  All proteins in living organisms are produced in ribosomes that facilitate
the translation of genetic information into a sequence of amino acid residues.
During translation, the ribosome undergoes stages of initialization,
elongation, termination, and recycling. In fact, peptide bonds are formed only
during the elongation stage, which comprises periodic association of transfer
RNAs and multiple auxiliary proteins with the ribosome, and the addition of an
amino acid to the nascent polypeptide one at a time. The protein spends a
considerable amount of time attached to the ribosome. Here, we conceptually
divide this portion of the protein lifetime into three epochs. We define the
epochs on the basis of the position of the N-terminus of the nascent
polypeptide within the ribosome exit tunnel and the context of the catalytic
center. We argue that nascent polypeptides experience a variety of forces that
determine how they translocate through the tunnel and interact with the tunnel
walls. We review current knowledge about translocation and identify several
white spots in our understanding of the birth of proteins.
