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

  

<<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)

  

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

  

<<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

  

<<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

  

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

  

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

  

<<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.