  Synthesis of protein molecules in a cell are carried out by ribosomes. A
ribosome can be regarded as a molecular motor which utilizes the input chemical
energy to move on a messenger RNA (mRNA) track that also serves as a template
for the polymerization of the corresponding protein. The forward movement,
however, is characterized by an alternating sequence of translocation and
pause. Using a quantitative model, which captures the mechanochemical cycle of
an individual ribosome, we derive an {\it exact} analytical expression for the
distribution of its dwell times at the successive positions on the mRNA track.
Inverse of the average dwell time satisfies a ``Michaelis-Menten-like''
equation and is consistent with the general formula for the average velocity of
a molecular motor with an unbranched mechano-chemical cycle. Extending this
formula appropriately, we also derive the exact force-velocity relation for a
ribosome. Often many ribosomes simultaneously move on the same mRNA track,
while each synthesizes a copy of the same protein. We extend the model of a
single ribosome by incorporating steric exclusion of different individuals on
the same track. We draw the phase diagram of this model of ribosome traffic in
3-dimensional spaces spanned by experimentally controllable parameters. We
suggest new experimental tests of our theoretical predictions.
