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TRANSLATION

TRANSLATION. The reading of the mRNA transcript and the creation of the polypeptide chain. RIbosomes: Composed of a small subunit (40S) and a large subunit (60S) INITIATION: Ribosome recognizes the 5' cap on the mRNA transcript and binds to the RNA

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TRANSLATION

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  1. TRANSLATION • The reading of the mRNA transcript and the creation of the polypeptide chain. • RIbosomes: • Composed of a small subunit (40S) and a large subunit (60S) • INITIATION: • Ribosome recognizes the 5' cap on the mRNA transcript and binds to the RNA • The subunits act as a clamp (80S) around the mRNA http://www.mtmi.vu.lt/pfk/funkc_dariniai/nanostructures/images/0901whitesides_ribo.jpg http://www.modares.ac.ir/elearning/Dalimi/Proto/Lectures/week2/ribosome_1%5B1%5D.gif

  2. GENETIC CODE • There are 20 amino acids found in proteins, only 4 bases in mRNA (U C A G) • Codons: sequences of three bases used to code for an a.a. • 43=64 possible codons (some amino acids have more than one codon) • Ex. UUU UUC, UCU, UCC all code for phenylalanine (a.a.) • This redundancy helps to reduce errors • AUG: start codon (Methionine) used 99% of the time • UAA, UAG, UGA: stop codons • The mRNA transcript is read in sets of 3 nucleotides (one codon) to determine which a.a. is next.

  3. http://anthro.palomar.edu/biobasis/images/protein_synthesis_at_ribosomes.gifhttp://anthro.palomar.edu/biobasis/images/protein_synthesis_at_ribosomes.gif

  4. ELONGATION of the POLYPEPTIDE CHAIN • AUG is the first codon read for every protein (start codon) • Ensures the correct reading frame (phase in which the mRNA is read) is used by the ribosome • The large subunit of the ribosome has two sites for tRNA • A (acceptor) site • P (peptide) site • tRNA carrying the methionine enters the P 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. • The ribosome moves over one codon. The second a.acid moves to the P site, a tRNA brings the next a.acid to the A site and it 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.

  5. http://sun.menloschool.org/~birchler/cells/animals/ribosome/synthesis.GIFhttp://sun.menloschool.org/~birchler/cells/animals/ribosome/synthesis.GIF

  6. tRNA • tRNA delivers amino acids to the ribosome • Looks like a cloverleaf • Has an anticodon – sequence of three bases that recognize the mRNA codon (complementary to) • 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 • Aminoacyl t-RNA synthases add the amino acids to tRNA at the binding site, the tRNA is now a aminoacyl-tRNA

  7. TERMINATION • Ribosome will reach a stop codon (UGA, UAG, UAA) • No tRNA 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. MODIFICATIONS • 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.

  8. 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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