The double helix of DNA is formed through a combination of base pairing, hydrogen bonding, and phosphodiester linkages. Here’s how it happens step by step:
1. Backbone Formation (Phosphodiester Bonds)
- DNA is made of nucleotides, each consisting of a phosphate group, a deoxyribose sugar, and a nitrogenous base (A, T, C, or G).
- Nucleotides link together through phosphodiester bonds, forming a sugar-phosphate backbone.
- These bonds occur between the 5' phosphate of one nucleotide and the 3' hydroxyl (-OH) of the next.
2. Base Pairing (Hydrogen Bonds)
- Two DNA strands run in opposite directions (antiparallel).
- The nitrogenous bases pair using complementary base pairing:
- Adenine (A) pairs with Thymine (T) through two hydrogen bonds.
- Cytosine (C) pairs with Guanine (G) through three hydrogen bonds.
- These hydrogen bonds help hold the two strands together.
3. Helical Twist (Stacking Interactions)
- The hydrophobic nature of the nitrogenous bases causes them to stack on top of each other, creating a twisted ladder shape.
- The hydrophilic sugar-phosphate backbone stays on the outside, interacting with water.
- The twisting is further stabilized by van der Waals forces between stacked base pairs.
4. Right-Handed Helix
- The natural form of DNA (B-DNA) twists into a right-handed helix due to structural constraints.
- One turn of the helix contains about 10 base pairs and spans about 3.4 nm in length.
This unique double-helical structure makes DNA stable and ideal for storing genetic information while allowing replication and transcription.
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