1. Part 3 DNA Replication 24-1

2. Learning Targets 5. Understand base pairing and its role in replication, transcription and translation. 6. Understand that replication is semi-conservative and occurs on a lagging and leading strand. 7. Know the role of enzymes in replication including helicase, primase, DNA polymerase and ligase. 24-2

3. 24-3 DNA Replication Found only in the nucleus Origin of replication Multiple origins on a chromosome Occurs during S phase of the cell cycle Semiconservative  each new DNA strand has 1 new/1 old strand

4. DNA Replication Enzymes DNA helicase unzips DNA  unwinds it At “replication fork”  “unzipped” bases form H bonds with “free” nucleotides Primase assembles RNA Primer “seats” DNA Polymerase DNA polymerase bonds sugar to phosphate  makes new strand of DNA (DNA Ligase (see Okazaki Fragments)) 24-4

5. 24-5 Accuracy DNA Polymerase also “proofreads” Very accurate (only 1 error every billion nucleotides!) 20 repair enzymes “edit” goofs  mutagens (heat, chemicals, radiation, etc.) EXACT SEQUENCE is VERY IMPORTANT

6. 24-6 Okazaki fragments DNA is “anti-parallel” Sugar/phosphate groups “point” in diff. direction 5’  3’ “leading strand” 3’  5’ “lagging strand” DNA Polymerase only assembles new strand in 5’  3’ direction Leading strand is OK Lagging strand forms in segments  Okazaki fragments DNA Ligase ties sugar/phosphates together