Old DNA strands stay whole when DNA is copied
- The idea
- Predict
- See it
- Why
- Check
The idea
“When DNA is copied, the old DNA gets broken up and mixed into every new strand.”
Each copy is half old, so it's easy to picture old bits stirred through it like paint. The old strands actually stay in one piece.
The short version
The mistake: When DNA is copied, the old DNA gets broken up and mixed into every new strand.
What's actually true: Each new DNA molecule is one whole old strand paired with one whole new strand, which is called semiconservative replication.
- Helicase unzips the helix. Each old strand is a template, and DNA polymerase builds a new strand along it by base pairing.
- So every new molecule is one old strand plus one new one. The old strand is never split between copies.
- Density experiments confirm it. Bacteria moved from heavy to light nitrogen had half their DNA fully light after two rounds. Patchwork would leave none fully light.
On the exam: Old strands stay whole. Two old strands means only two copies ever hold old DNA.