exam2_notes - Chapter 7DNA Structure (9-30-05) Transforming...

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Chapter 7 DNA Structure (9-30-05) (Problems 1, 3, 4, 5, 16, 17, 26) Transforming Principle Frederick Griffith-1928 (Figure 7-2) Streptococcus pneumonia Causes pneumonia in humans, lethal in mice. Strain 1 (S) normal, virulent, smooth colony from polysaccharide coat Strain 2 (R) mutant, avirulent, rough colony without polysaccharide coat 1) Boil S (dead) 2) Mix boiled S with live R inject mice 3) Mice die 4) Live S cells recovered from dead mice. Interpretation cell debris from S strain converted live R strain to live S strain. This process is called transformation. Avery, MacLeod and McCarty-1944 (Figure 7-3) What is the transforming principle? Experiment Separated various classes of compounds found in boiled S and tested each class for the ability to transform the R strain to the S strain. Only DNA caused transformation. DNA encoded the smooth phenotype. First demonstration that genes are composed of DNA. Many scientists still thought protein was the hereditary material. (Figure 7-4) Blender Experiment Phosphorous (P) in DNA; Sulfur (S) in protein. Incorporate 32 P into bacteriophage T2 DNA or 35 S into bacteriophage T2 protein. Infect E. coli . 32 P found inside the cell but not 35 S. Conclusion DNA is the hereditary material.
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Structure of DNA DNA is composed of 4 nucleotides (nt) each containing a different nitrogenous base linked to an identical deoxyribose sugar and a phosphate. (Figure 7-5) Adenine (A); Guanine (G) purines Cytosine (C); Thy mine (T py rimidines 1) Chargaff’s rules A) T + C = G + A B) T = A; G = C C) A + T G + C 2) X-ray crystal data indicating that DNA is long, skinny and helical with two similar parts that are parallel to each other. 3) (Nobel prize) Double helix Each helix is composed of two antiparallel strands. (i.e., 5' ' and 3' 5') (Figure 7-8) Each strand is composed of nucleotides held together by phosphodiester bonds that form between the phosphate from one nt and the deoxyribose sugar from the neighboring nt. A pairs with T (2 hydrogen bonds). G pairs with C (3 hydrogen bonds). Purines pair with pyrimidines. One strand is complementary to the other. B-form DNA, right hand helix. Major and minor grooves. (Figure 7-9)
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Implications of DNA structure 1) Suggested a way for DNA replication. 2) Suggested that a genetic code in DNA would specify the amino acid sequence in proteins. DNA Replication (Figure 7-11) (i.e., each daughter duplex contains one parental and one newly synthesized daughter strand).
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exam2_notes - Chapter 7DNA Structure (9-30-05) Transforming...

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