Chapter 19: Translation and Proteins
Conversion of the genetic information encoded in mRNA into
the amino acids of a protein is accomplished by the interaction
of the mRNA with amino acids, and two other components of
the cytoplasm: ribosomes and transfer RNAs (tRNAs).
Ribosomes are cellular organelles composed of two subunits,
each containing RNA (rRNA) and proteins, and are the site of
protein synthesis. This adaptor has two functions: (1) to bind
to the appropriate amino acid and (2) to recognize the proper
codon in mRNA using the tRNA anticodon sequence. Since
there are twenty different amino acids, twenty different types
of tRNA are required to accomplish these tasks. Amino acids
are linked together on the ribosome by the formation of
covalent peptide bonds. Two linked amino acids form a
dipeptide, three form a tripeptide and ten or more make a
polypeptide. After translation, polypeptides fold into a
three-dimensional conformation to form a protein, whose
function produces a phenotype. Translation takes place in a
series of steps: initiation, elongation and termination. These
steps are shown in the following animations.
While the basic mechanism of translation is conserved
between prokaryotes and eukaryotes, there are important
differences between the two groups. In general, we have
summarized the process from E. coli. In eukaryotes,
transcription and translation are compartmentalized;
transcription occurs in the nucleus, translation occurs in the
cytoplasm. The ribosomes of eukaryotes are larger and more
complex than those of prokaryotes. Finally, eukaryotic
mRNA only encodes a single polypeptide, while those of
prokaryotes can encode two or more polypeptides. These
differences can be exploited, since they allow the inhibition of
prokaryotic translation with out disrupting our own. Antibiotics
as streptomycin, puromycin, chloramphenicol, and
erythromycin all inhibit the prokaryotic translation apparatus at
one point or another.