Biology Reference
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maintained throughout subsequent steps in the translation process. Protein syn-
thesis is often regulated at the level of initiation, making it an important step.
Because the RNA of the larger ribosomal subunit carries out the formation of
the peptide bonds, the ribosome is a ribozyme , meaning that the RNA acts as an
enzyme. The RNA carries out the key peptidyl transferase reaction ( Cech 2000,
Nissen et al. 2000 ). By contrast, the proteins in the ribosome are structural units
and help to organize key catalytic RNA elements.
Translation of the genetic information in eukaryotes begins when an mRNA
molecule binds to the surface of a ribosome and the initiation codon (AUG) is
selected. The Met-tRNA interacts with the AUG start codon of the mRNA. More
than nine eukaryotic initiation factors have been identified and several are com-
posed of multiple polypeptide chains. The large number of polypeptides indi-
cates that protein-protein interactions play an important role in initiation of
translation ( Dever 1999 ).
tRNAs carry an amino acid to the ribosome where they bind to the mRNA
molecule attached to the ribosome. tRNAs have a three-base sequence, the anti-
codon , that is complementary to a specific codon in the mRNA. At the other end
of the tRNA is a site to which a specific amino acid is bound ( Figure 2.6 ). Binding
between the mRNA codon and tRNA anticodon occurs by hydrogen bonds.
Proteins within the ribosome function as cofactors, buttressing, stabilizing, and
orienting the floppy ribosomal RNA into a specific, active ribozyme. Peptide
bonds are made between the successively aligned amino acids until the stop
codon at the end of the mRNA is reached (UAA, UAG, or UGA) and the com-
pleted protein is released. The polypeptide is thus synthesized from the amino
end toward the carboxyl end.
tRNAs are small, single-stranded molecules ranging from 70 to 90nt
( Figure 2.6 ). Internal complementary-base sequences allow the molecule to form
short double-stranded regions, thereby yielding a folded molecule (sometimes
called a cloverleaf structure) in which open loops are connected to each other
by double-stranded stems. The three-dimensional structure of tRNA molecules is
important. One significant region is the anticodon region; this region consists of
three bases that can base-pair with the codon in the mRNA. A second critical site
is the 3'OH end of the molecule where the amino acid attaches.
A specific enzyme called aminoacyl tRNA synthetase matches each amino
acid with the tRNA attachment site. tRNA molecules and their synthetases are des-
ignated by giving the name of the amino acid that is specific to each particular
tRNA molecule. Thus, leucyl-tRNA synthetase attaches leucine to tRNA Leu . If an
amino acid is attached to a tRNA molecule, it is “charged.” Usually, one, and only
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