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Copying the Master: DNA Replication

Before a cell divides it must duplicate its DNA exactly. See how the double helix unzips and each old strand templates a new one — the elegant trick called semiconservative replication.

Two strands that already know the answer

The double helix is two strands wound together, and they are not random partners. Wherever one strand has an A, the other has a T; wherever one has a G, the other has a C. This is base pairing, and it means the two strands carry the same information twice, written as mirror images. Each is the complementary strand of the other.

Here is the beautiful consequence: if you separate the two strands, each one alone is enough to rebuild its lost partner. You read off A and you know to add T across from it. The molecule contains its own backup. That single fact is why copying DNA is reliable enough to last billions of years of life.

Unzip, match, build

The whole job is called DNA replication, and the workhorse enzyme is DNA polymerase. It slides along an opened strand, reads each base, and snaps the matching nucleotide into place on the growing copy. The rules never change: A pairs with T, G pairs with C.

  1. Unzip. Helper proteins break the rungs between paired bases and open the helix into two single strands, like undoing a zipper.
  2. Match. Each exposed base calls for its one legal partner — A wants T, G wants C — so the sequence of the new strand is forced by the old one.
  3. Build. DNA polymerase links the incoming nucleotides into a continuous new strand and proofreads as it goes, catching most mistakes on the spot.
  4. Finish. Each old strand now sits paired with a brand-new partner, giving two double helices identical to the original.
SEMICONSERVATIVE REPLICATION — each daughter keeps one old strand

  PARENT helix          unzip            two NEW helices

  5'-A T G C C A-3'      5'-A T G C C A-3'  (old)
     | | | | | |   ───▶     | | | | | |
  3'-T A C G G T-5'        new ▶ T A C G G T-3'
                       
                          5'-A T G C C A-3'  ◀ new
                             | | | | | |
                          3'-T A C G G T-5'  (old)

  Each new helix = ONE old strand + ONE new strand.
  That half-old, half-new pattern is why it's called 'semiconservative'.
Why we say 'semiconservative': every copy conserves one original strand.

Half old, half new — and why that name

Notice what each finished helix is made of: one strand from the parent, one freshly built. The cell did not make a brand-new copy from scratch; it kept half of the original in each product. This is semiconservative replication — 'semi' for half, 'conservative' for keeping. Scientists confirmed it in the 1950s by labeling DNA with heavy atoms and watching the density of the copies fall exactly halfway, just as this model predicts.