BIO SCI 93 Lecture Notes - Lecture 15: Cytosine, Guanine, Thymine
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BIO SCI 93 Full Course Notes
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Now that we"ve looked at the general structure of dna, we should take a closer look at the structures that make up nucleotides. The four nitrogen bases found in dna are adenine, cytosine, guanine, and thymine. Each of these bases are often abbreviated a single letter: a (adenine), c (cytosine), g (guanine), t (thymine). The bases come in two categories: thymine and cytosine are pyrimidines, while adenine and guanine are purines (). The pyrimidine structure is produced by a six-membered, two-nitrogen molecule; purine refers to a nine-membered, four-nitrogen molecule. As you can see, each constituent of the ring making up the base is numbered to help with specificity of identification. The nitrogen bases form the double-strand of dna through weak hydrogen bonds. The nitrogen bases, however, have specific shapes and hydrogen bond properties so that guanine and cytosine only bond with each other, while adenine and thymine also bond exclusively. This pairing off of the nitrogen bases is called complementarity.