13.3: Translation - mRNA to Polypeptide

translation

ribosomes synthesize proteins using the information encoded in mRNA - mRNA sequence converted into an amino acid sequence to form a polypeptide chain.

ribosome structure

small and large subunits - both complexes of rRNA and numerous ribosome protein and has a, e and p site

initiation in pro

small subunit recognises start codon by first binding to Shine-Delgado sequence in mRNA - tRNA with methionine placed in correct position and large subunit associates with small subunit to form complete ribosome

initiation in eu

no Shine-Delgado sequence, instead small subunit recognises the 5' prime cap and scans mRNA to identify a start codon before recruiting large subunit

translation elongation

ribosomes move in 5’ to 3’ direction

1. charged tRNAs enter A site and their anticodon binds to complementary mRNA codon

2. rRNA in large subunit catalyse the formation of peptide bonds between amino acids in p and a site

3. uncharged tRNAs move to and released from e site as ribosome shifts by 1 codon = growing chain moves to p site

4. Process repeats

termination

release factors bind to stop codons causing release of newly synthesised proteins and dissociation of ribosomes = can reinitiate translation to generate more proteins

differences in pro and eu gene expression

pro - transcription and translation occurs at same time in cytoplasm, DNA sequence aligns perfectly with mRNA sequence and usually no mods to mRNA after transcription

eu - transcription in nucleus and translation in cytoplasm, regions of DNA in gene don’t align with mRNA due to introns, which are spliced and undergoes mod with 5’ cap and 3’ poly A tail