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Chapter 13.2. The Discovery of DNA’s Structure. AP Biology Fall 2010. DNA’s Building Blocks. DNA consists of 4 kinds of nucleotides, each of which consists of: A five carbon sugar: deoxyribose A phosphate group One of four bases. DNA’s Building Blocks. Adenine (A) Guanine (G)

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chapter 13 2

Chapter 13.2

The Discovery of DNA’s Structure

AP Biology

Fall 2010

dna s building blocks
DNA’s Building Blocks
  • DNA consists of 4 kinds of nucleotides, each of which consists of:
    • A five carbon sugar: deoxyribose
    • A phosphate group
    • One of four bases
dna s building blocks3
DNA’s Building Blocks
  • Adenine (A)
  • Guanine (G)
  • Thymine (T)
  • Cytosine (C)
dna s building blocks4
DNA’s Building Blocks
  • The nucleotides are similar
  • T and C are single ring pyrimidines
  • A and G are double ring purines
dna s building blocks5
DNA’s Building Blocks
  • In 1949, Edwin Chargaff noted two critical bits of data
  • The 4 kinds of nucleotide bases making up DNA molecule differ in relative amounts from species to species
  • The amount of A = T and the amount of G = C
dna s building blocks6
DNA’s Building Blocks
  • Rosalind Franklin used X-ray diffraction techniques to produce images of DNA
    • X-ray diffraction uses a beam of electrons directed at a molecule which scatters the x-ray in a pattern
    • Franklin used x-ray diffraction with a “wet” form of DNA
    • DNA exists as a long, thin molecule of uniform diameter
    • The structure is highly repetitive
    • DNA is helical
patterns of base carriers
Patterns of Base Carriers
  • The following features were incorporated into their models
    • Single ringed thymine was hydrogen bonded with double-ringed adenine, and single ringed cytosine with double ringed guanine, along the entire length of the molecule
    • The backbone was made of chains of sugar-phosphate linkages
    • The molecule was double stranded and looked like a ladder with a twist to form a double helix
patterns of base carriers11
Patterns of Base Carriers
  • The base pairing is constant for all species, but the sequence of base pairs in a nucleotide strand is different from one species to the next
    • This is the basis for life’s diversity