-If tRNA molecules carrying the correct amino acids are the key to reading the genetic code, the critical
question becomes: How are specific amino acids attached to specific tRNA molecules?
-Translation of the genetic code requires: 1) tRNA molecules, and 2) aminoacyl-tRNA synthetases, which
are enzymes that “charge” a tRNA with a particular amino acid.
-The CCA terminus of the tRNA molecule is “charged” with a particular amino acid which binds to either
the 3’ or 2’ Carbon of the ribose and form ester linkages, which we have to break in order to form a
polypeptide chain. The term is called “charging” because amino acids are positively charged (the amino
group), which adds a +1 charge to the tRNA molecule.
-Different aminoacyl-tRNA synthetases recognize and charge different tRNAs and this specificity comes
from the recognition of different anticodon loops and acceptor stems of tRNAs, and in some cases,
-Aminoacyl-tRNA synthetases translate the genetic code with high fidelity through two different
proofreading schemes: 1) amino acid activation, in which amino acids that are two big are excluded from
the activation site (so sometimes amino acids that are smaller than the right one can enter), and 2) editing
at discrete sites, in which amino acids that are smaller than the right amino acid which are incorrectly
placed in the activation site will be removed.
-Aminoacyl-tRNA synthetases have three binding sites: 1) tRNA binding site, 2) Amino acid sites, which
are specific for specific amino acids, and 3) binding site for ATP, because tRNA charging requires
-The charging cycle occurs in different steps: 1) a specific amino acid with an ATP molecule bind to the
aminoacyl-tRNA synthetase in the two binding sites, and the two phosphate groups are removed, leaving
an “activated” amino acid with a bound AMP group, 2) a tRNA specific for the specific amino acid binds
to the tRNA binding site, and 3) the AMP group dissociates and the amino acid binds to the tRNA, and
the charged tRNA molecule is dissociated from the aminoacyl-tRNA synthetase.
-Ribosomes are required for translation and can be visible as small spheres in a eukaryotic cell’s
-In prokaryotes, transcription and translation are coupled, and while transcription is occurring, multiple
ribosomes (polysomes) bind to the mRNA and translate it.
-Ribosomes read the instructions for protein synthesis from mRNA molecules in the 5’
3’ direction (so
that translation can occur shortly after transcription) and the amino acid chain is polymerized from the N-
terminus to the C-terminus.
-In E. Coli, ribosomes contain two subunits, the large 50S subunit and the small 30S subunit. They also
contain three aminoacyl-tRNA sites: 1) E (Exit) site, in which a tRNA that has been added to the A site
and had its amino acid polymerized in the growing polypeptide chain exits, 2) P (Peptydil) site, in which a
newly synthesized polypeptide chain is attached to the Peptidyl tRNA, and 3) A (Aminoacyl) site, where