Older mRNA have shorter poly A tails. It seems to correlate with stability. It provides the ability to withstand and avoid degradation. 5' cap is added by enzyme post-transcriptionally. The cap structure is what is recognized by ribosomes to bind and start protein synthesis on eukaryotic mRNAs.
Restriction enzymes are examples of endonucleases.
S value - sedimentaion coefficient, a measure of sedimentation rate in a centrifuge. Despite very different sequences from eukaryotic rRNAs, much of the structure is conserved. This suggests that it is the structure that is important for the function. As well, there is no prokaryotic equivalent to the eukaryotic 5.8 S rRNA, but the 5' end of the 23 S prokaryotic rRNA is similar to the 5.8S rRNA found in eukaryotes. If they aren't methylated, ribosomes don't function correctly.