Introduction

After Watson and Crick published their paper proposing their double helix model of DNA in the journal Nature in 1953, the scientific community was then sure of the structure of DNA.  However there was still no conclusion as to how DNA replicates.  As we know, 'structure and function are correlated at all levels of biological organization'.  The beauty of the new model of DNA was that it's structure suggested the basic mechanism of its replication. 

The relationship between structure and function is certainly apparent in the double helix.  The idea of specific base pairing rules contributed to Watson and Crick's correct double helix model  (In other words, they saw structural significance of these rules).  Just the same, they also immediately saw the functional significance of the base pairing rules.  In fact, they ended their one page paper by stating, "it has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material."

In a second paper, Watson and Crick detailed their hypothesis for a semi-conservative model of DNA replication.   Semi-conservative was not the only model for DNA replication.  There were also two other hypotheses for the model of DNA replication.  One was a conservative model and the other was a dispersive model, proposed by Max Delbruck.

Hypotheses Details and Definitions

Semi-Conservative DNA Replication

The semi-conservative model proposed by Watson and Crick means that after each round of DNA replication, when a double helix replicates, each new daughter molecule consists of one old strand, which came from the parent DNA molecule, and one new complementary strand.

Definitions-2

Conservative Model for DNA Replication

The conservative model for DNA replication, which was proposed by David Bloch in Proceedings of the National Academy of Sciences of the United States of America in 1955, suggests that the parent molecule is somehow reformed after replication.  In other words, it is conserved or left intact, hence use of the word "conservative".  Also, a completely new daughter molecule is generated, composed of entirely new DNA.  In this model, the bases of both strands are exposed for hydrogen bonding as a result of histones bound to the DNA.

Definitions-3

Dispersive Model for DNA Replication

In a third model, the dispersive model proposed by Max Delbruck in Proceedings of the National Academy of Sciences of the United States of America in 1954 , following replication, both strands of each daughter molecule contains a mixture of new DNA and old DNA for the parent molecule.  Max Delbruck recognized three ways  of separating the daughter duplexes.  One is by slipping them past each other longitudinally, another by unwinding the two duplexes from each other, and a third by breaks and reunions.  He dismissed the first two possibilities as "too inelegant to be efficient".  That's why he proposed the dispersive model.