1. DNA Replication

2. DNA replication depends on specific base pairing – DNA replication Starts with the separation of DNA strands – Then enzymes use each strand as a template To assemble new nucleotides into complementary strands

3. DNA Replication Replication occurs simultaneously at many sites (replication bubbles) on a double helix  Allows DNA replication to occur in a shorter period of time

4. DNA Replication Process 1.Helicase unwinds the double helix to expose DNA nucleotides http://www.nature.com/nature/journal/v439/n7076/images/439542a-f1.2.jpg

5. DNA Replication Process 2.Primase lays down an RNA primer to provide a 3’ OH group http://www.nature.com/nature/journal/v439/n7076/images/439542a-f1.2.jpg

6. DNA Replication Process 3.DNA polymerase attaches complementary DNA nucleotides to the 3’ end of a growing daughter strand Can only add bases to the exposed 3’-OH group Therefore, DNA Replication always occurs in the 5’→ 3’ direction http://www.nature.com/nature/jour nal/v439/n7076/images/439542a- f1.2.jpg http://www.mun.ca/biochem/courses/3107/images/S tryer/Stryer_F31-23.jpg

7. DNA Replication Process 4.DNA polymerase then removes the RNA primer and replaces it with complementary DNA nucleotides 5.DNA Ligase creates a covalent bond between the DNA fragments http://www.nature.com/nature/jour nal/v439/n7076/images/439542a- f1.2.jpg http://porpax.bio.miami.edu/~cmallery/150/chemistry/sf3x14a.jpg

8. DNA Replication “Problem” DNA Polymerase can only replicate in the 5’→ 3’ direction One of the template strands would require replication in the 3’→ 5’ direction (WON’T WORK) So, one daughter strand is made continuously while the other strand is made in short pieces called Okazaki fragments Overall Direction of Replication-toward the replication fork

9. DNA Replication

10. Assures that daughter cells will carry the same genetic information as each other and as the parent cell.  Each daughter DNA has one old strand of DNA and one new strand of DNA  Semiconservative Replication

11. Checking for Errors 1/1,000,000,000 chance of an error in DNA replication – Can lead to mutations DNA polymerases have a “proofreading” role – Can only add nucleotide to a growing strand if the previous nucleotide is correctly paired to its complementary base If mistake happens, DNA polymerase backtracks, removes the incorrect nucleotide, and replaces it with the correct base