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82 Biomolecules
RNA Messenger RNAs (mRNAs) transfer genetic
information from the cell nucleus to the cyto-
Ribonucleic acids (RNAs) are polymers con- plasm. The primary transcripts are substan-
sisting of nucleoside phosphate components tially modified while still in the nucleus
that are linked by phosphoric acid diester (mRNA maturation; see p. 246). Since mRNAs
bonds (see p. 80). The bases the contain are have to be read codon by codon in the ribo-
mainly uracil, cytosine, adenine, and guanine, some, they must not form a stable tertiary
but many unusual and modified bases are also structure. This is ensured in part by the at-
found in RNAs (B). tachment of RNA-binding proteins, which pre-
vent base pairing. Due to the varying amounts
of information that they carry, the lengths of
A. Ribonucleic acids (RNAs)
mRNAs also vary widely. Their lifespan is usu-
RNAs areinvolved inall theindividualsteps of ally short, as they are quickly broken down
gene expression and protein biosynthesis (see after translation.
pp. 242–253). The properties of the most im- Small nuclear RNAs (snRNAs) are involved
portant forms of RNA are summarized in the in the splicing of mRNA precursors (see
table. The schematic diagram also gives an p. 246). They associate with numerous pro-
idea of the secondary structure of these mol- teins to form “spliceosomes.”
ecules.
In contrast to DNA, RNAs do not form ex- Phe
tended double helices. In RNAs, the base pairs B. Transfer RNA (tRNA )
(see p. 84) usually only extend over a few The transfer RNAs (tRNAs) function during
residues. For this reason, substructures often translation (see p. 250) as links between the
arise that have a finger shape or clover-leaf nucleic acids and proteins. They are small
shape in two-dimensional representations. In RNA molecules consisting of 70–90 nucleoti-
these, the paired stem regions are linked by des, which “recognize” specific mRNA codons
loops. Large RNAs such as ribosomal 16S- with their anticodons through base pairing. At
rRNA (center) contain numerous “stem and thesametime, at their 3 end (sequence
loop” regions of this type. These sections are .. CCA-3 ) they carry the amino acid that is
again folded three-dimensionally—i. e., like assigned to the relevant mRNA codon accord-
proteins, RNAs have a tertiary structure (see ing to the genetic code (see p. 248).
p. 86). However, tertiary structures are only Thebase sequenceand thetertiarystruc-
known of small RNAs, mainly tRNAs. The dia- ture of the yeast tRNA specific for phenylala-
grams in Fig. B and on p. 86 show that the nine (tRNA Phe )is typical of alltRNAs. The
“clover-leaf” structure is not recognizable in molecule (see also p. 86) contains a high pro-
a three-dimensional representation. portion of unusual and modified components
Cellular RNAs vary widely in their size, (shaded in dark green in Fig. 1). These include
structure, and lifespan. The great majority of pseudouridine (Ψ), dihydrouridine (D), thymi-
them are ribosomal RNA (rRNA),which in dine (T), which otherwise only occurs in DNA,
severalforms is a structuraland functional andmanymethylatednucleotides such as 7-
7
component of ribosomes (see p. 250). Riboso- methylguanidine (m G) and—in the anti-
2
mal RNA is produced from DNA by transcrip- codon—2 -O-methylguanidine (m G). Numer-
tion in the nucleolus, and it is processed there ous base pairs, sometimes deviating from the
and assembled with proteins to form ribo- usual pattern, stabilize the molecule’s confor-
some subunits (see pp. 208, 242). The bacte- mation (2).
rial 16S-rRNA shown in Fig. A,with 1542 nu-
cleotides (nt), is a component of the small
ribosomae subunit, while the much smaller
5S-rRNA (118 nt) is located in the large sub-
unit.
Koolman, Color Atlas of Biochemistry, 2nd edition © 2005 Thieme
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