Translation
Protein Biosynthesis
Central Dogma
DNA
transcription
translation
RNA
protein
mRNAs are exported for translation
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Through nuclear pore complex
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Recognizes and transports ONLY
completed mRNA
Translation
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Process by which the sequence of
nucleotides in an mRNA directs the
incorporation of amino acids into a protein
Necessary Components for Translation
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mRNA
tRNAs covalently linked to amino acids
Ribosome
Three phases of Translation
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Initiation
Elongation
Termination
The mRNA strand is “read” and amino
acids are linked together to make a
protein by the ribosome
mRNA
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Carries the genetic information from
the chromosomes to the ribosomes
How is the language of nucleic acid
sequences translated into the amino
acid language of proteins?
mRNA is decoded in sets of three
nucleotides = codons
Genetic Code
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Each codon specifies either an amino acid
or stop signal to translation
There are only 20 amino acids and there
are 64 possible codons
The genetic code is degenerate - i.e.
there are "synonyms" (multiple codons)
for some amino acids
Three codons (UAG, UGA, and UAA)
encode translation "stop" signals rather
than amino acids
mRNA must be read in the correct reading
frame to be decoded into the protein
Redundancy or degenerate coding
Reference page 367
Amino acids
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Organic molecule containing both an
amino group and a carboxyl group
Building blocks of proteins
Are added to the C-terminal end of a
growing polypeptide chain by the
formation of a peptide bond
Peptide bonds – between the carboxyl
group at the end of growing chain and a
free amino group of incoming amino acid
Proteins are synthesized from its Nterminus to its C-terminus
tRNA
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Adapter molecule that mediates recognition of the
codon sequence in mRNA and allows its
translation into the appropriate amino acid.
~ 80 nucleotides long
Folds into 3D structure
It has sites for amino-acid attachment and codon
recognition
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The codon recognition is different for each tRNA and
is determined by the anticodon region, which
contains the complementary bases to the ones
encountered on the mRNA.
Each tRNA molecule binds only one type of amino
acid, but because the genetic code is degenerate,
more than one codon exists for each amino acid.
Ribosomes
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Small and Large subunits
The site of translation
Helps to maintain the correct reading
frame and to ensure accuracy
Complex catalytic machine made up of 50
different proteins and several RNA
molecules (rRNAs)
Produced in nucleolus
Millions exist in cell
tRNA structure
tRNA Molecule
Aminoacyl tRNA Synthetase
In Eukaryotes Unique synthetase for each
amino acid

Proper Amino Acid by affinity or fit
Corrected by Hydrolytic editing
The Ribosome
mRNA binding
• two subunits, a large and a small
• the mRNA binds to the small subunit
• there are three sites of activity and
tRNA binding within the large subunit
E
P
A
How do Ribosomes work?
Via 4 binding sites for RNA molecules
The ribosome attaches to the RNA and scans for AUG,the start codon
The ribosome reads the mRNA three nucleotides at a time
Each group of three nucleotides is a single codon
Each codon specifies an particular amino acid
codon
C G
codon
codon
A U C A A U
G C G
codon
C G
codon
A U C
A A
codon
U A C
Start
codon
TRANSLATION INITIATION
Initiation
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Translation begins with the codon AUG
A special tRNA is required to initiate
translation
Initiator tRNA always carries the
amino acid methionine
Initiator tRNA is loaded onto the small
subunit of the ribosome with the aid of
additional proteins (eIFs) which are
attached to GTPs
Initiation, cont.
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Initiator tRNA binds small ribosomal subunit
Small subunit then binds to 5’ end of an
mRNA molecule (recognized by 5’ cap)
The small subunit then moves along mRNA
(5’-3’) searching for the first AUG
eIF2 hydrolyzes GTP to GDP and detaches
Large Subunit then assembles and
elongation can begin
Bacteria use Shine-Dalgarno sequences to
initiate translation at any point on the
mRNA.
Shine Dalgarno sequence
Ribosome docking sequences
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Upstream of AUG consensus sequence
Once the ribosome recognizes the start
codon, protein synthesis begins
The ribosome promotes a chemical
reaction to occur that joins two amino
acids with a peptide bond
 Amino acids are transferred to the
ribosomes by tRNA molecules
 tRNAs have an anticodon on one end
and an amino acid on the other
 The anticodon is a sequence of three
nucleotides that complement a codon
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Cont.
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The anticodon determines which amino acid
it carries to the ribosome
EF-Tu (EF-1) helps the fidelity of the
process.
Each of the twenty amino acids pairs up
with between 1 and 4 anticodons
The process continues, the ribosome moves
along the mRNA to the next codon with the
help of EF-G (EF-2)
A new tRNA recognizes the next codon.
• This continues until the ribosome reaches a STOP codon,
which indicates the end of the gene
•The ribosome & last tRNA fall off the mRNA & the amino
acid chain is complete!
U C
A G U A A U
G U C
U C
A G C
A A G A C
Anti-codon
tRNA
Amino acid
Met
TRANSLATION ELONGATION
Elongation
Termination
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One of the three STOP codons mark
the end of translation
The stop codons are recognized by
proteins known as release factors
that do not specify any amino acids
The release factor triggers an
addition of water to the end of the
polypeptide chain the release of the
new protein.
Protein Folding
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Begins while Protein is still being
synthesized
Guided by and made more efficient
by molecular chaperones
Every protein has a unique order of amino acids
The amino acid chain folds up into a 3-dimensional
structure dictated by the order of the amino acids.
This unique structure gives the protein its unique
function and allows it to do its work
Proteins have many functions
Protein example: Antibiotics
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Some antibiotics are peptides,
others glycopeptides, others are
amino acid derivatives
Inhibitors of prokaryotic translation,
allowing for discrimination between
prokaryotic and eukaryotic cells
Examples: Tetracycline,
Streptomycin, Chloramphenicol,
Erythromycin