Translation

tRNA and rRNA

Each kind of tRNA has an anticodon that is complementary to an mRNA codon and also carries one specific arnino acid.

After the mRNA arrives in the cytoplasm, an anticodon on a tRNA bonds to the codon on the mRNA, and thus a correct amino acid is brought into place.

Three Stages of Translation

In initiation, a complex forms in this sequence: initiator tRNA + small ribosomal subunit + mRNA + large ribosomal subunit.

In elongation, a start codon on mRNA defines the reading frame. A series of tRNAs deliver amino acids in sequence by codon-anticodon matching. A peptide bond joins each amino acid to the next in sequence.

With termination, a stop codon is reached. A stop codon in the mRNA moves onto the ribosomal binding site. No tRNA has a corresponding anticodon. Proteins called release factors bind to the ribosome. mRNA and the polypeptide chain is released into the cytoplasm or enters the cytomembrane system for further processing.

In cells that are rapidly making proteins, polysomes consisting of many ribosomes are translating the same mRNA transcript to make polypeptides in a hurry. The transcript threads through the multiple ribosomes like the thread of a beaded necklace.

Newly synthesized polypeptides either join the cytoplasm's pool of proteins or enter the endoplasmic reticulum and move through the cytomembrane system in preparation for export.

This animation (Audio - Important) describes translation.

REVIEW: Ribosomes function as
Answer two-part units.