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1.3: The Structure of DNA

The experiments outlined in the previous sections proved that DNA was the genetic material, but very little was known about its structure at the time.

1.3.1 Chargaff’s Rules

When Watson and Crick set out in the 1940’s to determine the structure of DNA, it was already known that DNA is made up of a series four different types of molecules, called bases or nucleotides: adenine (A), cytosine (C), thymine (T), guanine (G).  Watson and Crick also knew of Chargaff’s Rules, which were a set of observations about the relative amount of each nucleotide that was present in almost any extract of DNA.   Chargaff had observed that for any given species, the abundance of A was the same as T, and G was the same as C. This was essential to Watson & Crick’s model.

Figure 1.8:  Chemical structure of two pairs of nucleotides in a fragment of double-stranded DNA.  Sugar, phosphate, and bases A,C,G,T are labeled.  Hydrogen bonds between bases on opposite strands are shown by dashed lines. Note that the G-C pair has more hydrogen bonds than A-T. The numbering of carbons within sugars is indicated by red numbers.  Based on this numbering the polarity of each strand is indicated by the labels 5’ and 3’. (Wikipedia-M. Strock-GFDL)

1.3.2 The Double Helix

Using proportional metal models of the individual nucleotides, Watson and Crick deduced a structure for DNA that was consistent with Chargaff’s Rules and with x-ray crystallography data that was obtained (with some controversy) from another researcher named Rosalind Franklin.  In Watson and Crick’s famous double helix, each of the two strands contains DNA bases connected through covalent bonds to a sugar-phosphate backbone (Fig 1.8, 1.9).   Because one side of each sugar molecule is always connected to the opposite side of the next sugar molecule, each strand of DNA has polarity: these are called the 5’ (5-prime) end and the 3’ (3-prime) end, in accordance with the nomenclature of the carbons in the sugars.  The two strands of the double helix run in anti-parallel (i.e. opposite) directions, with the 5’ end of one strand adjacent to the 3’ end of the other strand.  The double helix has a right-handed twist, (rather than the left-handed twist that is often represented incorrectly in popular media).  The DNA bases extend from the backbone towards the center of the helix, with a pair of bases from each strand forming hydrogen bonds that help to hold the two strands together.  Under most conditions, the two strands are slightly offset, which creates a major groove on one face of the double helix, and a minor groove on the other.  Because of the structure of the bases, A can only form hydrogen bonds with T, and G can only form hydrogen bonds with C (remember Chargaff’s Rules).  Each strand is therefore said to be complementary to the other, and so each strand also contains enough information to act as a template for the synthesis of the other.   This complementary redundancy is important in DNA replication and repair.

How can this molecule, DNA, contain the genetic material?

Figure 1.9:
DNA double helix structure. (Original-unknown-?)

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