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are explained as a consequence of gravity, and respiration as an effect of red blood cells that carry oxygen from the lungs to various parts of the body.

Third, a hypothesis must be examined for its consistency with hypotheses and theories commonly accepted in the particular field of science and to see whether it represents any advance with respect to well-established alternative hypotheses. Lack of consistency with other theories is not always ground for rejection of a hypothesis, although it will often be. Some of the greatest scientific advances occur precisely when it is shown that a widely held and well-supported hypothesis is replaced by a new one that accounts for the same phenomena that were explained by the preexisting hypothesis, and other phenomena it could not account for. One example is the replacement of Newtonian mechanics by the theory of relativity, which rejects the conservation of matter and the simultaneity of events that occur at a distance, 2 fundamental tenets of Newton’s theory.

Examples of this kind are pervasive in rapidly advancing disciplines, such as molecular biology at present. The so-called “central dogma” holds that molecular information flows only in one direction, from DNA to RNA to protein. The DNA contains the genetic information that determines what the organism is, but that information has to be expressed in the enzymes (and other proteins) that guide all chemical processes in cells. The information contained in the DNA molecules is conveyed to proteins by means of intermediate molecules, called messenger RNA. David Baltimore (1970) and Howard Temin (Temin and Mizutani, 1970) were awarded the Nobel Prize for discovering independently that information could flow in the opposite direction, from RNA to DNA, by means of the enzyme reverse transcriptase. They showed that some viruses, as they infect cells, are able to copy their RNA into DNA, which then becomes integrated into the DNA of the infected cell, where it is used as if it were the cell’s own DNA.

Other examples are the following. Biochemists assumed that only the proteins known as enzymes could catalyze the chemical reactions in cells. However, Thomas Cech (1985) and Sidney Altman received in 1989 the Nobel Prize for independently showing that certain RNA molecules act as enzymes and catalyze their own reactions. One more example concerns the so-called “colinearity” between DNA and protein. Molecular biologists thought that the sequence of nucleotides in the DNA of a gene is expressed consecutively in the sequence of amino acids in the protein. This conception was shaken by the discovery that genes come in pieces, separated by intervening DNA segments that do not code for protein; Richard Roberts and Philip Sharp received the 1993 Nobel Prize for this discovery (Crick, 1979; Chambon, 1981).

These revolutionary hypotheses were published after their authors had subjected them to severe empirical tests. Theories that are inconsistent

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