On the right is the DNA double helix. Each side is a sugar–phosphate backbone; between them the bases reach in and pair up — the rungs of a twisted ladder. Notice the strict pairing: A–T (blue–gold) and G–C (green–red). The two backbones run in opposite directions — they are antiparallel. ▶ Play to let the helix turn, then ⏭ Step a rung at a time.
Stay on this tab and look at the flattened ladder view: each rung is one base pair, joined by weak hydrogen bonds — count them: 2 bonds for A–T, 3 bonds for G–C. Because A only ever pairs with T and G only with C, the two strands are complementary: read one strand and you know the other. ⏭ Step to add the rungs one pair at a time and read off the sequence.
Switch to the Replication tab. The enzyme helicase breaks the hydrogen bonds and unzips the double helix, opening a Y-shaped replication fork. Each old strand is now exposed as a template. ▶ Play and watch the fork travel along, splitting the parent molecule into two single strands ready to be copied.
Now watch free nucleotides float in and pair onto each exposed template by the rule — A with T, G with C. DNA polymerase links them into a new sugar–phosphate backbone (the leading strand runs continuously, the lagging strand in short pieces). The result: two daughter helices, each with one old and one new strand — that is semi-conservative. ⏭ Step to add nucleotides one at a time.
Open the Transcription & translation tab. First, transcription: the gene unzips and RNA polymerase reads the template strand, building a single strand of mRNA by the pairing rule — but RNA uses U (uracil) opposite A. ▶ Play to watch the mRNA grow base by base, then peel off and head out of the nucleus toward the ribosome.
Now translation at the ribosome. The mRNA is read in triplets called codons. For each codon a tRNA arrives whose anticodon is complementary, carrying the matching amino acid. The ribosome links each amino acid to the last by a peptide bond, so the polypeptide grows. ▶ Play to slide the ribosome along, codon by codon, and watch the protein chain build to the stop codon.