BIOLOGY · MOLECULAR GENETICS

DNA structure and replication

Exploration tips

Structure

Move from Double helix to Base pair and One nucleotide. Tap a component for details, then Inspect close-up when needed.

Open DNA replication and trace an old strand or select an active enzyme. Drag the model to stop fork-following camera movement.

Switch to Practice. Predict before submitting, then compare with the model.

Preparing the 3D scene…

ATGCSimplified model

Sample sequence

A 24-base-pair illustration. Tap a component or select it below.

Strand to inspectNucleotide position

Follow one sugar–phosphate backbone from one end to the other. Both strands coil around a common axis.

What is it?: Nucleotide
Where does it connect?: Part of a strand, with its base facing the complement.
What does it do?: A DNA building block contains deoxyribose sugar, a phosphate group and one A, T, G or C base.

Old strand · 1 · position 12

Hydrogen bonds

Model notes

B-DNA is right-handed, with about 10.5 base pairs per turn and major and minor grooves between its sugar–phosphate backbones. This is idealized geometry, not an atomic model. Replication opens out one fork for clarity; the viewing-window ends lie within an internal DNA segment.
The 24 base pairs, two-nucleotide primers and six-nucleotide Okazaki fragments are shortened for visibility. Enzyme sizes, distances, counts and playback speed are not to scale.
Steps are separated for learning. In cells, unwinding, priming, elongation and fragment processing overlap. Primer replacement at a viewing boundary uses adjacent DNA outside the window.
The model is error-free and omits proofreading, supercoiling, single-strand binding proteins, telomeres and organism-specific enzyme machinery. Strand direction and complementary pairing remain consistent.

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