Download Translation and the Genetic Code

A. Amino Acid and Protein Structure
B. Formation of Aminoacyl tRNAs
C. Ribosome structure
D. Stages of Translation
E. Relationship between DNA, mRNA, and Protein
Sequences
1.
Amino acid structure
 Four different groups are attached to the central carbon
atom (α-carbon)
 Hydrogen atom
 Amino group (-N+H3)
 Carboxylic acid group (-COO-)
 Side chain group (“-R”): 20 different amino acids, each with a
different side chain, are encoded by codons on mRNA
2.
Peptides and Proteins
 Peptides are formed when a covalent peptide bond (an
amide bond) is formed between the carboxylic acid
group of one amino acid and the amino group of another
amino acid.
2.
…
 Proteins are long peptides, over 50 amino acids long and
typically much longer (in the low 100s), and typically
associated with some biological function
 The peptide chain of a protein folds into a specific threedimensional shape necessary for the activity of the
protein.
 The folding of the protein, and the chemistry of the
protein’s active site, are dependent on the amino acid
sequence of the protein.
Amino acids are covalently attached to the 3’ end of
the appropriate tRNAs. This is called the acceptor
end.
2. The anticodon is a 3-base sequence on the anticodon
loop of the tRNA. It is complementary to the
sequence of the codon on the mRNA. The 5’ position
is referred to as the “wobble base,” meaning that it
may pair up with more that one partner.
3. The reaction is catalyzed by an aminoacyl tRNA
synthase. Each tRNA has its own specific synthase
enzyme.
1.
4.
This is the reaction:
Amino acid + ATP + tRNA 
aminoacyl tRNA + AMP + PPi
5.
tRNA has a distinctive 3-D structure, described as a
“cloverleaf,” with hairpin loops and nonstandard
bases
http://en.wikipedia.org/wiki/Transfer_RNA
1.
Prokaryotic ribosomes
 Large subunit: 50 S
 Small subunit: 30 S
 Total size: 70 S
2.
Eukaryotic ribosomes
 Large subunit: 60 S
 Small subunit: 40 S
 Total size: 80 S
http://en.wikipedia.org/wiki/Ribosome
1.
Initiation
Requires the aid of initiation factor proteins
b. The small ribosome subunit binds to the 5’ end of
mRNA. The proper orientation is believed to be
established by a sequence in the leader region called
the Shine-Dalgarno sequence (in prokaryotes) or
similar sequences.
c. An initiation codon (AUG) is oriented on the small
ribosome subunit. AUG is the codon for the amino acid
methionine. Please note: Sequences on mRNA are
listed, by convention, in the 5’  3’ direction
a.
…
1.
d.
e.
f.
A molecule of methionyl tRNA (met-tRNA) binds to the
initiation codon through codon-anticodon base pairing.
This step requires GTP as an energy source.
The large subunit binds to the small subunit to complete the
initiation complex. All initiation factors are released.
Some interesting facts:



In prokaryotes, the methionine on the initiating methionyl tRNA
is formylated (f-met-tRNA). In eukaryotes, it is not.
Not every AUG codon can be an initiation codon. Sequences in
the mRNA leader seem to indicate which AUG codons are
initiation codons.
The initial methionine may be removed after translation
(posttranslational modification), so not every protein begins with
a methionine.
2.
Elongation
The ribosome/mRNA complex has two sites: the P site
(to which the growing peptide chain is attached) and
the A site (where the next aminoacyl tRNA binds). At
the beginning of elongation, the met-tRNA occupies
the P site. The A site is ready to receive the next
aminoacyl tRNA.
b. The next aminoacyl tRNA binds to the
ribosome/mRNA complex at the A site.
c. An enzyme activity in the ribosome, peptidyl
transferase, forms a peptide bond between the
carboxyl end of the growing peptide (on the P site) and
the amino end of the next amino acid (on the A site).
a.
2.
…
d. The tRNA on the P site, no longer attached to an amino
acid, is released.
e. Another enzyme activity in the ribosome, called
translocase, moves the ribosome so that the peptidyl
tRNA is transferred from the A site to the P site. This
process requires a GTP molecule as an energy source.
f. Now the A site is ready to accept the next aminoacyl
tRNA.
3.
Termination
When the ribosome encounters a termination codon
on the mRNA (UAA, UAG, or UGA), elongation ceases.
b. Termination factors cause the ribosome, tRNA, and
mRNA to dissociate from the nascent protein chain.
a.
Sequence
Nontemplate DNA strand:
5’ ATG TTT GCT AAG GAC ATC TAA 3’
Template DNA strand:
3’ TAC AAA CGA TTC CTG TAG ATT 5’
mRNA sequence:
5’ AUG UUU GCU AAG GAC AUC UAA 3’
Amino Acid Sequence:
(Amino end)
Met Phe Ala Lys Asp Ile
(Carboxyl end)
Be certain that you can read the genetic code table.
Be certain that you can read the genetic code table.