Cell Metabolism - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 483

small and simple cells. On the other hand, eukaryotic cells do not produce polygenic
messages.
In prokaryotic cells, there is no physical separation of the chromosome from the cyto-
plasma and ribosomes. Often an m-RNAwill bind to a ribosome and begin translation imme-
diately, even while part of it is still being transcribed! However, in eukaryotes, where the
nuclear membrane separates chromosomes and ribosomes, the m-RNA is often subject to pro-
cessing before translation (see Fig. 10.8 ). The DNA can encode for a transcript with an inter-
vening sequence (called an intron) in the middle of the transcript. This intron is then cut out of
the transcript at two specific sites. The ends of the remaining fragments are joined by
a process called m-RNA splicing. The spliced message can then be translated into an actual
protein. The part of the transcript forming the intron is degraded and the monomers
recycled. When m-RNA is recovered from the cytosol it will be in the mature form, while
m-RNA within the nucleus has introns. Many eukaryotic genes contain “nonsense DNA,”
which encodes for the intronic part of the transcript. The word “nonsense” denotes that
particular sequence of DNA does not encode for amino acids. The presence of introns compli-
cates the transfer of eukaryotic genes to protein production systems in prokaryotes such
as E. coli.
As shown in Fig. 10.9 , two other m-RNA processing steps occur in eukaryotic cells that do
not occur in prokaryotes. One is RNA capping, in which the C5 end is modified by the addi-
tion of a guanine nucleotide with a methyl group attached. The other is polyadenylation,in
which a string of adenine nucleotides are added to the C3 end. This tail of adenines is often
(a)
(b)
(c)
(d)
(e)
(f)
FIGURE 10.8 In eukaryotes, RNA splicing is important. The presence of introns is a complication in cloning
genes from eukaryotes to prokaryotes.