Computers have made us familiar with the concept of information as a
measurable quantity a million bytes (corresponding to about 200 pages of
text) on a floppy disk, 600 million on a CD-ROM, and so on. They have also
made us uncomfortably aware that the same information can be recorded in
many different physical forms. A document that is written on one type of
computer may be unreadable on another. As the computer world has evolved,
the discs and tapes that we used 10 years ago for our electronic archives have
become unreadable on present-day machines. Living cells, like computers,
deal in information, and it is estimated that they have been evolving and
diversifying for over 3.5 billion years. It is scarcely to be expected that they
should all store their information in the same form, or that the archives of one
type of cell should be readable by the information-handling machinery of
another. And yet it is so. All living cells on Earth, without any known
exception, store their hereditary information in the form of double-stranded
molecules of DNA long unbranched paired polymer chains, formed always
of the same four types of monomers A, T, C, G. These monomers are strung
together in a long linear sequence that encodes the genetic information, just as
the sequence of 1s and 0s encodes the information in a computer file. We can
take a piece of DNA from a human cell and insert it into a bacterium, or a
piece of bacterial DNA and insert it into a human cell, and the information will
be successfully read, interpreted, and copied. Using chemical methods,
scientists can read out the complete sequence of monomers in any DNA
molecule extending for millions of nucleotides and thereby decipher the
hereditary information that each organism contains.
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