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📘 A copied chromosome begins as one opened helix

At 3 a.m. inside a dividing cell, a DNA replication fork is already moving. Helicase separates the parental strands, and each exposed strand becomes a template for a new partner. The old DNA is not discarded; it guides the copy. Follow the

3
lessons
~15 min
to learn
Adults
level
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What you’ll learn

  1. The double helix opensExplain how replication origins, helicase, primers, and complementary base pairing prepare two templates for copying.Replication begins at selected origins. Helicase opens the parental duplex, stabilizing proteins hold the templates apart, and primase gives DNA polymerase a chemical starting point.
  2. One fork, two building stylesExplain why antiparallel DNA strands require continuous leading-strand synthesis and discontinuous lagging-strand synthesis.Because polymerase extends only 5-prime to 3-prime, one daughter strand follows the fork continuously while the other is assembled from repeatedly primed Okazaki fragments.
  3. Finishing and checking the copyTrace primer removal, ligation, proofreading, and chromosome-end management to the finished daughter molecules.Replication is completed by replacing temporary primers, sealing the backbone, checking new bases, and protecting the special ends of linear chromosomes.

Questions this course answers

Why does each daughter DNA molecule contain one old strand and one new strand?

The parental strands separate, and each one guides construction of a complementary partner, producing two mixed-age double helices.

Put these replication-fork events in the order they occur for a new lagging-strand fragment.

The lagging strand is repeatedly primed and extended, then its temporary RNA is replaced and the finished pieces are sealed together.

Match each replication protein or structure to its job.

The replication fork is a coordinated team: one component opens DNA, one starts synthesis, one extends the copy, and one joins finished pieces.

Explain why the leading and lagging strands are made differently even though they copy the same DNA molecule.

Opposite strand orientations collide with polymerase's one-direction rule, so one daughter strand is continuous and the other is assembled in pieces.

Grounded in trusted sources

  • NCBI Bookshelf, The Cell: DNA Replication, https://www.ncbi.nlm.nih.gov/books/NBK9940/
  • NCBI Bookshelf, Genome Replication, https://www.ncbi.nlm.nih.gov/books/NBK21113/
  • NCBI Bookshelf, Molecular Biology of the Cell: DNA Replication Mechanisms, https://www.ncbi.nlm.nih.gov/books/NBK26850/
  • NCBI/PMC, Okazaki Fragment Metabolism, https://pmc.ncbi.nlm.nih.gov/articles/PMC3552508/
  • NCBI/PMC, Starting DNA Synthesis: Initiation Processes during the Replication of Chromosomal DNA in Humans, https://pmc.ncbi.nlm.nih.gov/articles/PMC10969946/

Every Wunder lesson is built from real, reputable sources — never invented.

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