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Encyclopedia of Physical Science and Technology EN017F-788 August 3, 2001 16:27

34 Translation of RNA to Protein

B. Prokaryotic mRNA
In organisms that do not have a nucleus (prokaryotes), pre-
mRNA usually undergoes little or no modification so that
pre-mRNAandmRNAareverysimilarifnotidentical.Be-
cause pre-mRNA is colinear with DNA, DNA and proteins
are usually colinear in these organisms. Gene expression
in prokaryotes usually involves the co-transcription of sev-
eral adjacent genes, and translation of mRNA sequences

into polypeptides may begin at the 5 end of mRNA while

transcription is still in progress at the 3 end.

C. Eukaryotic mRNA
In cells with a nucleus (eukaryotes) the genetic informa-
tion is stored mainly in the nucleus and to a minor degree
FIGURE 2 Structure of the peptide bond. (a) Two L-amino acids in some organelles (mitochondria and chloroplasts). The
with different side chains, R 1 and R 2 ; (b) A dipeptide formed from description that follows pertains only to nuclear genes.
the two amino acids shown in (a). [From Cox, R. A., and Arnstein, Eukaryotic genes are more complicated than prokaryotic
H. R. V. (1995). In “Encyclopedia of Molecular Biology and Mole-
cular Medicine” (R. A. Meyers, ed.), Volume 6, pp. 108–125. VCH genes because the coding region is often discontinuous:
Publishers, New York. With permission.] the coding sequences or exons are interrupted by inter-
vening sequences (introns). Thus, genes and proteins are
usually not colinear in eukaryotes. In the nucleus, a com-
involves base-pairing between three bases (designated a plicated set of splicing reactions removes all the introns
codon) in the mRNA and the three-base anticodon of a from pre-mRNA and fuses the exons into a continuous
transfer RNA (tRNA). In a separate reaction, each tRNA coding sequence. Other processing steps involve adding a
is first linked to a particular amino acid and thus the pairing “cap” to the 5 end of the mRNA and a poly(A) “tail” to

of mRNA with tRNA determines the sequence of amino the 3 end. After completion of these nuclear maturation

acids in the resulting protein. steps the mRNA is transported to the cytoplasm where it is

FIGURE 3 Structure of typical mRNAs. (a) Prokaryotic mRNA. At the beginning of the transcript there are two ad-

ditional phosphate residues linked by pyrophosphate bonds to the 5 phosphate group of the terminal nucleotide

(see Fig. 1). The 5 untranslated sequence often contains a ribosome binding site (the Shine–Dalgarno sequence)
which increases the efficiency of translation. AUG and UAA are representative initiation and termination codons,
respectively. Cistrons (gene sequences) are separated by short (typically 12 to 24 nucleotides) noncoding inter-

cistronic sequences. (b) Eukaryotic mRNA. Typically, the processed transcripts are monocistronic. The 5 end is
usually modified by the addition of 7-MeG, known as a cap structure, which is linked by two pyrophosphate groups

to the terminal nucleotide of mRNA. The coding sequence is located between 5 and 3 untranslated sequences.

In the majority of cases the 3 end is modified by the addition of 25 to 250 adenylate residues, termed the poly(A)
tail. [From Arnstein, H. R. V., and Cox, R. A. (1992). “Protein Biosynthesis,” Oxford University Press, Oxford. With
permission.]