DNA REPLICATION Three theories were suggested Conservative replication

DNA REPLICATION Three theories were suggested: Conservative replication a Dispersive replication produce two DNA molecules with sections of both old and new DNA interspersed along each strand. Semi-conservative replication produce molecules with both old and new DNA - each molecule would be composed of one old strand one new one.

DNA Replication is semi-conservative Experimental Proof (1957) Mathew Meselson and Franklin Stahl grew the bacterium Escherichia coli on medium that contained 15 N in the form of ammonium chloride The 15 N became incorporated into DNA (nitrogenous bases). The resulting heavy nitrogen-containing DNA molecules were extracted from some of the cells.

When subject to density gradient centrifugation, they accumulated in the high-density region of the gradient. The rest of the bacteria were transferred to a new growth medium in which ammonium chloride contained the naturally abundant, lighter 14 N isotope.

The newly synthesized strands were expected to be less dense since they incorporated bases containing the lighter 14 N isotope. The DNA from cells isolated after one generation had an intermediate density, indicating that they contained half as many 15 N isotope as the parent DNA. This finding supported the semi-conservative model - each double helix would contain one previously synthesized strand a newly synthesized strand.

The nucleotides lining up by complementary base pairing are deoxynucleoside triphosphates As the phosphodiester bond forms between the 5' phosphate group of the new nucleotide and the 3' OH of the last nucleotide in the DNA strand, two of the phosphates are removed providing energy for bonding

DNA polymerase enzymes are only able to join the phosphate group at the 5' carbon of a new nucleotide to the hydroxyl (OH) group of the 3' carbon of a nucleotide already in the chain. As a result, DNA can only be synthesized in a 5' to 3' direction while copying a parent strand running in a 3' to 5' direction

Bacteria have 5 known DNA polymerases: Pol I: DNA repair has 5'→ 3' (Polymerase) activity both 3' → 5' (proof reading) and 5' → 3' exonuclease activity (in removing RNA primers). Pol II: involved in repair of damaged DNA has 3' → 5' exonuclease activity Proof that this is not the main polymerase Pol III: the main polymerase in bacteria (elongates in DNA replication) → has 3' 5' exonuclease proofreading ability. is the principal replicative enzyme is highly processive 2. catalyses polymerization at a high rate.