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The genetic information that governs all living organisms resides within the DNA strands of their chromosomes. A DNA molecule is a double‑helix composed of nucleotides—each nucleotide comprising a phosphate group, a deoxyribose sugar, and a nitrogenous base. Because the sugar‑phosphate backbone is asymmetrical, the two strands run in opposite directions.
The sugar‑phosphate backbone is built from a five‑carbon deoxyribose (C1′‑C5′). The 5′ carbon attaches to a phosphate group, while the 3′ carbon bears a hydroxyl group. When nucleotides link, the 5′ phosphate of one sugar covalently bonds to the 3′ hydroxyl of the next, establishing a continuous 5′→3′ strand. The complementary strand runs 3′→5′.
Base pairing follows strict rules: adenine pairs with thymine (A‑T) and cytosine pairs with guanine (C‑G). These pairs form the rungs of the helix.
During S‑phase, helicase unwinds the double helix, and DNA polymerase III (in prokaryotes) or DNA polymerase δ/ε (in eukaryotes) begins synthesis. Polymerases can only add nucleotides in the 5′→3′ direction, so the strand oriented 5′→3′—the leading strand—can be copied continuously as the replication fork progresses. The opposite strand, oriented 3′→5′, is the lagging strand and must be synthesized in short, reverse‑direction segments called Okazaki fragments.
Each fragment is initiated by a primer laid down by primase, then elongated by polymerase. When the replication fork advances, the next fragment is produced, and this cycle repeats until the entire lagging strand is completed.
Once all Okazaki fragments are formed, DNA ligase catalyzes the formation of phosphodiester bonds between adjacent 3′‑OH and 5′‑phosphate termini, yielding a continuous strand. The result is two identical double helices, each composed of one parental strand and one newly synthesized strand.
