1. DNA STRUCTURE, REPLICATION, AND REPAIR

2. DNA-Structure  Double helix Hydrophobic nitrogen bases on the inside Hydrophilic phosphate-sugar backbones on outside Nitrogenous bases pair in specific manner  Purine pairs with pyrimidine to make “rungs” of the ladder equal widths

3. DNA-Structure

5. DNA structure

6. DNA-Structure  Nitrogen base pairs 2 hydrogen bonds between Adenine and Thymine 3 hydrogen bonds between guanine and cytosine  Between each “rung” of the ladder, Van der Waals forces hold the stacks together

7. DNA REPLICATION  Look at animations—they really help!  Biologycorner.com  Google search: DNA replication animation—there’s lots of them!  biocoach

8. Meselson and Stahl experiment

10. DNA REPLICATION  Starts at origin of replication “bubble forms”  In long pieces of DNA, there are many origins of replication  At the ends of each bubble is a replication fork

11. DNA REPLICATION

12. DNA REPLICATION--Elongation  The bubbles unzip (open) HELICASE enzyme untwists the double helix as replication occurs  DNA replication occurs in both directions, but by different process in each direction.  DNA polymerases catalyze the process of making new strands

13. DNA REPLICATION  DNA strands are antiparallel

14. DNA REPLICATION  One strand runs in 3’  5’ direction; other strand runs in 5’  3’ direction 3’ carbon has a –OH group 5’ carbon has the phosphate group

15. DNA REPLICATION

16.  DNA polymerase can only add nucleotides to the 3’ end of the growing strand Continuous elongation can only occur in the 5’  3’ direction This new strand is called the LEADING STRAND

17. DNA REPLICATION—leading strand

18. DNA REPLICATION  The other new strand is called the LAGGING STRAND (3’  5’ direction) Replicates in short pieces, called OKAZAKI FRAGMENTS DNA LIGASES then join together all of the Okazaki fragments to create the complete strand

19. DNA REPLICATION

20.  DNA polymerase adds to the 3’ end of a strand, but it can’t START the process An RNA primer is necessary to start replication process Only one primer is required for leading strand Each Okazaki fragment requires an RNA primer (on the lagging strand)

21. Priming DNA synthesis

22. DNA REPLICATION

23. DNA REPLICATION: eukaryotes  The ends of the strands have telomeres Short, repetitive, non-coding nucleotide sequences Protects from progressive shortening of DNA strands

24. Telomeres:why are they necessary?

25. Telomeres

27. DNA REPAIR  DNA polymerase acts as a “proofreader” Checks new nucleotide against original If incorrect, the nucleotide is removed, replaced, and then replication continues on This system is not 100% accurate

28. DNA REPAIR  MISMATCH REPAIR Enzymes fix incorrectly paired nucleotides Can repair nucleotides damaged by environmental agents Monitoring is continuous

29. DNA REPAIR  NUCLEOTIDE EXCISION REPAIR NUCLEASE  Enzyme that “cuts out” the damaged segment DNA POLYMERASES AND LIGASES  Fill in the gaps

30. NUCLEOTIDE EXCISION REPAIR