 DNA replication: DNA making DNA  Why must DNA replicate? › So that when a cell divides, each new cell gets a copy of the DNA  When does DNA replication take place? › During the S phase of interphase › When chromosomes go from individual chromosomes to double chromosomes

 DNA replication where the replicated DNA consists of one old strand and one newly made strand

 Origin of replication › Sites where replication of DNA begin  Replication fork/bubbles › Openings in DNA where replication takes place

 Occurs in the Nucleus 1. Helicase binds to the origin of replication to unwind and unzip DNA; Breaking what bond?  Hydrogen Bonds – helicase breaks hydrogen bonds 2. DNA polymerase brings in complementary nucleotides attaching them in a 5’ to 3’ direction (phosphate of new incoming nucleotide attaches to the 3’ last nucleotide in the strand

3. Ligase – joins sugars and phosphates of newly formed strands of DNA 4. DNA polymerase – proofreads DNA, checking of spelling errors  Replication ends with 2 identical molecules of DNA; each with 1 old strand and 1 new stand

 Direction of Replication › 5’ to 3’ – DNA polymerase needs to have an open 3’ carbon on deoxyribose so that it can add a new nucleotide › Leading vs Lagging  Leading strand is “continuously” replicated into the the replication fork runs 5’ to 3’  Lagging strand is “discountinuosly” replicated; out of the replication fork runs 3’ to 5’

 Okazaki Fragments - short stretches of nucleotides formed on the lagging strand of DNA › Why do we need Okazaki Fragments? › They are needed to get replication going in the correct direction on the lagging strand