Structure of Ribosomes

Ribosomes comprise a Small Subunit (SSU) on top of a Large Subunit (LSU). Each subunit contributes to a Peptidyl ("P") site and and an Aminoacyl ("A") site. There is also an Exit ("E") site (not shown). The P site is typically shown on the left, but remember that "top" & "left" are arbitrary conventions to make rRNA transactions easier to describe.