rRNA / tRNA / mRNA complex
Ribosomes comprise a Small
(SSU) and a Large Subunit
(LSU) that includes rRNA molecules. Each subunit
contributes to an
Exit ("E"), Peptidyl
and an Aminoacyl ("A")
site. During translation, tRNA molecules enter the right-hand A site, pass to the middle P site, and are released from the
left-hand E site. To ensure
that the mRNA will be read 5'3', it is convenient to
think of the ribosomal complex as moving along a fixed mRNA molecule in the 5'3' direction.
Note however that this
motion is relative: one can also think of the ribosome as fixed, and
with the 5'
sites may be drawn left to right, or right to left, and the LSU and
SSU above or below each other:
remember that "top" & "left" are arbitrary
conventions to make 5'3' rRNA transactions
easier to describe.
Figure © 2010 PJ
Russell, iGenetics 3rd ed.; all
text material © 2010 by Steven M.