Nucleic acid structure
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Nucleic Acid Structure. Structure of nucleotides Nitrogenous bases Pentose sugars Nucleosides Nucleotides Nucleotide chains Structure of B-DNA Images accompanying this lecture may be found at Select chapter 10. A. Structure of Nucleotides.

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Nucleic acid structure l.jpg
Nucleic Acid Structure

  • Structure of nucleotides

  • Nitrogenous bases

  • Pentose sugars

  • Nucleosides

  • Nucleotides

  • Nucleotide chains

  • Structure of B-DNA

    Images accompanying this lecture may be found at chapter 10

A structure of nucleotides l.jpg
A. Structure of Nucleotides

  • A nucleotide is composed of

    • A nitrogenous base

    • A pentose sugar

    • A phosphate group

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Nitrogenous Bases

  • Two different classes of aromatic carbon-nitrogen heterocycles

    • Purines

      • Adenine (Found in DNA & RNA)

      • Guanine (Found in DNA & RNA)

    • Pyrimidines

      • Cytosine (Found in DNA & RNA)

      • Thymine (Found in DNA)

      • Uracil (Found in RNA)

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C. Pentose Sugars

  • Ribose

    • Found in RNA

    • Forms a 5-atom ring structure in aqueous solution

    • Carbons are numbered 1´ (one-prime), 2´, 3´, 4´, 5´

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C. Pentose Sugars

  • Deoxyribose

    • Found in DNA

    • Identical to ribose, except that the “-OH” group on the 2´ carbon is replaced with an “-H”

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D. Nucleosides

  • Nucleoside

    • A pentose sugar molecule with a nitrogenous base attached to the 1´ carbon

    • Nucleosides are named by using the root of the base name, plus the suffix “-osine” (for purines) or “-idine” (for pyrimidines)

    • Nucleosides with deoxyribose sugars are designated with the prefix “deoxy-”

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E. Nucleotides

  • Nucleotide

    • A nucleoside with one, two, or three phosphate groups attached to the 5´ carbon

    • Nucleotides are named using the name of the nucleoside plus “monophosphate,” “diphosphate,” or “triphosphate” depending on the number of phosphates

    • Nucleotides with one phosphate may also be named by changing the nucleoside suffix to “-ylic acid”

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F. Nucleotide Chains

  • The 5´ carbon of one nucleotide can be linked to the 3´ carbon of another nucleotide via a phosphodiester bond

  • An oligonucleotide chain (polynucleotide chain) is a linear chain of nucleotides linked in this fashion

  • The oligonucleotide chain has two ends: 5´ and 3´

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G. Structure of B-DNA

  • In the 1940s, Chargaff discovered that the DNA isolated from most sources exhibited a 1:1 molar ratio of A:T, and a 1:1 ratio of G:C (as compared to RNA, in which the A:U and G:C ratios are random)

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G. Structure of B-DNA

  • In the early 1950s, Watson, Crick, and Franklin studied the X-ray diffraction patterns of crystalline DNA fibers, and determined that DNA had a symmetrical 3-D structure in the form of a helix

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G. Structure of B-DNA

  • Watson & Crick knew of Chargaff’s ratios, and realized that they could build a helical model for DNA structure, consistent with the X-ray data

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G. Structure of B-DNA

  • The Watson and Crick model is known as a B-DNA helix, and it is believed to be the native conformation of most DNA found in living organisms

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G. Structure of B-DNA

  • Features of the B-DNA helix:

    • Two oligonucleotide strands

    • The sugar-phosphate backbones of the strands are on the outside, and twist around a central axis to form a helix

    • The helical twists form two “grooves”that turn around the axis: the major groove and the minor groove

    • Therefore, DNA is a pretty groovy molecule

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G. Structure of B-DNA

  • The two strands are in antiparallel orientation (one strand goes from 5´  3´ and the other strand goes from3´  5´

  • These are the dimensions of the helix:20 Å diameter10 bases per turn34 Å per turn

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G. Structure of B-DNA

  • The bases are located in the center of the helix, with the flat planes of the bases perpendicular to the axis of the helix

  • The bases between the two strands are “paired” with an “A” on one strand paired with a “T” on the other strand, and “G” paired with “C”

  • This property of the strands is called complementarity (the two strands are said to be complementary to each other, thank you very much)

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G. Structure of B-DNA

  • The strands are held together by noncovalent “hydrogen bonds” between the complementary pairs of bases:A – T pairs have two hydrogen bondsG – C pairs have three hydrogen bonds

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G. Structure of B-DNA

  • The two strands may be separated by heating (“melting”) the DNA, or by raising the pH with alkaline treatment

  • Two pieces of single-stranded DNA will spontaneously form a helix if the strands have enough base complementarity

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G. Structure of B-DNA

  • An interactive tutorial on DNA structure can be found at