Transcript 10.3 Protein Synthesis

Protein Synthesis
Using RNA to make proteins
Going from DNA to Proteins
• Let’s review what we’ve done so far:
• We take our DNA and convert it into RNA in
a process called ________________.
• This happens in the _____________.
• Our proteins are made from Amino Acids so
we need a bridge to go from the RNA we
just made to amino acids…
The Genetic Code
• The language of mRNA is called the Genetic Code (A, G, U,
C)  (contains only 4 letters)
• It is the matching of the RNA sequence to the correct amino
acid to make proteins.
• It is based on codons, which are 3 bases together on an
mRNA chain.
• Each codon codes for a specific amino acid
• There are 64 codons and 20 amino acids so several codons
will code for the same amino acid
• NOTE: The Genetic Code is Universal: Most organisms follow
this code to produce protein demonstrating that we are all
evolutionary related.
Special Codons: Start/Stop Condons
• Start Codon: AUG
• Signals the beginning of protein
synthesis/translation
• Protein synthesis can only begin at this codon.
• Also codes for the Amino Acid: Methionine
• Stop Codons: UAA, UAG, UGA
• Signals protein synthesis to stop as soon as
they are reached on the mRNA strand.
• Similar to the period at the end of a sentence or
the end of a polypeptide chain.
Genetic Code Square
Genetic Code Circle
What do we have after transcription?
RNA molecules:
•mRNA with the DNA code
•tRNA waiting with the amino acid
•rRNA making ribosomes for translation
How do we get from RNA to Proteins?
Translation!!
Changing from the language of nucleic acids to the
language of proteins in the RIBOSOME
OR
process of converting mRNA to amino acids
All 3 types of RNA are
used in protein synthesis
tRNA and Anticodons
• Amino acids are free floating in the
cytosol and are transferred to the
ribosome by tRNA molecules
• How is tRNA able to do this job? 
it contains 4 binding sites
• Top part of loop allows for 1 AA to
bind
• Bottom part of loop is the anticodon
(sequence of 3 nucleotides) that
pairs with its complementary
matching mRNA
• Ex. If the mRNA: UUU then
anticodon : AAA
Note: the pairing of a anticodon
with a codon ensures that the
Amino Acids are being joined
in the correct order
Setting the stage for
Translation…
We’ve made our mRNA and its moving
into the ribosome
Our tRNA’s are transporting the amino
acids over to the ribosomes
OK. We are ready to Translate!
Steps 1-4 of Translation
• Step 1: Ribosomes attach to the start codon (AUG) of
the mRNA.
• Step 2: A tRNA with the anticodon of UAC and a
methionine binds to the mRNA inside the ribosome.
• Step 3: In the 2nd binding site, the next codon is read
and a tRNA is attached
• Step 4: When the second tRNA attaches, the amino
acids attached to the 1st tRNA releases from the tRNA
and binds to the new amino acid with a peptide bond
Steps 5-6 of Translation
• Step 5: Then the
1st tRNA is
released and the
ribosome moves
to another codon
(3 nucleotides).
• Step 6: When a
stop codon is
reached,
translation stops
and the
ribosome
releases the
mRNA and the
created protein.
Protein synthesis analogy
Animations
http://biology.kenyon.edu/slonc/bio3/ribo/ri
bo3.html
http://www.youtube.com/watch?v=NJxobg
kPEAo