Semi Conservative Replication

• 1. The double helix is untwisted.
  2. The enzyme DNA helicase breaks the hydrogen bonds between the complementary nitrogenous bases, allowing the two strands to separate and expose the nucleotide bases.
  3. Both strands act as a template.
  4. Free complementary nucleotide bases hydrogen bond to the exposed bases according to the complementary base pairing rule.
  5. Phosphodiester covalent bonds are formed by a condensation reaction between nucleotides to join the sugar phosphate backbone using the enzyme DNA polymerase.
  6. This process is called semi-conservative replication.
• 1. The double helix is untwisted.
  2. The enzyme DNA helicase breaks the hydrogen bonds between the complementary nitrogenous bases, allowing the two strands to separate and expose the nucleotide bases.
  3. Both strands act as a template.
  4. Free complementary nucleotide bases hydrogen bond to the exposed bases according to the complementary base pairing rule.
  5. Phosphodiester covalent bonds are formed by a condensation reaction between nucleotides to join the sugar phosphate backbone using the enzyme DNA polymerase.
  6. This process is called semi-conservative replication.
• 1. The double helix is untwisted.
  2. The enzyme DNA helicase breaks the hydrogen bonds between the complementary nitrogenous bases, allowing the two strands to separate and expose the nucleotide bases.
  3. Both strands act as a template.
  4. Free complementary nucleotide bases hydrogen bond to the exposed bases according to the complementary base pairing rule.
  5. Phosphodiester covalent bonds are formed by a condensation reaction between nucleotides to join the sugar phosphate backbone using the enzyme DNA polymerase.
  6. This process is called semi-conservative replication.
• A DNA molecule consists of one 'conserved' old strand and one newly built strand.
• The process of semi-conservative replication was demonstrated by an experiment carried out by Meselsohn and Stahl. 
• • Meselsohn and Stahl firstly grew bacteria in a medium in which the only source of nitrogen was a heavy N 15 isotope. The DNA in the offspring of this bacteria was denser than usual.
  • The bacteria containing N 15 was transferred to a medium containing N 14 (light nitrogen) and was allowed to replicate for a few generations. Samples of bacteria were taken from each generation and centrifuged. The DNA formed visible bands in the tube.
  • Centrifugation separates the DNA off of the basis of mass. Heavier DNA will be lower down the tube.
• • Generation 1: One band of N 15 at the bottom of the tube
  • Generation 2: One band slightly higher in the tube (one N 15 strand and one N 14 strand)
  • Generation 3: One lighter band of N 14 higher up in the tube and one hybrid DNA band of N 15 and N 14.
• The conservative model of DNA replication was disproved by Meselson and Stahl. It stated that each replication of DNA would result in one DNA molecule consisting of both original strands of DNA and one molecule consisting of two entirely new strands of DNA.
• The dispersive model was disproved by the Meselson-Stahl experiment. It stated that each replication resulted in two molecules of DNA that are combinations of both original and new DNA.