To carry out its information-storage function, DNA must do more than copy
itself before each cell division by the mechanism just described. It must also
express its information, putting it to use so as to guide the synthesis of other
molecules in the cell. This also occurs by a mechanism that is the same in all
living organisms, leading first and foremost to the production of two other key
classes of polymers: RNAs and proteins. The process begins with a templated
polymerization called transcription, in which segments of the DNA sequence
are used as templates to guide the synthesis of shorter molecules of the closely
related polymer
of translation, many of these RNA molecules serve to direct the synthesis of
polymers of a radically different chemical class the
In RNA, the backbone is formed of a slightly different sugar from that of
DNA ribose instead of deoxyribose and one of the four bases is slightly
different uracil (U) in place of thymine (T); but the other three bases A, C,
and G are the same, and all four bases pair with their complementary
counterparts in DNA the A, U, C, and G of RNA with the T, A, G, and C of
DNA. During transcription, RNA monomers are lined up and selected for
polymerization on a template strand of DNA in the same way that DNA
monomers are selected during replication. The outcome is therefore a polymer
molecule whose sequence of nucleotides faithfully represents a part of the
cell's genetic information, even though written in a slightly different alphabet,consisting of RNA monomers instead of DNA monomers.
The same segment of DNA can be used repeatedly to guide the synthesis of
many identical RNA transcripts. Thus, whereas the cell's archive of genetic
information in the form of DNA is fixed and sacrosanct, the RNA transcripts
are mass-produced and disposable (Figure 1-5). As we shall see, the primary
role of most of these transcripts is to serve as intermediates in the transfer of
genetic information: they serve as messenger RNA (
synthesis of proteins according to the genetic instructions stored in the DNA.
RNA molecules have distinctive structures that can also give them other
specialized chemical capabilities. Being single-stranded, their backbone is
flexible, so that the polymer chain can bend back on itself to allow one part of
the molecule to form weak bonds with another part of the same molecule. This
occurs when segments of the sequence are locally complementary: a ...
GGGG... segment, for example, will tend to associate with a ...CCCC...
segment. These types of internal associations can cause an RNA chain to fold
up into a specific shape that is dictated by its sequence (Figure 1-6). The shape
of the RNA molecule, in turn, may enable it to recognize other molecules by
binding to them selectively and even, in certain cases, to catalyze chemical
changes in the molecules that are bound. As we see later in this book, a few
chemical reactions catalyzed by RNA molecules are crucial for several of the
most ancient and fundamental processes in living cells, and it has been
suggested that more extensive catalysis by RNA played a central part in the
early evolution of life (discussed in Chapter 6).mRNA) to guide theribonucleic acid, or RNA. Later, in the more complex processproteins (Figure 1-4).
EmoticonEmoticon