BIO 151 1nd Edition Lecture 15 Outline of Last Lecture - Genetics- Charles Darwin- Gregor Mendel- Thomas Hunt Morgan- Alfred Hershey & Martha Chase- DNAOutline of Current Lecture - Erwin Chargaff’s rules- DNA structureo Double helixo DNA-DNA Hybridizationo Nucleic acidsCurrent Lecture- Erwin Chargaff’s analysis of DNA (1950)o Hypothesis: If DNA is genetic material, then different species will have varying ratios of nucleotideso Conclusion: Chargaff’s Rules: Different species have different nucleotide ratios Ratio of A:T=1 and C:G=1- DNA structureo Consensus in 1950s: DNA is the genetic materialo If know structure, can figure out how it’s inheritedo Rosalind Franklin: convert molecule into crystal shine xray to scatter moleculesget photograph of DNA diffraction Revealed dimensional info: dark area tells repetitive structureo Pauling’s Alpha Helix: describes structural motif of proteins; alpha helix and beta sheets (of proteins); published before Franklin; actually incorrect thought it was atriple helixo James Watson and Francis Crick: 1953; published article and proposed double helix modelThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute. Made idea their own since Franklin’s work wasn’t yet published- Building double helixo Nucleotides (ATCG) covalently linked into a strand in any ordero Nucleotides on one strand form H bonds to complementary nucleotides on otherstrand A forms 2 H bonds with T C forms 3 bonds with G  H bonds are weaker than the covalent bonds linking backbone  explains Chargaff’s ratioo Strands are directional 3’5’ always read this wayo Strands of helix run in opposite directions- H bonds reveal evolutionary patterns: fundamentals reveal new informationo Example: H bonds between nucleotides of complementary DNA reveal evolution Species that diverge more recently have fewer differences in their DNA sequence that diverged longer ago- Similar time, similar DNA sequence- Know common ancestry, can figure sequence of divergence- DNA-DNA hybridizationo Melt strands: melt double strands lots of energy split into two strands  determine melting point (T m)o Make Hybrids: more mismatches, few H bonds able to make bondso Melt Hybrids: more similar hybrids, melt at higher temp, calculate hybrid meltingtemp (T h ) Compare melting points: dT=Tm-Th Greater dT, more evolutionary distance- Semiconservative model (1953): o Parental moleculeo Separation of parental strands into templateso Formation of new strands complementary to template strands- Matthew Meselson and Franklin Stahl Tested 3 Molecules of Replication (1957)o Semiconservative model: parental strands separate and serve as template for new strando Conservative Model: parental DNA intact all new copy madeo Dispersive Model: parental DNA gets chopped up, daughter strands contain mix of old and new- Messelson and Stahl Experiment (1957)o Bacteria cultured in medium with N15 (Heavy isotope labels parental DNA)o Bacteria transferred to medium with N14 (lighter isotope to label copied DNA)o DNA sample centrifuged after first replication; Replication 1: Intermediate bando DNA sample centrifuged for second time; Replication 2: intermediate + top Conclusion: semiconservative model (Watson and Crick) - Where and how does Replication happeno Replication begins at origin of replication: where DNA strands separateo Origin of replication has specific nucleotide sequence in humans, high GC content- Prokaryotes (bacteria): 1 circular chromosome=1 origin of replication- Eukaryotes: multiple, bigger; linear chromosomes= multiple ORIo Origins will split DNA strand  bubble  2 daughter DNA molecules 1 parent strand + 1 new strando Preparing for DNA replication, NEED: DNA template “Free” nucleotides to build new strands proteins to perform specific