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BIOLOGY 1P03 Lecture Notes - Ribosome, Methionine, Ribosomal Rna

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Lovaye Kajiura

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Initiation: Translation begins when tRNA and RNA bind to a ribosome
- the first amino acid in all proteins is a methionine (AUG codon)
- an initiation complex a small ribosomal subunit, a methionine tRNA, and a methionine
amino acid binds to an AUG codon in an mRNA molecule
- a large subunit of the ribosome joins the complex to complete the assembly of the ribosome
- the methionine tRNA binds to the first binding site on the large ribosome subunit
Elongation: amino acids are added one at a time to the growing protein chain
- Assembled ribosomes have two binding sites and a catalytic site
- The first binding site has methionine and its rRNA attached.
- The second biding site accepts another tRNA with an anticodon complementary to the
codon on the mRNA associated with the second biding site
- The catalytic site forms peptide bond between the two amino acids
- The ribosome moves to the next codon on mRNA and shifts the growing amino acid
chain from the second to the first biding site
- The third amino acid is then added to the chain
- The ribosome moves along mRNA, adding one amino acid to the next
Termination: a stop codon signals the end of translation
- Ribosome encounters a stop codon in the mRNA sequence that signals that protein
synthesis is complete.
- Stop codons do not bind tRNA, but rather they bind proteins that cause the ribosomes
to release the complete amino acid chain
- The large and small subunits of the ribosome also come apart once the stop codon is
Summary: Transcription and Translation
- Transcription of a protein-coding gene produces an mRNA that is complementary to the
template strand of the DNA for the gene
- Enzymes in the cytoplasm attach the appropriate amino acid to each tRNA
- The mRNA moves from the nucleus to the cytoplasm
- tRNAs carry their attached amino acide to the ribosome
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