RNA Structure and Function 289
Figure 7.41 The secondary and tertiary structures of pseudoknots. (a) Many RNA molecules contain pseudoknots
that typically form a straight, coaxial stack of the two helices. (b) The mRNA of retroviruses contains
sequences that form an unusual type of pseudoknot. Due to the presence of unpaired nucleotides (bulge)
at the junction between stems 1 and 2, these pseudoknots bend, thereby causing a disruption of ribo-
somal reading frame
(Reprinted from Ref. 72. © (2000), with permission from Elsevier)
Figure 7.42 Translation initiation of eukaryotic mRNAs (top) and Hepatitis C RNAs (bottom). Eukaryotic translation
is initiated through binding of the 40S ribosomal subunit (light grey ellipse) to the cap structure (red
ellipsoid) at the 5-end of mRNA molecules. Hepatitis C viral RNA is not capped and therefore it initiates
translation by promoting interactions between the 40S subunit and an unusual stem-loop structure,
called an internal ribosome entry site (IRES)