Question

How did the scientific community learn that DNA replication takes place in a semi-conservative fashion?

Short answer

The conclusion was reached because of Meselson's research with E. coli cultured in 15N.

Step-by-step solution

Introduction 

Each of the two strands that make up the double helix acts as a template from which new strands are copied during DNA replication. The parental or "old" strands will complement the new strands. The identical DNA copies that result from the formation of two daughter cells have the same sequence.

The explanation for the answer. 

The DNA extracted from cells cultured for two generations in 14N in Meselson's research with E. coli created two bands: one band was at the intersection of 15N and 14N, while the other band matched the band of 14N DNA. Only a semi-conservative replication of DNA may account for these findings. The other two models were consequently eliminated as a result of this.