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5.4: Base Pairing in DNA and RNA

  • Page ID
    4742
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    Rules of Base Pairing

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    Figure 5.4.1: Base Pairing

    The rules of base pairing (or nucleotide pairing) are:

    • A with T: the purine adenine (A) always pairs with the pyrimidine thymine (T)
    • C with G: the pyrimidine cytosine (C) always pairs with the purine guanine (G)

    This is consistent with there not being enough space (20 Å) for two purines to fit within the helix and too much space for two pyrimidines to get close enough to each other to form hydrogen bonds between them. But why not A with C and G with T? The answer: only with A & T and with C & G are there opportunities to establish hydrogen bonds (shown here as dotted lines) between them (two between A & T; three between C & G). These relationships are often called the rules of Watson-Crick base pairing, named after the two scientists who discovered their structural basis.

    Table 5.4.1: Relative Proportions (%) of Bases in DNA
    Organism A T G C
    Human 30.9 29.4 19.9 19.8
    Chicken 28.8 29.2 20.5 21.5
    Grasshopper 29.3 29.3 20.5 20.7
    Sea Urchin 32.8 32.1 17.7 17.3
    Wheat 27.3 27.1 22.7 22.8
    Yeast 31.3 32.9 18.7 17.1
    E. coli 24.7 23.6 26.0 25.7

    The rules of base pairing tell us that if we can "read" the sequence of nucleotides on one strand of DNA, we can immediately deduce the complementary sequence on the other strand. The rules of base pairing explain the phenomenon that whatever the amount of adenine (A) in the DNA of an organism, the amount of thymine (T) is the same (called Chargaff's rule). Similarly, whatever the amount of guanine (G), the amount of cytosine (C) is the same. The C+G:A+T ratio varies from organism to organism, particularly among the bacteria, but within the limits of the experimental error, A=T and C=G.


    This page titled 5.4: Base Pairing in DNA and RNA is shared under a CC BY 3.0 license and was authored, remixed, and/or curated by John W. Kimball via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.

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