ABSTRACT
As mentioned in the introductory chapter, RNA and DNA are
chemically very similar. Nevertheless, inside a living cell they behave
radically different. Why? This reflects the fact that DNA molecules
are always paired which results in a stiff double helix, whereas RNA
chains are single stranded. That any DNA strand comes together
with its complementary strand is automatically ensured by the DNA
replication process, as depicted in Fig. 1.2. On the other hand, RNA is
produced as a single-strandedmolecule, see Fig. 1.3. Suchmolecules
are rather flexible with an effective bond length of a few bp-as
we have discussed in Section 4.4 on DNA melting. Since the RNA
backbone is flexible, the molecule can easily fold onto itself. If a
sequence of a few bases finds its counterpart somewhere else along
the chain (e.g., if there is a sequence AUGGC and somewhere else
GCCAU), these two stretches can hybridize with each other and
form a short piece of RNA double helix. This might be biologically
irrelevant for a messenger RNA en route to a ribosome, but in other
cases the folding of an RNA chain is of vital importance, e.g., for
transfer RNAs, the adapters between the RNA and the proteinworlds
that can do their job only because they are folded in a specific way,
see Fig. 1.4.
