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TRANSLATION

TRANSLATION. The reading of the mRNA transcript and the creation of the polypeptide chain. Occurs on the Ribosomes : Composed of a small subunit (40S) and a large subunit (60S) Made up of ribosomal RNA and protein

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TRANSLATION

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  1. TRANSLATION • The reading of the mRNA transcript and the creation of the polypeptide chain. • Occurs on the Ribosomes: • Composed of a small subunit (40S) and a large subunit (60S) • Made up of ribosomal RNA and protein • The mRNA transcript is read by the ribosome in sets of 3 nucleotides to determine the amino acid sequence. • tRNA deliver the appropriate amino acid to the ribosome where the polypeptide chain is built.

  2. Genetic Code • There are 20 amino acids found in proteins, but only 4 bases in the code (U C A G of mRNA) • Codons: sequences of three nucleotide bases used to code for an amino acid. Ex. UAC codes for Tyrosine • 43=64 possible codons • therefore some amino acids can have more than one codon • Ex. UUU UUC, UCU, UCC all code for phenylalanine • This redundancy helps toreduce errors

  3. The Genetic Code • AUG: start codon used 99% of the time (codes for Methionine) • UAA, UAG, UGA: stop codons

  4. tRNA • tRNA delivers amino acids to the ribosome • Looks like a cloverleaf • Has an anticodon – sequence of three bases that recognize (are complementary to) the mRNA codon • ex. The GCC codon specifies the a.a. alanine • The anticodon on the tRNA carrying alanine would be CGG • Opposite arm carries the amino acid binding site • Amino acids are added by Aminoacyl t-RNA synthases to make aminoacyl-tRNA

  5. 1) Initiation • The small subunit of the ribosome recognizes the 5' cap on the mRNA transcript and binds to the RNA. • The ribosome will position itself at AUG (the first codon read for every protein) • Ensures the correct reading frame (phase in which the mRNA is read) is used by the ribosome • The large subunit binds and the subunits act as a clamp (80S) around the mRNA.

  6. Sites on the Ribosome • The large subunit of the ribosome has two sites that the mRNA moves through: • A (acceptor) site: where new amino acids enter the ribosome • P (peptide) site: where the polypeptide chain grows as the amino acids are combined. • At a third site, the catalytic site, the peptide bonds are made between the amino acid in the A site and the P site.

  7. 2) Elongation of the Polypeptide Chain • The AUG codon will be in the P site, tRNA carrying the corresponding amino acid (methionine) enters this site. • The next tRNA carrying the required a.a. enters the A site. • At the catalytic site (b/w A and P) a peptide bond forms between the two amino acids.

  8. The ribosome moves over one codon. The second tRNA moves to the P site, a new tRNA brings the next a.a. to the A site and the a.a. is added to the chain. • The first tRNA is released to be recycled for use with another amino acid. • The process repeats with the polypeptide chain trailing in the cytoplasm.

  9. Translation

  10. Translation

  11. 3) Termination • Ribosome will reach a stop codon (UGA, UAG, UAA) • No tRNA or a.a. exists for a stop codon • A release-factor protein helps the two ribosome subunits to fall off the mRNA and the polypeptide chain is released.

  12. 4) Post-Translational Modifications • Polypeptide chains travel through the RER and are modified to become functional proteins. • Some amino acids may be glycosylated (sugar added) or phosphorylated (phosphate added) or altered in another way. • Enzymes may cleave (cut) the chain at specific places.

  13. Wobble Hypothesis • In codons the third base may differ between 2 codons that code for the same amino acid (UAU and UAC both code for tyrosine). • If the tRNA's anticodon is AUA it can still bind to UAC. • This flexibility allows for the correct amino acid to be added to the polypeptide chain despite errors in the gene sequence. • The proposal that tRNA can recognize more than one codon by unusual base pairing is known as the “wobble hypothesis”.

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