Protein molecules, like DNA and RNA molecules, are long unbranched
polymer chains, formed by the stringing together of monomeric building
blocks drawn from a standard repertoire that is the same for all living cells.
Like DNA and RNA, they carry information in the form of a linear sequence
of symbols, in the same way as a human message written in an alphabetic
script. There are many different protein molecules in each cell, and leaving
out the water they form most of the cell's mass.
The monomers of protein, the amino acids, are quite different from those of
DNA and RNA, and there are 20 types, instead of 4. Each amino acid is built
around the same core structure through which it can be linked in a standard
way to any other amino acid in the set; attached to this core is a side group that
gives each amino acid a distinctive chemical character. Each of the protein
molecules, or polypeptides, created by joining amino acids in a particularsequence folds into a precise three-dimensional form with reactive sites on its
surface (Figure 1-7A). These amino acid polymers thereby bind with high
specificity to other molecules and act as enzymes to catalyze reactions in
which covalent bonds are made and broken. In this way they direct the vast
majority of chemical processes in the cell (Figure 1-7B). Proteins have a host
of other functions as well maintaining structures, generating movements,
sensing signals, and so on each protein molecule performing a specific
function according to its own genetically specified sequence of amino acids.
Proteins, above all, are the molecules that put the cell's genetic information
into action.
Thus, polynucleotides specify the amino acid sequences of proteins. Proteins,
in turn, catalyze many chemical reactions, including those by which new DNA
molecules are synthesized, and the genetic information in DNA is used to
make both RNA and proteins. This feedback loop is the basis of the
autocatalytic, self-reproducing behavior of living organisms (Figure 1-
8).
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