Brent Cornell

    
DNA replication is a semi-conservative process, because when a new double-stranded DNA molecule is formed:

• One strand will be from the original template molecule
• One strand will be newly synthesised

Semi-Conservative DNA Molecule

This occurs because each nitrogenous base can only pair with its complementary partner

• Adenine (A) pairs with thymine (T)
• Cytosine (C) pairs with guanine (G)
  

Consequently, when DNA is replicated by the combined action of helicase and DNA polymerase:

• Each new strand formed will be identical to the original strand separated from the template
• The two semi-conservative molecules formed will have an identical base sequence to the original molecule

Conservation of Sequence by Complementary Base Pairing

    
The theory that DNA replication was semi-conservative was confirmed by the Meselson-Stahl experiment in 1958

Prior to this experiment, three hypotheses had been proposed for the method of replication of DNA:

• Conservative Model – An entirely new molecule is synthesised from a DNA template (which remains unaltered)
• Semi-Conservative Model – Each new molecule consists of one newly synthesised strand and one template strand
• Dispersive Model – New molecules are made of segments of new and old DNA

Three Proposed Models of DNA Replication

Meselson and Stahl were able to experimentally test the validity of these three models using radioactive isotopes of nitrogen

• Nitrogen is a key component of DNA and can exist as a heavier ¹⁵N or a lighter ¹⁴N 

DNA molecules were prepared using the heavier ¹⁵N and then induced to replicate in the presence of the lighter ¹⁴N

• DNA samples were then separated via centrifugation to determine the composition of DNA in the replicated molecules

The results after two divisions supported the semi-conservative model of DNA replication

• After one division, DNA molecules were found to contain a mix of ¹⁵N and ¹⁴N, disproving the conservative model
• After two divisions, some molecules of DNA were found to consist solely of ¹⁴N, disproving the dispersive model

Results of the Meselson-Stahl Experiment