SECTION 25.6
Genetic Code
571
F I G U R E 2 5 - 9
Diagram of the conalbumin primary transcript and the processed mRNA. The 16 introns, which are excised from the
primary transcript, are shown in color.
Before RNA splicing was discovered, the nucleus was
observed to contain a significant amount of seemingly
untranslated RNA. The collection of RNA molecules of
widely varied sizes was given the name
heterogeneous
nuclear RNA (hnRNA),
a term that is still sometimes used
for nuclear RNA.
Splicing and Ribozymes
Different snRNPs are found in eukaryotic cells which
function in removing introns from primary RNA tran-
scripts. The association of small RNAs, nuclear proteins,
and the introns that they attach to is referred to as a
spliceo-
some.
Small nuclear RNAs (designated U 1 through U
6
)
provide specificity to different spliceosomes so that they
recognize different classes of introns. Introns are distin-
guished according to their three-dimensional structures
and each class of introns is spliced out by a different
mechanism.
Class I introns
were originally discovered in ciliated
protozoa and subsequently were found in fungi, bacterio-
phages, and other organisms. The RNA itself in a class I in-
tron has catalytic activity and class I introns remove them-
selves from primary RNA transcripts by a self-splicing
reaction. Class I introns are not true enzymes in that they
function only once. The nucleotides in the intron that is
spliced out are recycled in the cell.
Class II introns
are removed from RNA by a self-
splicing reaction that proceeds through an intermediate
structure called a lariat. The removal of class II introns
from RNAs also results in the splicing together of exons
on either side of the intron. The ability of class II introns to
specifically bind to a 5' exon has led to their use as reagents
to construct novel RNA molecules. Chemical derivatives
of class II introns have been constructed that can carry out
the reverse of the splicing reaction. When these introns
insert themselves into RNA, they can be used to shuffle
sequences or to link one RNA molecule to another.
Class II introns, which are found in bacteria, plant or-
ganelles, yeast, and fungi, also are capable of reintegrat-
ing into DNA after being excised from an RNA molecule.
Class II introns encode a multifunctional intron-encoded
protein (IEP) that has reverse transcriptase activity, RNA
splicing activity, and DNA endonuclease activity. These
three enzymatic activities allow the intron to be excised
from RNA, copied into an RNA-DNA heteroduplex, and
inserted into DNA at specific target sites recognized by the
IEP-intron complex. Recently, class II introns have been
modified so that they can be targeted to specific genes
in any DNA molecule. This novel molecular mechanism
raises the possibility that suitably engineered class II in-
trons can be used therapeutically in gene therapy and pos-
sibly to inactivate viruses such as HIV by integration and
disruption of essential genes.
Ribonuclease P (RNase P)
consists of both protein and
an RNA component that has catalytic activity. RNase P
functions in eukaryotic cells to process the 5' end of pre-
cursor tRNA molecules. RNase P also can be directed to
cleave any RNA molecule when the target is complexed
with a short complementary oligonucleotide called an
external guide.
Ribozymes
refer to catalytic RNA molecules that rec-
ognize specific target sequences in other RNA molecules.
This activity and specificity makes ribozymes potentially
useful therapeutic agents. For example, a ribozyme could
be directed to silence the expression of a deleterious gene
by destroying the mRNA transcript before it can be ex-
pressed. Or ribozymes might be constructed that would
inactivate the mRNA or oncogenes or the expression of
genes in RNA viruses such as HIV. Laboratory studies
have supported the feasibility of ribozyme therapy but its
application in clinical practice is still far in the future.
25.6 Genetic Code
“Universal” Genetic Code
Production of proteins from mRNA requires translation of
the base sequence into an amino acid sequence. The col-
lection of base sequences (codons) that correspond to each
amino acid and to signals for termination of translation is
the genetic code. The code consists of 64 triplets of bases
(Table 25-2). The codons are written with the 5' terminus
