Chapter 23: How Life Works

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How does life carry out its fundamental processes of growth and reproduction at the molecular level? "Enzymes" are a special kind of . "Catalysts" are molecules that participate in a chemical reation (to speed it up or slow it down), but are not consumed in the reactions. Enzymes are (organic, inorganic?) catalysts. Some enzymes put molecules together; other enzymes take molecules apart. Manganese dioxide (MnO2) is an (organic, inorganic?) catalyst that speeds up the decomposition of hydrogen peroxide (H2O2) into water and carbon dioxide. Enzymes can put together simple molecules (like amino acids), but they are NOT "smart" enough to put together protein chains like sheep insulin. "Nucleic acids" are long chains. The links in the nucleic acid chains are called . The links are themselves made up of an organic base (one of five, labelled for simplicity as CATGU), a (either ribose or deoxyribose), and a group. Of the five organic bases, A (adenine) and (letter?) or U (uracil) stick together weakly at two points, but G (guanine) and (letter?) stick together at (number?) points. A single chain of nucleotides is called (ribonucleic, deoxyribonucleic?) acid. In RNA, the sugar is always (ribose, deoxyribose?) and uracil appears instead of (letter?) . A double chain of nucleotides (in the form of a twisted helix) is called (ribonucleic, deoxyribonucleic?) acid. In DNA, the sugar is always (ribose, deoxyribose?) and thymine appears instead of (letter?) . The genetic instructions for building protein are coded in the RNA and DNA. The code is now understood to be in the ordering of the bases, taken (how many?) at a time. The code is the same for all living things, with very few exceptions. This implies that life has a common origin. It explains why genes from one organism (such as a human being) can be transplanted into something as different as a petunia and still function. How does life work? To build protein for growth, a living cell must have energy; it must have plans ("blueprints"); it must have raw materials, and ; a building site. Proteins are built at the of a cell. Raw materials come into the cell through its walls from the nutrient bath in which the cell floats. are the raw materials (links) of proteins. (What kind of?) RNA (tRNA) attaches to and transfers the amino acids to the ribosomes. There is a different tRNA molecule for each of the (2, 20, 200, 2000?) amino acids found in proteins. The code for the amino acid that a tRNA molecule picks up is written on one end of the tRNA molecule. (What kind of?) RNA (mRNA) carries an "anticopy" of the plans as they are written on the DNA and carries it from the nuclelus (in cells that have one) to the ribosomes. Assembly is accomplished at the ribosomes. Energy is carried from the to the ribosomes by ATP molecules. When enough protein has been fashioned, there is enough for two cells and reproduction occurs. Enzymes split the DNA into two halves and other enzymes reproduce the missing side until two DNA molecules exist instead of one. Each of the two DNA molecules goes to a separate cell. There are about 100 billion atoms in a human DNA molecule, which is about the number of stars in the Milky Way galaxy. And all of the carbon atoms in the molecule have conspired to have (how many?) valence electrons.

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