Base pairs

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In Watson and Crick's model of DNA, the double helix, the two strands of DNA are joined to one another by hydrogen bonds between complementary nitrogenous bases. These hydrogen bonds have a strength of 4-21 kJ mol-1[1].

In DNA adenine always pairs with thymine and cytosine always pairs with guanine. In RNA uracil replaces thymine, therefore in RNA adenine always pairs with uracil. Thymine and uracil or adenine have two hydrogen bonds between them, whereas guanine and cytosine have three. Consequently, DNA with a larger proportion of guanine and cytosine is more stable and it takes more energy to break the two strands of DNA apart.   

Contents

Structure

The base pairing in the DNA helix helps to determine its structure. Due to the different interactions between the bases, the dsDNA helix completes a full turn on its axis every ten bases. Each base allows the helix to turn thirty-six degrees [2].

Purines and Pyrimidines

Adenine and guanine are both purine bases, this means that they have a double-ringed structure. Cytosine, uracil (only present in RNA) and thymine are pyrimidines and have single ringed structures. These bases contain nitrogen in their ring compounds.[3] Purines only ever pair with pyrimidines and pyrimidines only ever pair with purines. This is one of the reasons why a transversional base pairing change can have such disastrous effects on the structure of a protein as hydrogen bonds will not occur between two purines or two pyrimidines [4]. Before Watson and Crick presented the structure of DNA, Erwin Chargaff in the 1950s discovered a chemical technique in which he could determine the molar concentration of any one of the bases in a source of DNA. From what Chargaff discovered he noticed some patterns in the molar concentrations of the bases, from his results he devised some rules [5].

Chargaff's Rules

Base Stacking

In the DNA double helix, as well as the bases being complementary base-paired they are also stacked on top of one another. These bases also have interactions (Van der Waals) happening between one another which also contribute towards the DNAs structure. Base stacking in this way creates a hydrophobic core on the DNA [6].

References

  1. Hartyl, D. Jones, E.. (2005). DNA Structure and DNA Manipulation. In: Weaver, S. et al. Genetics, Analysis of Genes and Genomes. 6th ed. Sudbury,: Jones and Bartlett Publishers. p46-52.
  2. Berg, J. Stryer, L. Tymoczko, J.. (2007). DNA, RNA, and the Flow of Genetic Information. In: Ahr, K. et al. Biochemistry. 6th ed. New York: W.H. Freeman and Company. p107-112.
  3. Alberts et.al. (2007) Molecular Biology of the Cell 5th ed pg. 116
  4. Hartyl, D. Jones, E.. (2005). DNA Structure and DNA Manipulation. In: Weaver, S. et al. Genetics, Analysis of Genes and Genomes. 6th ed. Sudbury,: Jones and Bartlett Publishers. p46-52.
  5. Hartyl, D. Jones, E.. (2005). DNA Structure and DNA Manipulation. In: Weaver, S. et al. Genetics, Analysis of Genes and Genomes. 6th ed. Sudbury,: Jones and Bartlett Publishers. p46-52.
  6. Hartyl, D. Jones, E.. (2005). DNA Structure and DNA Manipulation. In: Weaver, S. et al. Genetics, Analysis of Genes and Genomes. 6th ed. Sudbury,: Jones and Bartlett Publishers. p46-52.

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