Origins of Replication

- starting sequence (initiation codon) AUG	1.  Recognize and Separate
- replication fork (location of new DNA)	2.  Elongation catalyzed by polymerases along parental DNA (pDNA) from 3' to 5'
- nucleoside triphosphate, DNA + 3 phosphates	3.  OH bonds to first phosphates (closest to sugar) & pyrophosphate made along with a nucleotide joined to new DNA

ATP vs. NTP == ribose vs. deoxyribose

Anti-Parallel DNA strands

The difference between 5' and 3' is 5' is a phosphate end & 3' is a hydroxyl end.  The leading strand is always the 5' to 3' strand.  DNA polymerase can only synthesize from 3' to 5' along the pDNA strand and knestles in the fork.  Polymerase must work backwards from 3' to 5' away from the replication fork.  Thus it can only synthesize segments (Okazaki fragments) that will be joined together by ligase.

Priming DNA synthesis

A primer is a short stretch of RNA.  Polymerase cannort initiate a polynucleotide strand.  Primase must make a RNA primer that would be continued by polymerase as DNA.  Then the primer is hydrolyzed and the 2 segments join.  Helicase is the enzyme that "unzips" the helix.  Single-strand binding proteins (SSBP) stabilize the unpaired pDNA strands by binding to it in chains of SSBP.

Enzymes proofread DNA

• Mismatch repair
  1. In bacteria
          Polymerase backs up and replaces mistakes as it goes along the strand.
  2. In eukaryotes
          Several proteins are required.
               Colon cancer results from a hereditary defect in one of these proteins, resulting in unproofread errors.
• Excision repair
       Polymerase and ligase fill in the gaps after a protein cuts out the problem area.
            Skin cells exposed to UV radiation are repaired by this.

Next:  "Codons."