University:
Western Sydney University
Course:
MEDI 2003 | Clinical Sciences 2
Academic year:

2024
Views:

224

Pages:

3
Author:

Mariam A. A.

Post-transcription and pre-translation • • • Pre-mRNA is processed: (1) introns are spiced out and the 3’ end gets a poly-A-tail, (2) mature mRNA will exit the nucleus through nuclear pores into the cytoplasm rRNA binds to proteins in the nucleus and exist the nucleus as ribosomes tRNA undergoes modifications and is exported from the nucleus into the cytoplasm Amanita phalloides – death cap mushrooms • Produces the toxin α-amanitin → binds to RNA polymerase II → can’t synthesise mRNA → liver failure → death Translation • The genetic code consists of codons that are three consecutive nucleotides on mRNA • A specific codon encodes for 1 specific mino acid e.g. CCA = Proline, GGC = Glycine • Degenerate: multiple codons can code for the same amino acid e.g. GCC, GCA, GCG = Alanine • Codons are read consecutively in the 5’ to 3’ direction – no codons are skipped in the ‘reading frame’ • Start (AUG) and stop (UAA, UAG, UGA) codons • Codons are nearly universal – shared by bacteria up to complex animals • Cell translates information in mRNA in a polypeptide (protein) with the help of tRNA and ribosomes • Requirements: mRNA, tRNA, amino acids, ribosomes (rRNA), initiation factors tRNA • Hairpin-like structure with ‘aa’ attached to the 3’ end • Specific tRNAs will attach to a specific aa • Triplet nucleotide at their base – anticodon (complementary to codon sequence in mRNA) • Aminoacyl-tRNA synthetase links the aa to a tRNA • 1 to 4 tRNAs for each of the 20 aa Ribosomes • Made up of rRNA and other proteins – large subunit and a small subunit • Site for mRNA and tRNA to come together • Three catalytic sites: A,P,E - Aminoacyl-tRNA binding site - Peptidyl-tRNA binding site - Exit site Initiation: Elongation: • Peptide bond formed between 2 aa at A and P site • Reaction uncouples the growing polypeptide chain from the tRNA in the P site and transfers it to the tRNA in the A site • Ribosomes then move one codon forward and the empty tRNA moves into the E site Termination: • Stop codon enters the A site – release factor binds to A site • Triggers release of polypeptide chain from tRNA in P site o Polypeptide breaks away at P • Empty tRNA moves into the E site and off the ribosome/mRNA complex • Termination is completed by release of the mRNA from the ribosome and the dissociation of the ribosome into subunits There will only be 1 polypeptide produced per ribosome, but many ribosomes can act on a single mRNA strand Understanding translation • Synthetic RNA was added to bacterial extracts to see what polypeptides were made Amino acids • R groups can be nonpolar, polar or electrically charged (+/-) → gives proteins diversity