Approximately there are more than 10 million—perhaps 100 million—living species on Earth today. Each species is different, and each reproduces itself faith-fully, yielding progeny that belong to the same species: the parent organism gives younger organism information specifying, in extraordinary detail, the characteristics that the offspring shall have.
This phenomenon called heredity is central to the definition of life and it distinguishes life from other processes including, growth of a crystal or the formation of waves on water in which orderly structures are generated but without the same type between the features of
parents and the features of progeny. Living creatures need to consume energy to create and maintain their organization. But life make use of the fee energy to organize and maintain a huge complex system of chemical processes that are specified by hereditary information.
Most living organisms are single cells. Others, such as ourselves, are vast multicellular cities in which groups of cells perform specialized functions linked by intricate systems of communication. But even for the aggregate of more than trillions of cells that form a human body, the whole organism has been generated by cell divisions from a single cell.
Living cells like computers, store information and it is estimated that they have been evolving and diversifying for over 3.5 billion years. All living cells on Earth 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. These monomers, chemical compounds known as nucleotides, have nicknames drawn from a four-letter alphabet—A, T, C, G—and they are strung together in a long linear sequence that encodes the genetic information.
It is possible to 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. By using chemical techniques, scientists are able to read out the
complete sequence of monomers in any DNA molecule.
The mechanisms that make life possible depend on the structure of the double-stranded DNA molecule. Each monomer in a single DNA strand—that is, each nucleotide—consists of two parts: a sugar (deoxyribose) with a phosphate group attached to it, and a base, which may be either adenine (A), guanine (G), cytosine (C), or thymine (T)
Each sugar is linked to the next via the phosphate group, creating a polymer chain composed of a repetitive sugar-phosphate backbone with a series of bases protruding from it. The DNA polymer is extended by adding monomers at one end. For a single isolated strand, these monomers can, in principle, be added in any order, because each one links to the next in the same way, through the part of the molecule that is the same for all of them. In the living cell, however, DNA is not synthesized as a free strand in isolation, but on a template formed by a preexisting DNA strand.
The bases protruding from the existing strand bind to bases of the strand being synthesized, according to a strict
rule defined by the complementary structures of the bases: A binds to T, and C binds to G. This base-pairing holds fresh monomers in place and thereby controls the selection of which one of the four monomers shall be added to the growing strand next. In this way, double-stranded structure is made, consisting of two exactly complementary sequences of As, Cs, Ts and Gs. The two strands twist around each other, forming a DNA double helix.
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