3.5/7.4 Translation


– takes place on ribosomes
– ribosomes are placed in the cytoplasm, outside the nucleus
– a ribosome has a small and large subunit
– mRNA binds to small subunit
– tRNA molecules carry the specific amino acid, corresponding the anti-codon
– tRNA binds to ribosome at the place where it’s anti-codon matches the anti-codon on the mRNA
– two tRNAs bind at once and the first one in transfer the growing polypeptide chain to the second one
– the ribosome moves along the mRNA and the process continues until a stop codon is reached  when the completed polypeptide is released

Details of Translation 

– translation always begins in the cytoplasm
(when proteins are destined to be in lysosomes or export:
– ribosomes bind to endoplasmic reticulum and complete the process of translation)
– ribosomes are composed of ribosomal RNA (rRNA) and other proteins
– each ribosome is composed of two subunits – small and large
– each ribosome has three tRNA binding sites – the E (exit) or P (peptidyl) or A (aminoacyl) sites
– more than one ribosome can be actively translating the same mRNA molecule at the same time
– polyribosome/polysome: complex ribosomes along a single mRNA

Initiation, elongation & termination

I: mRNA molecule binds to the small ribosomal subunit at an mRNA binding site
– and initiator tRNA molecule carrying methionine then binds at the start of codon AUG
– the large subunit then binds to the small one
– initiator tRNA is in the P site
– codon signals another tRNA to bind
– occupies A site
– peptide bond is formed between amino acids in the P and A sites
E: ribosome translocated three bases along mRNA
– moves the tRNA in the P site into the E site
– this allows tRNA with the appropriate anticodon to bind to the next codon and occupy the vacant A site
T: process continues until a stop codon is reached
– direction of movement is along the mRNA 5′ to the 3′ end

Data-based questions: interpreting electron micrographs

1) Translation – polysomes are in the image
2) Replication – there is the loop in the image
3) Transcription – relation to translation


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