DNA: Structure

Watson and Crick Model:

James Watson and Francis Crick proposed a structure for the DNA molecule that suggested the basic mechanism of DNA replication. The model proposes that DNA is composed of two strands of DNA running anti-parallel (remember the 5' and 3' ends!!) to each other. The hrdorphilic sugar(ribose) and phosphate groups of the nucleotides are face the outside of the molecule (where water would be) and the relatively hydrophobic nitorgenous bases are on the inside of the molecule, hidden from water. See Fig. 1. The nucleotides within each strand are held together by the phosphodiester bonds between the 5' carbon of one nucleotide and the 3 ' carbon of the adjacent nucleotdie. These strong covalent bonds form the the glue that holds the sugar/phosphate backbone together and thus act to keep each strand together.

The two strands of DNA are held together by weak hydrogen bonds (remember, something must hold them together!) between the nitrogenous bases. The interesting (and very important) aspect of the Watson-Crick model is that these hydrogen bonds are base specific. That is, A (adenine) can only form hydrogen bonds with T (thymine) and C (cytosine) can only form bonds with G (guanine). This means, that if you know the sequence of bases of one strand, you know the sequence of bases of the other strand. The strands is such a molecule are said to be complementary. There is one other aspect of the structure of DNA that is not shown in the figure: the two strands wrap around each other to form a double helix structure. At the present time, we can ignore this fact but we will need to remember it later when we are duplicating DNA.

There are two aspects of the Watson-Crick model that especially pleased Watson and Crick. One was that the strands are complementary. As stated above, if one knows the sequence of bases of one strand, one can deduce the sequence in the complementary one. This property explains how a cell can replicate a DNA molecule to produce two identical molecule. The cell needs to separate the strands and then simply synthesize a new strand for each by using the original strands as a template. You will see later that this is exactly what the cell does! Watson and Crick were also pleased that their model explained 'Chargraff's Rule' of DNA in which the amount of A in any DNA sample always equaled the amount of T and that the amount of G always equaled the amount of C.

Fig. 1: This image was taken from the Department of Energy's Primer on
Molecular Genetics .

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