handout

Translation Study Guide
Kelsey Volk, Jeremy Arnt, Trevor Turner, Justin Neasley, Sean Nielson
RIBOSOMES
A site – Aminoacyl Binding Site
P site – Peptidyl Binding Site (first binding site of tRNA)
E site – Exit Site
Composed of rRNA and proteins.
 All of the catalytic activity is carried out by rRNA.
 Proteins stabilize the structure since rRNA by itself is unstable.
Large subunit and a small subunit, both synthesized by the nucleolus.
Subunits join together when they each attach to mRNA.
 Small subunit attaches to mRNA first.
Prokaryotes have 70S ribosomes
 30S small subunit
 50S large subunit
Eukaryotes have 80S ribosomes.
 40S small subunit
 60S large subunit
Ribosomes in the mitochondria are similar to those in prokaryotes.
40S (small) subunit finds AUG (start codon) near the 5’ end, and translation begins when the
60S large subunits binds.
Transfer RNA (tRNA)

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The “adapter” molecule between
mRNA and a protein.
Cloverleaf shaped molecule
consisting of between 76 and 90
nucleotides.
o Cloverleaf shape created by
hydrogen bonds between
complementary bases
causing tRNA to fold.
o Further folding creates a
distinct “L” shape to fit
within the ribosome.

2 important regions of tRNA:
o Anticodon: 3 bases that are
complementary to an
mRNA codon. These are
anti-parallel to mRNA.
o Amino Acid Binding Site:
Located at the 3’ end of the tRNA molecule, this is where the specific amino acid
binds to the tRNA. The region always consists of the bases CCA.

Aminoacyl-tRNA synthetase:
o Enzyme that couples an amino acid to a specific tRNA based on its anticodon.
o When the amino acid is linked to the 3’ end of tRNA, the tRNA molecule is
considered “charged” to recognize a complementary mRNA codon.

Wobble Position
o In its association with tRNA, the 3rd position of the mRNA codon (which is the 1st
position of the tRNA anticodon), is not as strict as the first 2 positions. This
creates plasticity in what bases can work in this position.
Prokaryotic Initiation
1. To initiate translation, a 30S ribosomal subunit first binds to a sequence on the mRNA called
the Shine-Dalgarno sequence found upstream of start codon AUG.
2. Then, a charged tRNA having a complementary anticodon and carrying the amino acid fmethionine binds with the AUG start codon.
3. After the new complex is formed, the large 50S sub-unit binds with the complex and initiation
is completed.
Notes
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f-Met is the only tRNA that begins in the P slot of the ribosome
Initiation in prokaryotes happens simultaneously with transcription.
16S rRNA portion of the 30S sub-unit binds to the Shine-Dalgarno sequence
The large ribosomal subunit perform all catalytic activity
Eukaryotic initiation
1. To initiate translation, a charge tRNA molecule binds onto the 40S small ribosomal sub-unit.
Attached to the tRNA is a Met amino acid with the complementary anticodon that binds to the
AUG start codon.
2. Next, the tRNA and small sub-unit complex bind to translation initiation factors and attach to
the 5’ cap of the mature mRNA.
3. Next, the entire complex scans in the 3’ direction until it finds the AUG start codon of the
mRNA
4. Lastly, the 60S large sub-unit binds to the complex removing the initiation factors with the
tRNA starting in the P slot.
Notes:
 From the 5’ cap to the AUG start codon is called the untranslated region UTR.
 Translation initiation factors are needed for the small sub-unit to find the 5’ cap
 Eukaryotic initiation uses a Met amino acids not f-Met.
Elongation
-After initiation occurs with the first tRNA in the P site of the ribosome elongation
begins.
-Elongation is the process of lengthening the amino acid chain.
-mRNA is read 5’ to 3’
Elongation can be broken into a 5-step repeating process.
1. A charged tRNA enters the ribosome at the A site
2. The bond attaching the amino acid chain to the tRNA in the P site is broken and a
peptide bond is formed to the new tRNA amino acid in the A site moving the growing
chain from the P site to the A site.
3. The large subunit of the ribosome shifts one codon, moving the tRNA with the growing
amino acid chain back to the P site, and the tRNA that no longer has an amino acid
attached is moved to the E site. This is called Translocation.
4. The small subunit of the ribosome shifts into alignment with the large subunit. This
ejects the used tRNA from the E site.
5. Step 1 repeats. A charged tRNA enters the A site
Prokaryotic elongation factors
 EF-Tu: mediates the entry of the charged tRNA into a free site of the ribosome.
 EF-Ts: serves as the guanine nucleotide exchange factor for EF-Tu, catalyzing the release
of GDP from EF-Tu.
 EF-G: catalyzes the translocation of the tRNA and mRNA down the ribosome at the end
of each round of polypeptide elongation.
Eukaryotic elongation factors
 eEF-1: has two subunits, α and βγ.
o α acts as counterpart to prokaryotic EF-Tu, mediating the entry of the
charged tRNA into a free site of the ribosome.
o βγ acts as counterpart to prokaryotic EF-Ts, serving as the guanine nucleotide
exchange factor for α, catalyzing the release of GDP from α.
 eEF-2: This is counterpart to prokaryotic EF-G, catalyzing the translocation of the tRNA
and mRNA down the ribosome at the end of each round of polypeptide elongation.
Termination
 Termination occurs when a stop codon enters the A site.
o Stop codons are UAA, UAG, and UGA
 There are no tRNA that bind to the stop codons, instead release factors bind to the codon.
 These release factors trigger the hydrolysis of the bond in the peptidyl-tRNA and release
the newly synthesized protein from the ribosome.
 After the peptide chain is released the ribosome breaks apart.
Prokaryotic releasing factors
 RF1 – recognizes the UAA and UAG stop codons
 RF2 – recognizes the UAA and UGA stop codons
 RF3 – catalyzes the release of the RF1 and RF2 at the end of termination
Eukaryotic releasing factors
 eRF1 – recognizes all the stop codons
 eRF3 catalyzes the release the completed polypetide