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.