which this information is copied throughout the living world.
DNA replicationtemplated polymerization is the way in which this information is copied throughout the living world.
To understand the mechanisms that make life possible, one must understand
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
either adenine (A), guanine (G), cytosine (C) or thymine (T) (Figure 1-2). 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 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, there is a
constraint: 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,DNA replicationtemplated polymerization is the way in which this information is copied throughout the living world.
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, a double-stranded structure is created,
consisting of two exactly complementary sequences of As, Cs, Ts, and Gs. The
two strands twist around each other, forming a double helix (Figure 1-2E).
The bonds between the base pairs are weak compared with the sugarphosphate
links, and this allows the two DNA strands to be pulled apart
without breakage of their backbones. Each strand then can serve as a template,
in the way just described, for the synthesis of a fresh DNA strand
complementary to itself a fresh copy, that is, of the hereditary information
(Figure 1-3). In different types of cells, this process of
occurs at different rates, with different controls to start it or stop it, and
different auxiliary molecules to help it along. But the basics are universal:
DNA is the information store, and which this information is copied throughout the living world
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