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Lec 30

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University of Lethbridge
BIOL 1010
Igor Kovalchuk

Translation Lec 33 Ribodomes-free int eh cytosol or attached ot rough ER?  Messenger RNA for secretory proteins code for initial signal sequence of 16- 20 hydrophobic amino acids at the amino end of the newly forming polypeptide  When a ribosome beings to synthesizes a protein with a signal sequence, it moves to the ER membrane by a mechanism that involves two other components  Signal recognition particle (SRP) o SRPs are a complex of protein and RNA (SRP RNA) o They serve as adaptor between the translation complex and the ER o SRPs first attach tot the signal sequence of growing polypeptide and link the translation complex to a receptor protein on the ER membrane (SRP receptor) o The receptor is built into the ER membrane  The signal recognition particle docks with the receptor and the ribosome thus becomes bound to the ER  The ribosome continues protein synthesis and the leading end of the new polypeptide (N-terminus) threads into the cisternal space  The signal sequence is removed by an enzyme  Newly formed polypeptide is released from the ribosome and folds into its native conformation  If an mRNA does not code for a signal sequence, the ribosome remains free and synthesizes its protein in the cytosol o Different signal sequences may also dispatch proteins to specific sites other than the ER- eg. Newly formed proteins may be targeted to mitochondria or chloroplasts. In these cases translation is completed in cytosol The signal mechanism for targeting proteins to the ER 1. Polypeptide synthesis begins on a free ribosome in the cytosol 2. A signal-recognition particle (SRP) beings to the signal peptide 3. The SRP then binds to a receptor protein in the ER membrane. This receptor is part of a protein complex, here called a translocation complex, that also includes a membrane pore and a signal-cleaving enzyme. 4. The SRP is released, and the growing polypeptide translocate across the membrane. The signal peptide stays attached to the membrane 5. The signal-cleaving enzyme cuts off the peptide 6. The rest of the completed polypeptide leaves the ribosome and folds into its final conformation RNA plays multiple roles in the cell  1. Information carrier. Messenger RNA (mRNA) carries genetic information from DNA to ribosomes0 this genetic message specifics a proteins primary structure  2. Adaptor molecule- tRNA acts as an adaptor in protein synthesis by translating information from one form (mRNA nucleotide sequence) into another (protein amino acid sequence). SRP RNA helps direct translation complexes to the ER  3. Catalyst and structural molecule. o During translation ribosomal RNA plays a structural and probably enzymatic roles in ribosomes o Small nuclear RNA (snRNA) in rnsRNP particles also plays important roles within spliceosomes that catalyze RNA splicing reactions  4. Viral genomes. Some viruses use RNA as their genetic material. Protein synthesis in prokaryotes and in eukaryotes  While transcription and translation are similar in prokaryotes and eukaryotes, there are some notable difference sin the cellular machinery and in some details of the processes  The following differ: o RNA polymerases- those from eukaryotes depend on transcription factors. o Termination of transcription o Ribosomes o Location  Prokaryotes lack nuclei, so transcription is not segregated with translation  Consequently translation may begin as soona s the 5’ end of mRNA peels away from template DNA, even before transcription is complete 
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