Topic 2 and 7 ~ Nucleic acids The Molecular Basis of Inheritance

1. Topic 2 and 7 ~ Nucleic acids The Molecular Basis of Inheritance

2. History of DNA Discovery and Experiments

3. SKIP…Searching for Genetic Material, I • Mendel: modes of heredity in pea plants • Morgan: genes located on chromosomes • Griffith: bacterial work; transformation: change in genotype and phenotype due to assimilation of then unknown external substance by a cell • Avery: purified various molecules from cells and discovered that the transformation agent was DNA Griffith’s experiment

4. Skill: Analyze Hershey Chase Experiment • Hershey and Chase animation of experiment • Experiment: used phages with radioactively labeled sulfur and phosphorus; sulfur(S) is in protein, phosphorus (P) is in DNA; only P was found in host cell • DNA, not protein, is the hereditary material

5. Application: Rosalind Franklin and Maurice Wilkins X-Ray Diffraction investigation • X-ray crystallography (shooting crystallized DNA with X-rays and then analyzing the diffraction patterns) produced this image captured by Rosalind Franklin • Provided major clues into DNA’s structure • The cross indicated a helical shape • Angle of cross showed angle of helix • https://www.youtube.com/watch?v=u7RrXAjuNRk explanation • https://www.dnalc.org/view/15014-Franklin-s-X-ray-diffraction-explanation-of-X-ray-pattern-.html

6. Nature of science: Think about this… • Making careful observations—Rosalind Franklin’s X-ray diffraction provided crucial evidence that DNA is a double helix. (1.8)

7. Watson and Crick-1953 Using this x-ray diffraction data, Watson and Crick built the first accurate model of DNA structure.

8. DNA Structure • The Double Helix • Sugar/Phosphate backbone • Nitrogen bases = rungs • Monomer = nucleotide: • 3 parts of nucleotide • sugar (deoxyribose) • phosphate group • nitrogenous base (thymine, adenine, cytosine, guanine); • Antiparallel: sugar/phosphate backbones run in opposite directions

9. Nitrogen Bases in DNA • ratio of nucleotide bases (A=T; C=G) • base-pairing of complementary bases due to hydrogen bonding • A-T (2 hydrogen bonds) • C-G (3 hydrogen bonds)

10. DNA Be able to draw and label a simple diagram of the molecular structure of DNA Note : must show that strands are anti-parallel

11. Essential Idea: The structure of DNA is ideally suited to its function. Discuss: How is DNA’s structure ideal for its functions?

12. DNA Replication (Copying DNA) • Strands are complementary; • nucleotides line up on template strand according to base pair rules. • Meselson & Stahlreplication is semiconservative; Expt: varying densities of radioactive nitrogen Which is correct?

13. DNA Replication (general/ simplified) • C:\Documents and Settings\BBAUGHMAN\Desktop\bio powerpoints\Chapter 11 BDOL IC

14. DNA Replication: a closer look • Origin of replication: location on DNA where replication starts • Replication fork: ‘Y’-shaped region where DNA is unwinding. • DNA Replication is SEMICONSERVATIVE (each of the new DNA molecules is made up of one original strand and one new strand) • Note: only prokaryotic replication is expected by IB

15. Nature of science: • Obtaining evidence for scientific theories—Meselson and Stahl obtained evidence for the semi-conservative replication of DNA. • As you watch the animation, take down notes… You need to be able to analyze and explain the experiment …(anim..)

16. The Enzymes of Replication • Helicase:catalyzes the untwisting of the DNA at the replication fork • DNA Gyrase:  relieves strain while double-strand DNA is being unwound. (demo why necessary with ropes) • Single- strand binding protein: Stabilizes unwound strands of DNA • Primase – adds RNA primer (short stretch of RNA- ten nucleotides or so) necessary to get DNA polymerase started. (DNA Gyrase)

17. The Enzymes:continued… • DNA polymerase III: catalyzes the elongation of new DNA by adding new nucleotides to 3’ end of the growing strand. • DNA polymerase I: removes RNA Primers and replaces them with DNA nucleotides. • DNA ligase: Joins Okazaki fragments (DNA Gyrase)

18. DNA Replication: Direction Matters! • Remember: DNA strands are antiparallel. • 5’= phosphate end • 3’= hydroxyl end • DNA polymerase only adds nucleotides at the free 3’ end, forming new DNA strands in the 5’ to 3’ direction only

19. DNA Replication… leading vs. lagging strands • New strands are ALWAYS built in a 5’ to 3’ direction! • Leading strand: synthesis toward the replication fork (only in a 5’ to 3’ direction) • Lagging strand: synthesis away from the replication fork ( forms Okazaki fragments); • joined by DNA ligase

20. Nucleoside Triphosphates (not in new syllabus) • Nucleoside Triphosphate • a nucleotide but with three phosphates instead of one. • The extra phosphates provide the energy needed to add nucleotides to the chain.

21. DNA Replication: the leading strand and the lagging strand (Animation)http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter14/animations.html# http://www.rsc.org/education/teachers/learnnet/cfb/nucleicacids.htm Good extension material and review. Kinesthetic activity – Pop beads

22. End of IB Stuff

23. DNA Repair • Mismatch repair: fixing base pairing mistakes as DNA is replicated (accomplished by DNA polymerase and other enzymes) • Excision repair: • Nuclease cuts out damaged part of DNA strand • DNA Polymerase and ligase fill in the gap.

24. Telomeres *Problem: DNA polymerase can only add nucleotides to 3’ end of an RNA primer. This leaves a section of DNA (where the RNA primer was) that doesn’t get replicated!! (i.e. the DNA gets shorter each time it replicates) ** Solution: Eukaryotes have special nucleotide sequences at the ends of their DNA called telomeres. Telomeres do not have genes. They have a short, repeated sequence like TTAGGG…

25. Also telomerase lengthens telomeres in germ line cells that make gametes.