46 Chapter 2. What’s inside cells[[Student version, December 8, 2002]]
Figure 2.18:(Schematic molecular structure.) RNA is generally single-stranded, but it can contain local regions of
short complementary sequences, which can pair via hydrogen bonding as in DNA. The pairing between such regions
can fold the RNA up into a specific three-dimensional shape, as shown. Section 6.7 will discuss how the folding and
unfolding of RNA can be controlled by external forces. [Copyrighted figure; permission pending.]
“nucleosomes.” The nucleosomes in turn wind into higher-order structures, and so on up to the
level of entire condensed chromosomes (Figure 2.10).^5
Section 2.2.1 mentioned the formation of polypeptides. The genetic message in DNA encodes
only the polypeptide’sprimary structure,orlinear sequence of amino acids. After the polypeptide
has been synthesized, though, it folds into an elaborate three-dimensional structure—aprotein—
such as those seen in Figure 2.4g,h,i,k,l,m. The key to understanding this process is to note that
unlike DNA, whose large, uniform negative charge makes it repel itself, individual amino acid
residues on a protein may attractorrepel each other (see Chapter 8). Thus the primary structure
determinesthe protein’s final, three-dimensional folded structure.
The lowest level of folding (thesecondary structure)involves interactions between residues near
each other along the polypeptide chain. An example that will interest us in Chapter 9 is thealpha
helix,shown in Figure 2.19. At the next higher level, the secondary structures (along with other,
disordered regions) assemble to give the protein’stertiary structure,the overall shape visible in the
examples of Figure 2.4. A simple protein consists of a single chain of 30–400 amino acids, folded
into a tertiary structure which is dense, roughly spherical, and a few nanometers in diameter (a
“globular” protein).
More complex proteins consist of multiple polypeptide chain subunits, usually arranged in a sym-
metrical array—thequaternary structure.Afamous example is hemoglobin, the carrier of oxygen
in your blood (Chapter 9), which has four subunits. Many membrane channels (see Section 2.3.1
below) also consist of four subunits.
Polysaccharidesform a third class of biopolymers (after nucleic acids and proteins). These are
long chains of sugar molecules. Some, like glycogen, are used for long-term energy storage. Others
help cells to identify themselves to each other.
2.2.4 Macromolecular assemblies
The previous subsection mentioned that individual protein chains can form confederations with def-
inite shapes, the quaternary structure of a protein assembly. Another possibility is the construction
of alineararray of polypeptide subunits, extending for an arbitrarily long distance. Such arrays
(^5) Simpler forms of DNA packaging have also been found recently in prokaryotic cells.