Three rules of DNA Structure

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File:Dna

The structure of DNA has been determined based upon three important rules. The first is known as 'Chargaff's rules'. Erwin Chargaff introduced a chemical technique which is used to measure the amount of base pairs present in a DNA molecule, prior to the work done by Watson and Crick in 1953 [1]. Chargaff's rules state that, for a particular species, the amount of Guanine is equal to the amount of Cytosine (both base pairs, through triple hydrogen bond formation). Therefore [G]=[C], and similarly, the amount of Adenine is equal to the amount of Thymine ([A]=[T]), through double hydrogen bond formation) [2]. The triple hydrogen bond between G=C has a stronger bond than A=T so it requires higher temperature to denature the bond. It also states that the amount of Adenine and Thymine bases varies in accordance with the amount of Guanine and Cytosine bases, however, the amount of purine bases ([A]+[G]) is always equal to the amount of pyrimidine bases ([T]+[C]).

The second rule was formed on the work carried out by Franklin and Wilkin. Due to their x-ray diffraction of DNA fibres, they were able to conclude that a DNA molecule has a double helix structure, formed by 2 strands of polymers twisted around each other [3].

The third, and probably the most important and well known rule, was made based on the observations of Watson and Crick in 1953 [4]. Like Franklin and Wilkin, James Watson and Francis Crick observed that DNA formed a helical structure by the twisting of the polynucleotide chains. They also found that the 2 strands were anti-parallel, therefore one strand runs from 3' to 5' and the other 5' to 3' [5], with the sugar phosphates on the outside and the nucleotide bases stacked on the inside, creating a stabilising force [6]. 1 turn of the helix is 3.4nm, with 10 bases per turn, therefore the rise per base is 0.34nm [7]. The diameter of the helix is 2nm in width. Interestingly, Watson and Crick found that Adenine always pairs up with Thymine, and Cytosine always pairs up with Guanine, which proves Chargaff's rule that the proportion of [A] is the same as [T], and similarly the amount of [G] is the same as [C]. They found that bases from either strand pair up using hydrogen bonds [8]. From this point in time onwards, scientists have been able to develop our understanding of DNA, in terms of mutation, replication and how it controls hereditary traits. All these rules have been invaluable in our understanding of DNA.

References

  1. International Edition - Genetics, Analysis of Genes and Genomes, 7th Edition, Hartl and Jones, Chapter 2 - DNA Structure and Genetic Variation, Page 42
  2. International Edition - Genetics, Analysis of Genes and Genomes, 7th Edition, Hartl and Jones, Chapter 2 - DNA Structure and Genetic Variation, Page 43
  3. Molecular Biology of the Cell, 5th Edition, Alberts, Johnson, Lewis, Raff, Roberts, Walter, Chapter 4: DNA, Chromosomes, and Genomes, page 197
  4. International Edition - Genetics, Analysis of Genes and Genomes, 7th Edition, Hartl and Jones, Chapter 1 -Genes, Genomes and Genetic Analysis, page 6
  5. International Edition - Genetics, Analysis of Genes and Genomes, 7th Edition, Hartl and Jones, Chapter 1 - Genes, Genomes and Genetic Analysis, page 8
  6. Molecular Biology of the Cell, 5th Edition, Alberts, Johnson, Lewis, Raff, Roberts, Walter, Chapter 4: DNA, Chromosomes, and Genomes, page 197
  7. International Edition - Genetics, Analysis of Genes and Genomes, 7th Edition, Hartl and Jones, Chapter 2 - DNA Structure and Genetic Variation, Page 43
  8. International Edition - Genetics, Analysis of Genes and Genomes, 7th Edition, Hartl and Jones, Chapter 2 - DNA Structure and Genetic Variation, Page 43
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