Science Common Sense
1639 - What is the historical development of DNA replication as a semi-conservative replication model
Imagine you have a big Lego bridge that you need to copy. There are three ways to do it:
- Conservative: Make a brand new bridge using all new Lego pieces.
- Semi-conservative: Break the old bridge into two halves, and then use each half to make a new bridge, using half-new Lego pieces.
- Dispersion: Break the old bridge into tiny pieces, mix them up, and then use those pieces to make two brand new bridges.
DNA replication is like copying a Lego bridge, but it's how cells make a copy of their DNA.
In the 1950s, Watson and Crick discovered the double helix structure of DNA. But they didn't know how cells make copies of it.
Meselson and Stahl (1957-1958) did an experiment to figure this out. They used a special kind of DNA that they could track. Here's what they did:
- They grew bacteria in a special nutrient that had a heavy version of a certain atom (called nitrogen).
- Then, they moved the bacteria to a nutrient that had a light version of that same atom.
- They let the bacteria grow for a while and then took a look at their DNA.
What they found was that the DNA of the bacteria had a mix of heavy and light atoms. This meant that the old DNA was being used as a template to make new DNA, and the new DNA was using half-old and half-new pieces. This is the semi-conservative model.
Their experiment proved that DNA replication works like the semi-conservative way of making a Lego bridge: break the old DNA into two halves, and use each half to make a new DNA, using half-new pieces.