6.4 DNA Replication

<<Semi Conservative Replication<<

  • ^^Matthew Meselon and Franklin Stahl^^ in 1958

  • DNA is double stranded

  • 2 parent strands are seperated

  • complementary strand is made from each parent strand

  • results in 2 double stranded molecules, each containing one old strand and one new strand

    Semi Conservative Replication

<<Step 1: Separation<<

  • Strands are unwound from ^^replication origins^^ by %%DNA helicase%%==(1)==.

    • Forms a ^^replication fork^^

  • %%Topoisomerases%%==(2)== cut and rejoin strands to keep them from tangling (supercoils)

    Topoisomerase

  • %%Single-strand binding proteins%%==(3)== attach to unwound strands to prevent them from rejoining (annealing)

    DNA separation

<<Replication Bubbles<<

  • because there are many replications going on, especially in Eukaryotic DNA

  • Helicase unwinds DNA in both directions from each origin, producing replication bubbles

  • Complementary strands are synthesized as the forks continue (step 2)

  • Bubbles meet and merge, producing 2 separate daughter strands

    Replication Bubbles

<<Step 2: Synthesis / Replication<<

  • %%Polymerases%% build complementary strands in 5’ → 3’ direction

    • can only add to 3’ carbon
    • which means its reading 5’
    • new strands are always 5’ → 3’

  • Nucleotides are added as ^^nucleoside triphosphates^^

    • a building block and energy source for replicating DNA
    • very similar to nucleotides in the finished DNA
    • DNA phosphate needs energy

  • Hydrolysis of extra phosphates provides energy for synthesis (from ATP)

  • makes the pair grow on the 3 prime side w ATP

  • the extra %%phosphates%% make up the phosphodiesther bonds on the sides

  • the released energy drives DNA synthesis

    Replication with Polymerase

RNA Primase

  • %%RNA primase%% ==(4)==builds small complementary RNA primers at the replication fork on its own, these short pieces are called ^^RNA primers^^

  • %%DNA polymerase III%% ==(5)== ( first one, last one discovered) synthesizes complementary strands in opposite directions (5’ → 3’), can only add to previously existing nucleotides

    DNA polymerase & RNA primase working

Leading & Lagging Strands

  • Leading strand is elongated toward the fork; efficient (5’)

  • ^^Lagging strand^^ is elongated away from the fork, requires multiple RNA primers: constantly being synthesized backwards (3’)

    • Results in segments of RNA primers & DNA called ^^Okazaki fragments^^
    • %%DNA polymerase I%% ==(6)== replaces RNA primers with DNA - purifies
    • %%DNA ligase%% ==(7)== joins Okazaki fragments into one continuous strand

    Leading and Lagging Strands

<<Step 3: Repair<<

  • Mismatched base pairs don’t bond properly and distort shape of the DNA molecule
  • %%DNA polymerase II%% ==(8)== and other enzymes search for distortions, removes portion of the stand with mismatch and fills in the gap.