-bonds
Guanine (G)
⟷ Cytosine (C)
··· 3 H-bonds
⚖ Chargaff's Rule
In any DNA sample: %A = %T and
%G = %C. The ratio of purines to pyrimidines
is always exactly 1:1.
⬡ Purine vs Pyrimidine
Purine
Adenine & Guanine
double ring structure
double ring structure
Pyrimidine
Thymine & Cytosine
single ring structure
single ring structure