Special Structures in DNA and RNA

In double-stranded DNA, the pairing of bases on opposite nucleotide strands provides stability and produces the helical secondary structure of the molecule. Single-stranded DNA and RNA (the latter of which is almost always single stranded) lack the stabilizing influence of the paired nucleotide strands; so they exhibit no common secondary structure. Sequences within a single strand of nucleotides may be complementary to each other and can pair by forming hydrogen bonds, producing double-stranded regions (IFIGURE 10.17). This internal base pairing imparts a secondary structure to a single-stranded molecule. In fact, internal base pairing within single strands of nucleotides can result in a great variety of secondary structures.

One common type of secondary structure found in single strands of nucleotides is a hairpin, which forms when sequences of nucleotides on the same strand are inverted complements. The sequence 5' TGCGAT 3' and 5' ATCGCA 3' are examples of inverted complements and, when these sequences are on the same nucleotide strand, they can pair

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