This biology tutorial is about the semiconservative model of DNA. Shortly after the double helix, nucleotide-pairing model of DNA was discovered, there was a question over how, exactly, DNA replicated.
Did an entire original strand act as a template for a completely new strand? Or was the DNA somehow broken up along the way?
Here's how Meselson and Stahl found an answer to the question:
My real purpose behind these biology tutorials is to help you understand the thought process and stages of discovery that lead to an accepted model or theory. Most textbooks do not do this.
You should never accept anything taught to you on faith alone. It is important to know how scientists arrive at their conclusions. This will help you to think clearly, and it will help you to arrive at your own conclusions as well.
Sometimes I'll show you a biology tutorial that is really just a statement of facts or a way to perform some kind of technique, such as plotting dihybrid crosses on a Punnet square.
But most of the time, it's important to understand how these "facts" were discovered.
You should always be asking yourself, "How?" and "Why?