Topic 7.3: Translation Flashcards Preview

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Flashcards in Topic 7.3: Translation Deck (18)
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1

Ribosomes

Site of polypeptide synthesis

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Composition of Ribosomes

1) Ribosomal RNA
2) Protein

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Subunits of Ribosomes

1) Small subunit (mRNA binding site)
2) Large subunit (tRNA binding sites (E, P, A))

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Polysome

Group of two or more ribosomes translating an mRNA sequence simultaneously

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Transfer RNA

Carries amino acids to the ribosome

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tRNA-activating enzymes

Catalyses the binding of a tRNA molecule with a specific amino acid in the cytoplasm

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Process of tRNA-activating enzymes

1) The enzyme joins ATP to an amino acid (‘charging’)
2) ‘Charged’ amino acid is linked to tRNA (AMP is released)

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Purpose of charging the amino acid (2)

1) Create a high energy bond that can be be used during translation
2) Ribosomes use this energy to synthesise peptide bonds

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Specificity of tRNA-activating enzyme

Specific to a particular amino acid, but may bind multiple tRNA (due to degeneracy)

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Translation

Process of polypeptide synthesis

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Steps of Translation (4)

1) Initiation
2) Elongation
3) Translocation
4) Termination

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Initiation (Component assembly) (3)

1) The small ribosomal subunit binds to mRNA and moves in a 5’ → 3’ direction to the START codon (AUG)
2) The complementary tRNA molecule binds to the START codon via its anticodoni
3) The large subunit aligns itself to the tRNA molecule at its P-site and forms a complex with the small subunit

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Elongation / Translocation (Polypeptide synthesis) (4).

1) A tRNA molecule pairs with the next codon (via A-site)
2) The ribosome covalently attaches the amino acid in the P-site to the amino acid in the A-site (via peptide bond)
3) The ribosome moves along one codon position and the deacylated tRNA molecule is released (from the E-site)
4) The elongation and translocation processes continue along the mRNA coding sequence in a 5’ → 3’ direction

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Termination (Component disassembly)

When a ribosome reaches a STOP codon, a polypeptide is released and the ribosome disassembles into subunits

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Translation in Prokaryotes

The absence of a nuclear membrane allows translation to occur immediately after transcription

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Transcription in Eukaryotes

After transcription, the mRNA must be transported from the nucleus (via nuclear pores) prior to translation by the ribosome

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Protein Destination in Eukaryotes

1) Free ribosomes (cytosolic) synthesise intracellular proteins
2) Bound ribosomes (rER) synthesise proteins destined for secretion from the cell or for use in lysosomes

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Determination of Protein Destination in Eukaryotes

1) Presence / absence of an initial signal sequence on a nascent polypeptide chain
2) Results in the recruitment of a signal recognition particle (SRP), which halts translation
3) The SRP-ribosome complex then docks at a receptor located on the ER membrane (forming rough ER)
4) Translation is re-initiated and the polypeptide chain continues to grow via a transport channel into the lumen of the ER
5) The synthesised protein is transported via a vesicle to the Golgi complex (for secretion) or the lysosome
6) Proteins targeted for membrane fixation (e.g. integral proteins) get embedded into the ER membrane
7) The signal sequence is cleaved and the SRP recycled once the polypeptide is completely synthesised within the ER