Part 11 Translation

5' UTR

not translated; allows ribosome to bind to the mRNA and positions the ribosome to interact properly with the start codon.

Shine-Dalgarno sequence

a prokaryotic ribosome-binding site on mRNA. It forms hydrogen bonds with a complementary sequence in the 16S rRNA of the small ribosomal subunit, helping initiate translation.

Kozak sequence

positions the ribosome to recognize the start codon. (eukaryotes)

7-mG cap

the eukaryotic ribosome binding site on mRNA.

Start codon

5'-AUG-3'

fmet

start codon encodes this amino acid in bacteria

met

start codon encodes this amino acid in eukaryotes.

coding region function

contain start and sense codons; converts codons into amino acids.

stop codon

serves as a signal to end translation; it is not part of the coding region.

3' UTR

3' untranslated region that plays a role in transcription termination.

Genetic code: unambiguous

each codon specifies only 1 amino acid

Genetic code: Commaless

there are no breaks or pauses between codons

genetic code nonoverlapping

the nucleotides are read in triplets (codons) without reuse.

genetic code universal

used in all organisms.

free amino (NH3+) group

amino or N-terminus; found closer to 5' end of mRNA

peptide bond

covalent bond formed between amino acids

carboxyl terminal or C-terminal

free carboxy group; found closer to 3' end of mRNA.

tRNA

adaptor molecule

tRNA function 1

tRNA recognizes and binds to a specific mRNA codon using its complementary anticodon, forming hydrogen bonds to ensure the correct amino acid is added to the growing protein.

tRNA function 2

tRNA carries a specific amino acid to the ribosome, where it is incorporated into the polypeptide chain based on the codon-anticodon match.

tRNA acceptor stem

amino acid attachment site

tRNA anticodon

Found in the anticodon loop of tRNA; helps bind to the mRNA codon in the ribosome.

charging tRNAs

attaching an amino acid to the 3' end of a tRNA.

charging of tRNAs energy source

ATP

tRNA charging steps

1. Amino Acid Binding - Aminoacyl-tRNA synthetase binds to its specific amino acid and ATP.

2. Activation - ATP is cleaved, releasing pyrophosphate (PP), and AMP attaches to the amino acid, activating the enzyme.

3. tRNA Recognition - The enzyme binds to a tRNA with the correct anticodon for the amino acid; incorrect tRNA is released.

4. Amino Acid Attachment - The amino acid is covalently attached to the 3' end (acceptor stem) of the tRNA.

5. AMP Release - AMP is released, completing the attachment process.

6. Charged tRNA Release - The fully charged tRNA is released and ready to be delivered to the ribosome for translation.

wobble

Flexibility in the base-pairing rules in which the nucleotide at the 5' end of a tRNA anticodon can form hydrogen bonds with more than one kind of base in the third position of a codon.

isoacceptor tRNAs

two or more tRNAs that differ at the wobble position but can recognize the same codon

advantages of wobble

1. Increased Translation Efficiency - Wobble allows one tRNA to recognize multiple codons, speeding up translation.

2. Energy Conservation - Cells only need 30-40 tRNAs instead of 61, saving energy in tRNA production.

prokaryotic ribosome

Size: 70S. Subunits: 30S (16S rRNA), 50S (5S, 23S rRNA). Function: Binds to mRNA, recognizes codons, breaks tRNA-amino acid bond, forms peptide bonds (23S rRNA).

eukaryotic ribosome

Size: 80S. Subunits: 40S (18S rRNA), 60S (5S, 5.8S, 28S rRNA). Function: Binds to mRNA, recognizes codons, breaks tRNA-amino acid bond, forms peptide bonds (28S rRNA).

eukaryotic vs prokaryotic ribosome

Prokaryotic ribosome is 70S, eukaryotic ribosome is 80S. Prokaryotic ribosome has 30S and 50S subunits; eukaryotic has 40S and 60S subunits.

translation initiation

The small subunit attaches to the mRNA upstream of the start codon (AUG). The large subunit then joins the complex, and the tRNA with the corresponding anticodon binds to the start codon, positioning itself in the P site.

translation elongation

During elongation, the ribosome moves along the mRNA, reading codons and matching them with the appropriate tRNA carrying amino acids. Peptide bonds are formed, creating a growing polypeptide chain.

tRNA binding sites and their function

Aminoacyl site (A site): Charged tRNAs enter the ribosome at the A site. Peptidyl site (P site): Holds growing polypeptide. Exit site (E site): Allows uncharged tRNA to exit.

translation termination

When the ribosome reaches a stop codon, translation ends. The mRNA and polypeptide are released, and the ribosomal subunits dissociate for recycling.

IF2 and IF3 function in translation initiation prokaryotes

Help bring the starting tRNA to the ribosome and assist ribosome assembly. Prevent premature binding of large subunit.

energy source translation initiation prokaryotes

GTP

elF2 and CBP1 function translation initiation eukaryotes

eIF2 brings tRNA (Met) to the 40S ribosomal unit (cleaves GTP). CBP1 recognizes the 7-mG cap on eukaryotic mature mRNAs.

energy sources eukaryotic translation initiation

GTP and ATP

translation elongation eukaryotes step 1

Charged tRNA Delivery: A charged tRNA is brought to the ribosome's A site by eEF1A, with GTP being cleaved in the process.

translation elongation eukaryotes step 2

Codon-Anticodon Checking: The anticodon of the tRNA forms hydrogen bonds with the mRNA codon. If mismatched, translation pauses until the incorrect tRNA is released.

translation elongation step 3

Peptide Bond Formation: The amino acid in the P site tRNA is transferred to the A site tRNA, forming a peptide bond. Catalyzed by 28S rRNA (peptidyltransferase).

translation elongation step 4

Ribosome Translocation: The ribosome moves one codon along the mRNA. The tRNA with the polypeptide chain moves to the P site, and the uncharged tRNA exits via the E site. eEF2 helps translocate the ribosome using GTP.

eukaryotic equivalent of 23S rRNA, EF-Tu, and EF-G

23S rRNA → 28S rRNA, EF-Tu → eEF1A, EF-G → eEF2.

RF1

recognizes stop codons UAA and UAG.

RF2

recognizes stop codons UAA and UGA.

energy source translation termination

GTP

eukaryotic translation termination

Uses a single eRF that recognizes all three stop codons.

prokaryotic transcription and translation is coupled

Prokaryotic cells lack a nucleus.

Why is it advantageous to have multiple ribosomes translating a mRNA molecule simultaneously?

More ribosomes make more copies of the protein at the same time, increasing efficiency and speeding up protein production.