3.1.5.2 DNA replication

• DNA replication


  is a process where a DNA molecule, which has a paired base structure, can copy itself
• DNA copies itself before cell division so that


  each new cell has the full amount of DNA
• Semi-conservative replication


  ● DNA replication that ensures genetic continuity between generations of cells ● as half of the strands in each new DNA molecule are from the original DNA molecule— e.g the cells produced by cell division inherit their genes from their parent cells
• A DNA molecule has a paired base structure, which makes it


  easy for DNA to copy itself
• The 1st Step of DNA Replication

  ● The enzyme DNA helicase breaks the hydrogen bonds between bases on the two polynucleotide DNA strands — This makes the helix unwind to form two single strands

  
• The 2nd Step of DNA Replication

  ● Each original single strand acts as a template for a new strand● Complementary base pairing means that free-floating DNA nucleotides are attracted to their complementary exposed bases on each original template strand — A with T and C with G.

  
• The 3rd Step of DNA Replication

  ● Condensation reactions join the nucleotides of the new strand together — catalysed by the enzyme DNA polymerase● Hydrogen bonds form between the bases on the original and new strands

  
• Each new DNA molecule contains


  ● one strand from the original DNA molecule ● one new strand

  
• Each DNA strand has a directional structure due to


  the ends of each strand being either :● a sugar that’s attached to the 5th carbon 5’ (5 prime) end● or a hydroxyl group attached to the 3’ (3 prime) end

  
• Each end of a DNA strand is slightly different in its structure

  ● One end is called the 3’ — three prime end ● One end is called the 5’ — five prime end

  
• At the 3' end there is


  a hydroxyl group (OH) that is attached to the pentose sugar of the nucleotide

  
• At the 5' end there is


  a phosphate group

  
• During DNA replication, the active site of DNA polymerase is only complementary to the


  ●3’ end of the newly forming DNA strand — so the enzyme can only add nucleotides to the new strand at the 3’ end ●means that the new strand is made in a 5’ to 3’ direction ●DNA polymerase moves down the template strand in a 3’ to 5’ direction

  
• Because the strands in the double helix are antiparallel


  the DNA polymerase working on one of the template strands moves in the opposite direction to the DNA polymerase working on the other template strand

  
• DNA polymerase is only complimentary to the 3' end of the template strand so


  ● it can only move along the template strand and add nucleotides in the 3’ to 5’ direction ● means the new strand is built 5’ to 3’ — because the strands are antiparallel
• While one strand is continuously built the other is


  ● built in the other direction in sections as the DNA is unwound● the DNA polymerase on the opposite template strand has to detach and re-attach — often moves more slowly

  
• Meselson and Stahl showed


  DNA is replicated using the semi-conservative method
• experiment used two isotopes of nitrogen (DNA contains nitrogen)

  ● heavy nitrogen (¹⁵N)● light nitrogen (¹⁴N)
• Step 1 : Meselson and Stahl's experiment


  ● Two samples of bacteria were grown for many generations — one in a nutrient broth containing light nitrogen— one in a broth with heavy nitrogen● As the bacteria reproduced, took up nitrogen from broth to help make nucleotides for new DNA— nitrogen gradually became part of the bacteria’s DNA
• Step 2 : Meselson and Stahl's experiment


  ● sample of DNA was taken from each batch of bacteria● spun in a centrifuge● DNA from the heavy nitrogen bacteria settled lower down the centrifuge tube than the DNA from the light nitrogen bacteria — because it’s heavier

  
• Step 3 : Meselson and Stahl's experiment


  Then the bacteria grown in the heavy nitrogen broth were taken out and put in a broth containing only light nitrogen. The bacteria were left for one round of DNA replication, and then another DNA sample was taken out and spun in the centrifuge

  
• Step 4 : Meselson and Stahl's experiment

  ● If replication was conservative, the original heavy DNA (would still be together) — would settle at the bottom — the new light DNA would settle at the top
• Step 5 : Meselson and Stahl's experiment

  ● If replication was semi-conservative, the new bacterial DNA molecules would contain :● one strand of the old DNA containing heavy nitrogen and ● one strand of new DNA containing light nitrogen— So the DNA would settle out between where the light nitrogen DNA settled out and where the heavy nitrogen DNA settled out

  
• Step 6 : Meselson and Stahl's experiment


  ● As it turned out, the DNA settled out in the middle— showing that the DNA molecules contained a mixture of heavy and light nitrogen● The bacterial DNA had replicated semi-conservatively in the light nitrogen
• Once Meselson and Stahl had confirmed that DNA replication in bacteria was semi-conservative, other scientists carried out experiments to


  show that it was the universal method for DNA replication in all living things