Semi-conservative replication

• The leading strand grows continuously towards the replication fork
• WHY IS IT CALLED SEMI-CONSERVATIVE REPLICATION?
• DNA REPLICATES BY A SEMI-CONSERVATIVE METHOD

  1. Stage 3: Helicase enzymes separate the two DNA strands

  2. Stage 2: Each strand of the DNA molecule is kept apart during replication by single-strand binding proteins

  3. Stage 3: The formation of new strands is catalysed by DNA polymerase, which uses each parent strand as a template, reading the sequence of bases and adding complementary bases to form new strands

  4. Stage 4: DNA polymerase reads the parent strands in a 3' to 5' direction and builds the leading strand in a 5' to 3' direction
• EVIDENCE FOR SEMI-CONSERVATIVE REPLICATION
• It prevents the occurrence of spontaneous mutations
• One strand from the original DNA and one newly formed strand
• In 1958, Matthew Meselson and Franklin Stahl showed that DNA replicates semi-conservatively
• Eventually, all individual segments of the new strands meet up and are joined together, creating 2 new DNA molecules, each one identical to the parent molecule
• This meant that all their DNA contained the heavy nitrogen
• It ensures the same sequence of nucleotides is produced, therefore conserving genetic information
• In contrast, the lagging strand is built in a 3' to 5' direction
• Then they fed the bacteria lighter nitrogen
• They took a sample of these bacteria
• The lagging strand grows discontinuously from the fork
• There are multiple replication forks along a single DNA molecule, which speeds up replication as replication occurs in both directions
• Finally, the bacteria were fed more lighter nitrogen, allowed time to replicate, and then another sample was taken
• Stage 1: The bacteria Escherichia coli were grown in a medium containing heavy nitrogen
• All the bases in DNA contain nitrogen
• Meselson and Stahl conducted an investigation into DNA replication


  1958
• Light bacteria will incorporate nitrogen from their growing medium into the new DNA they make
• The bacteria were allowed time to replicate. Then another sample was taken
• WHY IS COMPLEMENTARY BASE PAIRING NEEDED?
• Heavy bacteria incorporate nitrogen from their growing medium into the new DNA they make
• Each original strand acts as a template for a new strand
• Stage 1

  The bacteria Escherichia coli grown in a medium containing a heavy isotope undergoes several generations
• Stage 5


  The cells are left to grow and divide, and after a certain time, a sample is taken
• Stage 2

  The isotope will be incorporated into the bases A, G, T, and C and therefore incorporated into new DNA
• Stage 3

  A sample of the E. coli grown purely in N is saved for later
• Stage 7

  The DNA from two samples is added to a solution of cesium chloride
• Stage 6

  After the E. coli have divided, a second sample is taken
• Stage 4


  New DNA will be more other than N in the first generation
• Stage 9


  Comparing the position of the bands from the different E. coli samples provides strong evidence for the semi-conservative method of DNA replication
• Bacteria were fed more 'Sight' nogen
• Mother Messbon and Hardin State American biologists showed that DNA replicated semi-conservatively


  1958
• Sample was taken
• Stage 8


  The tubes are spun at high speed in a centrifuge, and the DNA collects in a band