University:
Felician University
Course:
BIO-104 | General Biology II
Academic year:

2019
Views:

264

Pages:

1
Author:

SpaceCadet101

TRANSLATION Ribosomes Translation Ribosomes are the site of polypeptide synthesis (translation) • They are composed of ribosomal RNA and protein Translation is the process of polypeptide synthesis and this process involves a repeated cycle of four key events Ribosomes consist of two subunits: • Small subunit contains an mRNA binding site • Large subunit contains three tRNA binding sites (E, P, A) Initiation (component assembly) • The small ribosomal subunit binds to mRNA and moves in a 5’ → 3’ direction to the START codon (AUG) • The complementary tRNA molecule binds to the START codon via its anticodon Multiple ribosomes can translate a single mRNA sequence simultaneously (these are collectively called a polysome) polysomes mRNA DNA Transfer RNA Transfer RNA (tRNA) carries amino acids to the ribosome • Amino acids are attached by tRNA-activating enzymes The tRNA-activating enzyme functions in two steps: • The enzyme joins ATP to an amino acid (‘charging’) • ‘Charged’ amino acid is linked to tRNA (AMP is released) The purpose of ‘charging’ the amino acid is to create a high energy bond that can be be used during translation • Ribosomes use this energy to synthesise peptide bonds Each tRNA-activating enzyme is specific to a particular amino acid, but may bind multiple tRNA (due to degeneracy) Protein Structure Proteins have four levels of structural organisation: • 1º structure = sequence and number of amino acids • 2º structure = folding into α-helix or β-pleated sheet • 3º structure = three-dimensional shape of a polypeptide • 4º structure = presence of multiple polypeptide chains • The large subunit aligns itself to the tRNA molecule at its P-site and forms a complex with the small subunit Elongation / Translocation (polypeptide synthesis) • A tRNA molecule pairs with the next codon (via A-site) • The ribosome covalently attaches the amino acid in the P-site to the amino acid in the A-site (via peptide bond) • The ribosome moves along one codon position and the deacylated tRNA molecule is released (from the E-site) • The elongation and translocation processes continue along the mRNA coding sequence in a 5’ → 3’ direction Termination (component disassembly) • When a ribosome reaches a STOP codon, a polypeptide is released and the ribosome disassembles into subunits Repeat E PA E P A E P A Translocation New tRNA E P A Peptide bond formation Protein Destinations In prokaryotes, the absence of a nuclear membrane allows translation to occur immediately after transcription In eukaryotes, translation will occur at one of two locations: • Free ribosomes (cytosolic) synthesise intracellular proteins • Bound ribosomes (rER) synthesise proteins destined for secretion from the cell or for use in lysosomes