The two polynucleotide chains of a DNA molecule point in opposite directions (antiparallel) and are held together by hydrogen bonds between complementary base pairs—adenine (A) bonds to thymine (T); cytosine (C) bonds to guanine (G).

Dna Triplets

called a codon (fig. 4.22c and table 4.1). Note that sixty-four possible DNA base triplets encode twenty different amino acids. This means that more than one codon can specify the same amino acid, a point we will return to soon. Table 4.1 compares DNA and RNA molecules.

Until the early 1980s, all enzymes were thought to be proteins. Then, researchers found that a bit of RNA that they thought was contaminating a reaction in which RNA molecules are shortened actually contributed the enzymatic activity. The RNA enzymes were named "ri-bozymes." Because certain RNA molecules can carry information as well as function as enzymes — two biologically important properties—they may have been a bridge between chemicals and the earliest cell-like assemblages on earth long ago.

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Essentials of Human Physiology

Essentials of Human Physiology

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