DNA Double Helix

Interactive 3D visualization of DNA double helix structure, base pairing, hydrogen bonds, and thermal denaturation

3D Double Helix Structure

Sugar-phosphate backbone, base pairs, hydrogen bonds, major/minor grooves

Helix Diameter: 2.0 nm
Helix Pitch: 3.4 nm/turn
Base Pairs: 20
GC Content: 50%
Melting Temperature T_m: 85.0°C

Legend

Sugar-Phosphate Backbone
Adenine (A)
Thymine (T)
Guanine (G)
Cytosine (C)
2 H-bonds (A-T)
3 H-bonds (G-C)

DNA Melting Curve

Fraction of single-stranded DNA vs Temperature

Current Temperature: 25.0°C
Denatured Fraction: 0.0%

DNA Sequence

5' to 3' direction, showing base pairs

5' → 3' Strand:
3' ← 5' Strand:

DNA Parameters

Structure

Temperature & Denaturation

Display Options

DNA Sequences

DNA Structure Formulas

Base Pairing: A-T (2 H-bonds), G-C (3 H-bonds)
Melting Temperature: T_m = 69.3 + 0.41×(%GC) - 650/length
Helix Dimensions: Diameter: 2.0 nm, Pitch: 3.4 nm/turn
Strand Direction: Antiparallel: 5'→3' opposite 3'←5'

What is DNA Double Helix?

The DNA double helix is the molecular structure of deoxyribonucleic acid (DNA), consisting of two complementary strands wound around each other in a spiral. Each strand is composed of a sugar-phosphate backbone with nitrogenous bases (adenine, thymine, guanine, cytosine) attached. The two strands are held together by hydrogen bonds between complementary base pairs: A pairs with T (2 hydrogen bonds), and G pairs with C (3 hydrogen bonds).

Double Helix Structure

The DNA double helix has a diameter of 2.0 nm and completes one full turn every 3.4 nm along its axis, containing approximately 10 base pairs per turn. The sugar-phosphate backbones form the outer framework, while the nitrogenous bases stack in the interior, perpendicular to the helix axis. This arrangement creates two grooves: the major groove (wide) and minor groove (narrow), which are important for protein binding. The two strands run in opposite directions (antiparallel): one strand runs 5'→3', while the complementary strand runs 3'→5'.

Base Pairing and Hydrogen Bonds

Complementary base pairing is fundamental to DNA structure and function. Adenine (A) always pairs with thymine (T) through two hydrogen bonds, while guanine (G) pairs with cytosine (C) through three hydrogen bonds. This specific pairing ensures accurate DNA replication and transcription. The G-C pair, with three hydrogen bonds, is more thermally stable than A-T, making GC-rich DNA sequences have higher melting temperatures.

DNA Denaturation and Melting

When DNA is heated, the hydrogen bonds between base pairs break, causing the double helix to separate into single strands. This process is called denaturation or melting. The melting temperature (T_m) is the temperature at which half of the DNA is denatured. T_m depends on DNA length and GC content: longer DNA and higher GC content result in higher T_m because more hydrogen bonds need to be broken. The process is reversible; when cooled, complementary strands can reanneal to reform the double helix.

DNA Replication

DNA replication is semi-conservative: when the double helix unwinds, each parental strand serves as a template for synthesizing a new complementary strand. This produces two daughter DNA molecules, each containing one original (parental) strand and one newly synthesized strand. The antiparallel nature of DNA strands is crucial for replication, with the leading strand synthesized continuously and the lagging strand synthesized discontinuously as Okazaki fragments.

Applications