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

History of DNA Discovery and Experiments

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

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

Provided major clues into DNA’s structure 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

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)

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

DNA Structure The Double Helix Monomer = nucleotide: 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

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)

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

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

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

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

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

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..)

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)

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)

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

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

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

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

End of IB Stuff

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.

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…

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