Bio130- Lecture 9 (pg. 366-390)
From RNA to Protein:
Most genes in cell produce mRNA molecules that serve as intermediaries on the pathway to
A mRNA Sequence is decoded in sets of three Nucleotides:
Once mRNA has been produced by transcription and processing, the information present in its
nucleotide is used to synthesize a protein.
Transcription information transfer. DNA RNA
Translation The conversion of information in RNA to proteins.
The nucleotide sequence of a gene, through the intermediary of mRNA, is translated into the
amino acid sequence of a protein by rules that are known as the genetic code.
The sequence of the nucleotides in the mRNA molecule is read in consecutive groups of three.
RNA is a linear polymer of four different nucleotides, therefore there are 64 different possible
combinations of three nucleotides.
Since there are only 20 amino acids, the code is redundant.
Each group of three nucleotides is called a codon.
Used in all present-day organisms
There are three possible reading frames: In the process of translating a nucleotide sequence into
an amino acid sequence, the sequence of nucleotides in an mRNA molecule is read in the 5 to 3
direction, and in consecutive sets of three nucleotides. In principle, therefore, the same RNA
sequence can specify three completely different amino acids, depending on the reading frame.
In reality however only one of these reading frames contains the actual message.
tRNA Molecules Match Amino Acid to Codons in mRNA:
The translation of mRNA into protein depends on adaptor molecules that can recognize and
bind both to the codon and the amino acid.
These adaptors cosist of a set of small RNA molecules known as transfer RNAs (tRNAs), whicha
re about 80 nucleotides in length.
Two regions of unpaired nucleotides situated at either end of the L-shaped molecule are crucial
to the function of tRNA in protein synthesis. One of these regions forms the anitcodon.
Anticodon: A set of three nucleotides that pairs with the complementary codon in an mRNA
The other region is a short single stranded region at the 3 end of the molecule; this is the site
where the amino acid that matches the codon is attached to the tRNA.
Since the genetic code is redundant; there are more than one tRNA for many of the amino acids,
or some tRNA molecules can base-pair with more than one tRNA and some tRNA are
constructed so that they require accurate base-pairing only at the first two positions of the
codon and can tolerate a mismatch at the third position. tRNAs Are Covalently Modified Before They Exit from the Nucleus:
Both bacterial and eukaryotic tRNAs are synthesized by RNA polymerase III.
Both bacterial and eukaryotic tRNAs are typically synthesized as larger precursor tRNAs, which
are then trimmed to produce the mature tRNA. Some tRNA precursors contain introns that must
be spliced out.
tRNA splicing uses a cut-and-paste mechanism that is catalyzed by proteins.
Specific Enzymes Couple Each Amino Acid to its Appropriate tRNA Molecule:
Recognition and attachment of the correct amino acid depends on enzymes called aminoacyl-
tRNA synthetases. Which covalently couple each amino acid to its appropriate set of tRNA
Most cells have different synthetase enzymes for each amino acid.
One of the many reactions coupled to the energy-releasing hydrolysis of ATP, and it produces
high energy bond between the tRNA and the amino acid.
Editing by tRNA Synthetases Ensures Accuracy:
The synthetase must first select the correct amino acid, and most synthetases so so by a two-
First, the correct amino acid has the highest affinity for the active site pocket of its synthetase
and is therefore favored over the other 19.
A second discrimination step occurs after the amino acid has been covalently linked to AMP
Amino acid activationAn amino acid is activated for protein synthesis by an aminoacyl-tRNA
synthetase enzyme in two steps. The energy of ATP hydrolysis is used to attach each amino acid
to its tRNA molecule in a high-energy linkage. The amino acid is first activated through the
linkage of its carboxyl group directly to an AMP moiety, forming an adenylated amino acid; the
linkage of the AMP, normally an unfavorable reaction, is driven by the hydrolysis of the ATP
molecule that donates the AMP. Without leaving the synthetase enzyme, the AMP linked
carboxyl group on the amino acid is then transferred to a hydroxyl group on the sugar at the 3
end of the tRNA molecule. This transfer joins the amino acid by an activated ester linkage to the
tRNA and forms the final aminoacyl-tRNA molecule.
Hydrolytic editing: tRNA synthetases remove their own coupling errors through hydrolytic
editing of incorrectly attached amino acids. The correct amino acid is rejected by the editing
site. The error-correction process performed by DNA polymerase shows some similarities;
however, it differs in so far as the removal process depends strongly on a mispairing with the
This hydrolytic editing, which is analogous to the exonucleolytic proofreading by DNA
polymerases, raises the overall accuracy of tRNA charging to approximately one mistake in
Amino acids are added to the C-terminal end of a growing polypeptide chain: The fundamental reaction of protein synthesis is the formation of a peptide bond between the
carboxyl group at the end of a growing polypeptide chain and a free amino group on an
incoming amino acid.
A protein is synthesized step-wise from its N-terminal end to its C-terminal end.
Throughout the entire process the growing carboxyl end of the poly peptide chain remains
activated by its covalent attachment to a tRNA molecule.
Each addition disrupts this high-energy covalent linkage, but immediately replaces it with an
identical linkage on the most recently added amino acid.
The RNA Message is decoded in Ribosomes:
The synthesis in proteins is guided by information carried by mRNA in molecules. TO maintain
the correct reading frame and to ensure accuracy, protein synthesis is performed in the
RIBOSOME, a complex catalytic machine made from more than 50 different proteins, and
several RNA molecules, the RIBOSOMAL RNAs (rRNAs).
Eukaryotic ribosome subunits are assembled at the nucleolus, when newly transcribed and
modified rRNAs associate with ribosomal proteins, which have been transported into the
nucleus after their synthesis in the cytoplasm. The two ribosomal subunits are then exported to
the cytoplasm, where they join together to synthesize protein.
Both Eukaryotic and prokaryotic ribosomes have similar designs and functions, and are
composed of one large and one small subunit that if together to form a complete ribosome with
a mass of several million Daltons.
When not actively s