Snyder and Champness Molecular Genetics of Bacteria. Tina M. Henkin

Snyder and Champness Molecular Genetics of Bacteria - Tina M. Henkin


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Schematic illustration of the Watson-Crick structure of DNA showing the helical sugar-phosphate backbones of the two strands held together by hydrogen bonding between the bases.

      The nucleotides found at the ends of a linear piece of DNA have properties that are biochemically important and useful for orienting the DNA strand. At one end of the DNA chain, a nucleotide will have a phosphate attached to its 5′ carbon that does not connect it to another nucleotide. This end of the strand is called the 5′ end or the 5′ phosphate end (Figure 1.3B). On the other end, the last nucleotide lacks a phosphate at its 3′ carbon. Because it has only a hydroxyl group (the OH in Figure 1.3B), this end is called the 3′ end or the 3′ hydroxyl end.

Schematic illustration of a DNA chain, showing the 3'-to-5' attachment of the phosphates to the sugars, forming phosphodiester bonds.; Schematic illustration of two strands of DNA bind at the bases in an antiparallel arrangement of the phosphatesugar backbones.

      It did not escape the attention of Watson and Crick that the complementary base-pairing rules essentially explain heredity. If A pairs only with T and G pairs only with C, then each strand of DNA can replicate to make a complementary copy, so that the two replicated DNAs will be exact copies of each other. Offspring containing the new DNAs would have the same sequence of nucleotides in their DNAs as their parents and thus would be exact copies of their parents.

Schematic illustration of the two complementary base pairs found in DNA. Two hydrogen bonds form in adenine-thymine base pairs. Three hydrogen bonds form in guanine-cytosine base pairs.

      Because the two strands of DNA are wrapped around each other to form a double helix, the helix has two grooves between the two strands (Figure 1.1). One of these grooves is wider than the other, so it is called the major groove. The other, narrower


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