Semi conservative DNA replication

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• DNA replication occurs in preparation for mitosis, when a parent cell divides to produce two genetically identical daughter cells
• DNA replication occurs during the S phase of the cell cycle (which occurs during interphase, when a cell is not dividing)
• Semi-conservative DNA replication


  1. Hydrogen bonds between the base pairs on the two antiparallel polynucleotide DNA strands are broken

  2. This 'unzips' or unwinds the DNA double helix to form two single polynucleotide DNA strands

  3. Each of these single polynucleotide DNA strands acts as a template for the formation of a new strand

  4. The original strand and the new strand then join together to form a new DNA molecule
• Semi-conservative replication


  Half of the original DNA molecule is kept (conserved) in each of the two new DNA molecules
• Semi-conservative replication was shown to be the method of replication by Meselson and Stahl in 1958
• Nucleoside triphosphates


  Free nucleotides to which two extra phosphates have been added
• DNA replication by DNA polymerase

  1. Bases of the free nucleoside triphosphates align with their complementary bases on each of the template DNA strands

  2. DNA polymerase synthesises new DNA strands from the two template strands

  3. DNA polymerase catalyses condensation reactions between the deoxyribose sugar and phosphate groups of adjacent nucleotides within the new strands, creating the sugar-phosphate backbone of the new DNA strands

  4. DNA polymerase cleaves (breaks off) the two extra phosphates and uses the energy released to create the phosphodiester bonds (between adjacent nucleotides)

  5. Hydrogen bonds then form between the complementary base pairs of the template and new DNA strands
• Leading strand

  The template strand that the DNA polymerase attaches to and moves towards the replication fork
• Lagging strand


  The other template strand created during DNA replication, where DNA polymerase moves away from the replication fork
• Synthesis of the complementary strands on the leading and lagging template strands

  1. DNA polymerase can only build the new strand in one direction (5' to 3' direction)

  2. On the leading strand, DNA polymerase attaches to the 3' end of the original strand and moves towards the replication fork, allowing continuous synthesis

  3. On the lagging strand, DNA polymerase moves away from the replication fork, so it can only synthesise the lagging DNA strand in short segments (Okazaki fragments)

  4. DNA ligase is needed to join these lagging strand segments together to form a continuous complementary DNA strand
• Leading strand

  The template strand created during DNA replication
• Lagging strand


  The other template strand created during DNA replication
• DNA replication on the lagging strand


  DNA polymerase moves away from the replication fork (from the 5' end to the 3' end)
• Lagging DNA strand


  DNA polymerase can only synthesise it in short segments (called Okazaki fragments)
• DNA ligase


  The enzyme needed to join the lagging strand segments together to form a continuous complementary DNA strand
• Phosphodiester bonds


  The bonds that DNA ligase catalyses to create a continuous sugar-phosphate backbone
• The synthesis of the complementary strands occurs slightly differently on the leading and lagging template strands of the original DNA molecule that is being replicated