1. DNA Structure & Replication AP Biology

2. What is a Nucleotide?

3. DNA exhibits complementary base pairing

4. The two strands of DNA are anti-parallel

5. The Mystery of DNA Replication Revealed by its Structure Complementary base pairing reveals a possible copying mechanism for genetic material—Watson & Crick

6. DNA Replication: Simple View

7. The Basic Principle: Base Pairing to a Template Strand Since the two strands of DNA are complementary, each strand acts as a template for building a new strand in replication In DNA replication, the parent molecule unwinds, and two new daughter strands are built based on base-pairing rules Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Animation: DNA Replication Overview Animation: DNA Replication Overview

8. Watson and Crick’s semiconservative model of replication predicts that when a double helix replicates, each daughter molecule will have one old strand (derived or “conserved” from the parent molecule) and one newly made strand Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

9. DNA Replication: A Closer Look The copying of DNA is remarkable in its speed and accuracy More than a dozen enzymes and other proteins participate in DNA replication Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

10. Fig. 16-12b 0.25 µm Origin of replicationDouble-stranded DNA molecule Parental (template) strand Daughter (new) strand Bubble Replication fork Two daughter DNA molecules (b) Origins of replication in eukaryotes Overview of DNA Replication

11. Step 1 Replication begins at special sites called origins of replication, where the two DNA strands are separated by proteins, opening up a replication “bubble” Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

12. Step 2 Replication proceeds in both directions from each origin, until the entire molecule is copied.

13. Detailed VIEW of DNA Replication

14. Enzymes & Proteins involved in DNA Replication Helicases: Untwist the double helix at the replication forks Single-strand binding protein: Binds to and stabilizes single-stranded DNA until it can be used as a template Topoisomerase: Corrects “overwinding” ahead of replication forks by breaking, swiveling, and rejoining DNA strands

15. Elongating a New DNA Strand DNA polymerase catalyzes the elongation of DNA at the replication fork DNA polymerase adds nucleotides to the growing end of the DNA strand

16. Antiparallel Elongation DNA polymerases add nucleotides only to the free 3  end of a growing strand; therefore, a new DNA strand can elongate only in the 5  to  3  direction Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

17. The “Leading Strand” is the strand of new DNA formed towards the replication fork. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Animation: Leading Strand Animation: Leading Strand

18. Lagging Strand The “Lagging Strand” is formed 5’ to 3’, in the direction away from the replication fork. The lagging strand does not elongate continuously, instead it is formed in a series of fragments called “Okazaki fragments”

19. Lagging Strand & Okazaki Fragments DNA ligase joins Okazaki fragments

20. DNA Replication Summary DNA Replication is a semi-conservative process Replication differs in the production of the leading and lagging strands and occurs bi- directionally http://www.mcb.harvard.edu/losick/images/trombonefinald.swf http://highered.mcgraw- hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120076/m icro04.swf::DNA%20Replication%20Forkhttp://highered.mcgraw- hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120076/m icro04.swf::DNA%20Replication%20Fork

21. DNA Proofreading & Repair DNA polymerases proofread newly made DNA, replacing any incorrect nucleotides In mismatch repair of DNA, repair enzymes correct errors in base pairing

22. DNA Proofreading & Repair In nucleotide excision repair, a nuclease cuts out and replaces damaged stretches of DNA Damage shown is a thymine dimer from UV Radiation

23. Replicating the Ends of DNA Molecules Linear (eukaryotic) DNA can’t be completely replicated by DNA polymerase b/c it only adds to the 3’ end of the molecule Result: DNA is shortened with each replication

25. Telomeres To the Rescue Eukaryotic chromosomal DNA molecules have at their ends nucleotide sequences called telomeres Telomeres do not prevent the shortening of DNA molecules, but they do postpone the erosion of genes near the ends of DNA molecules

26. Telomerase in Germ Cells Telomerase catalyzes the lengthening of telomeres in germ cells Telomeres in cancer cells are very short Some cancer cells have active telomerase…