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I.Properties of genetic material What was known about the genetic material prior to the 1940s? Chapter 10 DNA: The Chemical Nature of the Gene A. Replication—be able to give exact copy to daughter cells B. Mutability—be able to change C. Location—must be part of chromosomes D. Affect the phenotype is specific ways E. Why most scientist in the early 40s thought proteins were the genetic material: DNA has equal ratios of purines: pyrimidines suggested DNA has simple structure F. Evidence of nucleic acids as genetic material 1. Griffith (1925) laid the groundwork for later discoveries by demonstrating transformation in vivo. Proteins vary in composition and are complex – 20 amino acids 20 building blocks vs 4 F. Evidence of nucleic acids as genetic material 2. Avery, McLeod and McCarty (1944) demonstrated in vitro that the transforming factor was DNA. 1 F. Evidence of nucleic acids as genetic material 3. Hershey and Chase (1953) used T2 phage to demonstrate that DNA was the genetic material. Label proteins with S35. No label is recovered in progeny phage. Label DNA with P32. Label is recovered in progeny phage. 2 F. Evidence of nucleic acids as genetic material 4. Fraenkel-Conrat and Williams (1955) demonstrated that RNA can also be genetic material II. Chemistry of nucleic acids A. B. 1. 2. Nucleoside—C5 sugar + base at 1′ C Nucleotide—nucleoside + phosphate at 5′ C Purines—adenine, guanine Pyrimidines—thymine (in DNA), cytosine, uracil (in RNA) C. Phosphodiester bond hooks nucleotides together D. Structure—double-stranded helix (Watson-Crick) 1. X-ray data (Franklin and Wilkins) —DNA is highly ordered and has multiple chains. —Repeating structure every 3.4 angstroms. —Helical. 2. Chargaff's rule: A=T G=C purines = pyrimidines 3. Other information —DNA is made of nucleotides with 3' to 5' linkage. —DNA can be denatured w/out breaking covalent bonds: ➔ DNA is stabilized by H-bonds. 3 Watson-Crick model a. Strands held together by H-bonds b. A opposite T; G opposite C c. Strands antiparallel 4 F. Properties 1. Complexity—different sequences give potential for different proteins 2. Replication—model proposes strands separate “It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material.” 3. Location—nucleus or chromosome The Central Dogma: Flow of information in the cell The Central Dogma: modifications 5 G. DNA can take up different structures -- condition dependent -- sequence dependent B-DNA right-hand helix most common A-DNA right-hand helix has less H2O; shorter, wider Z-DNA has left-hand helix -- zigzag -- role in gene regulation Special structures in DNA and RNA Methylation DNA methylation in eukaryotes frequently occurs at CpG sites Prokaryotic – distinguish ‘self’ from ‘other’ Eukaryotic – gene regulation C methylation in animals 5% in plants 50% in yeast none in flies very very low 6