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PROTEIN SYNTHESIS - TRANSLATIONDNA, and in some cases RNA, is the primary source of heritable information.
TranslationConverts mRNA sequence information in to polypeptide sequences.
Occurs at the ribosome.
tRNATransfer RNA molecules.
Responsible for bringing amino acids to the ribosome
• Amino acids are added to tRNA molecules through the action of "amino-acyl tRNAsynthase" enzymes.
The Genetic CodeThe genetic code is interpreted as a series of 3-nucleotide codons. 64 possible codons, which code for all 20 amino acids, along with 1 START codon and 3 STOP codons.
“Redundant, Unambiguous, Punctuated”
Ribosomes• The site of protein synthesis.• The only "non-membrane" bound organelle.• All cells have ribosomes.• Composed of two subunits.Has three "sites":
• A site: "Aminoacyl"- where amino acids enter the ribosome
• P site: "peptidyl"- where the growing polypeptide is kept.
• E site: "exit"- where empty tRNA molecules leave.
Translation - Initiation• The mRNA attaches to the small
ribosomal subunit.• Methionine is brought to the start
codon (AUG) by the methionine tRNA.• The ribosome assembles so that the
start codon (AUG) is in the P-site.• This is called the "translation initiation
complex".tRNA binding at the ribosome is mediated by an "anti-codon" loop in the tRNA molecule
Translation - Elongation
The ribosome shifts ("Translocates"). The tRNA with the polypeptide is now in the P-site.The uncharged amino acid is now in the E-site.
The growing polypeptide is transferred to the new tRNA molecule. A peptide bond is formed.
The next codon determines the next amino acid to be brought to the ribosome. The incoming charged tRNAenters at the A-site.
The next codon is now available in the A-site for the next incoming charged tRNA
Translation - Termination
• When a stop codon (UAG, UAA, or UGA) is encountered, a release factor binds to the A-site.
• The polypeptide chain is released.
• The ribosome disassembles.
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