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.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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