Conservative & Semi-Conservative

• Conservative replication results in the bonding pattern shown.
• Scientists determine that DNA replication is semi-conservative by observing the bonding pattern.
• Scientists determine that DNA replication is semi-conservative by separating the DNA double helix into two polynucleotide strands, each strand is then replicated into a complementary new strand.
• In semi-conservative replication, each of the two copies contains one strand from the original DNA molecule plus one new strand.
• Scientists use the fact that all bases in DNA contain the element nitrogen and that nitrogen atoms exist in two main forms, nitrogen 14 and nitrogen 15.
• Nitrogen 14 is the most common isotope with over 99% of nitrogen atoms being nitrogen 14.
• Atoms of nitrogen 15 are slightly heavier than atoms of nitrogen 14.
• In an experiment to determine whether DNA replicates by conservative or semi-conservative replication, scientists took a sample of bacteria and extracted the DNA.
• The extracted DNA was placed in a solution and spun at very high speeds in a centrifuge, forming a bond which the scientists could detect.
• The position of the DNA bond depends on how heavy the DNA is because the nitrogen atoms in this DNA were almost all nitrogen 14, representing light nitrogen.
• The scientists cultured the bacteria in a growth medium which contained only nitrogen 15.
• When bacteria are grown on nitrogen 14, their DNA forms a bond near the top of the tube.
• When bacteria are grown on nitrogen 15, their DNA forms a bond near the bottom of the tube.
• When bacteria are transferred from nitrogen 15 to nitrogen 14, their DNA forms a bond near the top of the tube.
• When bacteria are allowed to replicate on nitrogen 14, their DNA produces a bond in between the two bands produced before, indicating that the DNA contains one strand with nitrogen 14 and one strand with nitrogen 15.
• In semi-conservative replication, the DNA produced contains one strand from the original DNA plus one completely new strand, so the replicated DNA has one strand containing nitrogen 15 and one strand containing nitrogen 14.
• If dna replicates conservatively rather than semi-conservatively, after one round of replication, there would be one molecule of DNA containing two original strands and one molecule of DNA containing two new strands.
• After two rounds of replication, there would still be one molecule of DNA containing only nitrogen 15, but there would be three molecules of DNA containing only nitrogen 14.
• In a conservative replication, there would be no molecules of DNA containing both nitrogen 14 and nitrogen 15.