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.
[1] They are principally during transcription (phenomena of RNA synthesis from DNA template) and translation (phenomena of amino acid assembly from RNA).
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.
[2] The signal peptide is cleaved off in the
endoplasmic reticulum.
[2] Preproproteins have both sequences (inhibitory and signal) still present.
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.
[3] Protein biosynthesis, although very similar, is different for
prokaryotes and
eukaryotes.