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Translation. Translation is the process of building a protein from the mRNA transcript. The protein is built as transfer RNA ( tRNA ) bring amino acids (AA), one at a time, to the ribosomal unit holding the mRNA. tRNA.
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Translation • Translation is the process of building a protein from the mRNA transcript. The protein is built as transfer RNA (tRNA) bring amino acids (AA), one at a time, to the ribosomal unit holding the mRNA.
tRNA • 3D structure has an attachment site for an AA on one side and the anticodon on the other • anticodon is the complimentary sequence to the mRNA codon • if the mRNA is AAA the tRNA is UUU the AA attached is phe (phenylalanine) • is "L" shaped with a 3' attachment site for an AA and a loop on the other side that carries the anticodon • each tRNA is specific for the AA it attaches to • 45 different versions in the cytoplasm at all times that correspond to the different AAs • should be 61 for the different codons • a few can bind to multiple codons due to a loosely attached 3' site - wobble
tRNA are joined to the correct AA by the enzyme aminoacyl-tRNAsynthetase • each synthetase fits ONLY the tRNA and AA it is specific for • 20 different versions in the cytoplasm at all times • catalyzes the attachment of the AA to the tRNA with the use of ATP • the released tRNA/AA complex is called an activated tRNA or aminoacyltRNA • the activated tRNA delivers its AA to the growing polypeptide chain
Ribosomes • Ribosomes facilitate the binding of the tRNA to the mRNA by aligning the active sites and holding them in place. • Structure • ribosomes are made of mostly ribosomal RNA (rRNA) that is made from DNA that resides in the nucleolus • consists of a large unit and a small unit • each is made of rRNA and proteins imported from the cytoplasm • once assembled the 2 units are exported to the cytoplasm via membrane pores in the nuclear envelope • has 3 binding sites • A - site binds next tRNA with AA atached • P - site binds the tRNA whose AA is being attached to the growing chain • E - site binds the leaving tRNA minus the AA (already attached to the growing chain)
Initiation of translation • Small ribosomal subunit binds with the mRNA and searches for the start codon (AUG) • initiation tRNA (Met) binds to the P-site of the small unit • union of the mRNA & tRNA establishing the reading frame • large unit attaches after the union is made completing the initiation complex - translation begins • proteins called initiation factors bring the units together • energy from GTP is used in the formation of the unit
Elongation of Growing Polypeptide • requires several proteins called elongation factors • 3 GTP are required for each translocation steps (APE) • mRNA moves from 5' to 3' • AAs are added N-terminus to C-terminus with new AAs added to the carboxyl end • elongation ends when a stop is reached - UAG, UAA, UGA • do not code for an AA but binds to a release factor that binds to the adjacent A-site • the release factor binds a water molecule to the end of the polypeptide hydrolyzing the chain • the ribosomal unit comes apart • may be translated by many ribosomes at the same time - ployribosomal unit
Post Translation Modifications • proteins begin to spontaneously fold as they are synthesized • some will require additional proteins (chaperonins) • free ribosomes make proteins destined for the cytosol • bound ribosomes make proteins for the membrane systems of the cell • (NucEnv, ER, Golgi, Lysosomes, Vacuoles, Plasma membrane) as well as secreted proteins (Insulin) • ALL start as free ribosomes and bind to the ER if a signal peptide is present • about 20 peptides that form the signal-recognition particle (SRP) which attaches to the ER • after attachment the SRP is removed by enzymes and translation of the membrane protein continues • each membrane system has its own unique SRP - much like a zip code