Eukaryotic
ribosome rRNA / tRNA / mRNA complex
Ribosomes comprise a Small Subunit (SSU) and a Large Subunit
(LSU) that includes rRNA molecules. Each
subunit contributes to an Exit
("E"), Peptidyl
("P") site,
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 the mRNA molecule,
starting with the 5'
end.
IMPORTANT: The EPA sites may be drawn left to right, or right to left (APE), and the LSU and SSU above or below each other. Remember that "top / bottom" & "left / right" are arbitrary conventions to make 5'3' rRNA transactions easier
to describe.
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 the mRNA molecule,
starting with the 5'
end. IMPORTANT: The EPA sites may be drawn left to right, or right to left (APE), and the LSU and SSU above or below each other. Remember that "top / bottom" & "left / right" are arbitrary conventions to make 5'
3' rRNA transactions easier
to describe.