<P> Protein synthesis is the process whereby biological cells generate new proteins; it is balanced by the loss of cellular proteins via degradation or export . Translation, the assembly of amino acids by ribosomes, is an essential part of the biosynthetic pathway, along with generation of messenger RNA (mRNA), aminoacylation of transfer RNA (tRNA), co-translational transport, and post-translational modification . Protein biosynthesis is strictly regulated at multiple steps . They are principally during transcription (phenomena of RNA synthesis from DNA template) and translation (phenomena of amino acid assembly from RNA). </P> <P> The cistron DNA is transcribed into the first of a series of RNA intermediates . The last version is used as a template in synthesis of a polypeptide chain . Protein will often be synthesized directly from genes by translating mRNA . However, when a protein must be available on short notice or in large quantities, a protein precursor is produced . A proprotein is an inactive protein containing one or more inhibitory peptides that can be activated when the inhibitory sequence is removed by proteolysis during posttranslational modification . A preprotein is a form that contains a signal sequence (an N - terminal signal peptide) that specifies its insertion into or through membranes, i.e., targets them for secretion . The signal peptide is cleaved off in the endoplasmic reticulum . Preproproteins have both sequences (inhibitory and signal) still present . </P> <P> In protein synthesis, a succession of tRNA molecules charged with appropriate amino acids are brought together with an mRNA molecule and matched up by base - pairing through the anti-codons of the tRNA with successive codons of the mRNA . The amino acids are then linked together to extend the growing protein chain, and the tRNAs, no longer carrying amino acids, are released . This whole complex of processes is carried out by the ribosome, formed of two main chains of RNA, called ribosomal RNA (rRNA), and more than 50 different proteins . The ribosome latches onto the end of an mRNA molecule and moves along it, capturing loaded tRNA molecules and joining together their amino acids to form a new protein chain . </P> <P> Protein biosynthesis, although very similar, is different for prokaryotes and eukaryotes . </P>

Where do the amino acids come from in protein synthesis