chap9 - DNA and Its Role in Heredity Chapter 9 2 DNA...

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DNA and Its Role in Heredity
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2 Chapter 9
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DNA Structure Reflects Its Role as the Genetic Material After identifying DNA as the genetic material, scientists hoped to answer two questions about the structure: 1. How is DNA replicated between cell divisions? 2. How does it direct the synthesis of specific proteins?
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DNA structure was discovered through the work of many scientists. One crucial piece of evidence came from X-ray crystallography. A purified substance can be made to form crystals; the pattern of diffraction of X rays passed through the crystallized substance shows position of atoms. Rosalind Franklin: Prepared crystallographs from uniformly oriented DNA fibers—her images suggested a spiral model
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X-Ray Crystallography Helped Reveal the Structure of DNA 5 Chapter 9
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DNA Structure Reflects Its Role as the Genetic Material Chemical composition also provided clues: DNA is a polymer of nucleotides: deoxyribose, a phosphate group, and a nitrogen-containing base. The bases form the differences: Purines: adenine (A), guanine (G) Pyrimidines: cytosine (C), thymine (T)
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DNA Structure Reflects Its Role as the Genetic Material In 1950 Erwin Chargaff found that in the DNA from many different species: Amount of A = amount of T Amount of C = amount of G Or, the abundance of purines = the abundance of pyrimidines— Chargaff’s rule .
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8 Chapter 9
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Francis Crick and James Watson used model building and combined all the knowledge of DNA to determine its structure. Franklin’s X-ray crystallography convinced them the molecule was helical . Modeling also showed that DNA strands are anti-parallel .
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DNA Is a Double Helix 10 Chapter 9
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Watson and Crick suggested that: Nucleotide bases are on the interior of the two strands, with a sugar- phosphate backbone on the outside. Per Chargaff’s rule, a purine on one strand is paired with a pyrimidine on the other. These base pairs (A-T and G-C) have the same width down the helix.
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DNA Structure Reflects Its Role as the Genetic Material Four key features of DNA structure: It is a double-stranded helix of uniform diameter. It is right-handed . It is antiparallel . Outer edges of nitrogenous bases are exposed in the major and minor grooves.
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DNA Is a Double Helix 13 Chapter 9
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Surfaces of A-T and G-C base pairs are chemically distinct . Binding of proteins to specific base pair sequences is key to DNA–protein interactions, and necessary for replication and gene expression.
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Base Pairs in DNA Can Interact with Other Molecules 15 Chapter 9
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double-helical structure is essential: Storage of genetic information —millions of nucleotides; base sequence encodes huge amounts of information Precise replication during cell division by complementary base pairing Expression of the coded information as the
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chap9 - DNA and Its Role in Heredity Chapter 9 2 DNA...

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