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● In a DNA, two polynucleotide chains are coiled to form a helix.
Sugar­phosphate forms backbone of this helix while bases project in

wards to each other.

● Complementary bases pair with each other through hydrogen bond.
Purines always pair with their corresponding pyrimidines. Adenine

pairs with thymine through two hydrogen bonds while guanine pairs
with cytosine through three hydrogen bonds.

○ The helix is right­handed.
Pitch − 3.4 nm
10 bp in each turn
○ The plane of one base pair stacks over the other in a double
helix. This provides stability to the helix along with hydrogen
bonding.

Packaging of DNA Helix