Nucleic Acids in Chemistry and Biology

structures of the complete 70S ribosome,^28 a complex of many proteins and 3 RNAs, as well as the 30S
and 50S ribosomal subunits (Section 7.3.3).5,29

7.2 RNA Processing and Modification

RNA molecules are transcribed from DNA by RNA polymeraseenzymes, which initiate transcription
after binding to specific DNA promotersequences. By use of the adjacent DNA antisense strand as a tem-
plate, polymerases synthesize RNA transcriptsfrom the 5
- to the 3
-terminus (Section 6.6). Each organ-
ism has at least one, and often several, distinct RNA polymerases. After it has been transcribed, an RNA
molecule is typically not functional until it has undergone RNA processing.^30 The attachment of modifi-
cations, the removal of long sequences, and in some cases changes to the base sequence itself, are often
required before an RNA can be transported to the proper cellular compartment and carry out its function.
There are many different types of RNA in the cell. For example, messenger RNA (mRNA) encodes pro-
tein sequences, transfer RNA (tRNA) acts at the ribosome to decode mRNA information to specify par-
ticular amino acids, ribosomal RNAs (rRNA) assemble into the ribosome where protein is manufactured,
small nuclear RNAs (snRNAs) tailor other RNAs to the proper size, and microRNAs (miRNAs) are tiny
sequences that bind and regulate the function of other RNAs. The cellular localization and biological func-
tion of RNA molecules dictate the type of processing that they undergo. For the sake of simplicity, the pro-
cessing of eukaryotic mRNA will be our major focus.

7.2.1 Protecting and Targeting the Transcript: Capping and Polyadenylation

As the 5
-terminus of a new mRNA emerges from a eukaryotic RNA polymerase, it is immediately pro-
tected by attachment of a trimethyl G cap(Figure 7.13). During this process, the terminal nucleotide of the
transcript is joined by a 5
–5
linkage to a guanosine triphosphate. The guanosine itself is methylated, and
often hypermethylated, to distinguish it from other guanosines in the cell. As a result of capping, the nas-
cent transcript is protected from 5
-exonucleases that would otherwise digest the RNA as it emerged.

RNA Structure and Function 263

N

N

N

O

N

N

O

O OH

O P

O

O

CH 3

H

H

O Gua

O OH

3'

OH

3

triphosphate

5'-5' linkage

RNA

transcript

methylated

guanosine cap

first nucleotide of

the nascent strand

H

Figure 7.13 The cap structure of eukaryotic mRNA molecules. A red circle indicates the site of methylation on N-7
of guanosine, while arrows indicate common sites of additional methylation. Note the unusual 5
-5

triphosphate linkage that joins the two guanosine nucleotides of the cap