(To the tune of “Chim Chim Cher-ee”)
I’m obsessed with A-A-G-C-T-T
Because it’s the binding site of Hin-d-III
Cutting up DNA most readily
The ends are not blunt when they’re cut up you see
Five prime overhangs of A-G-C-T
Bacteria don’t have an immune system so
They must fight off phages or they will not grow
Protection by chopping is their strategy
And one of the cutters we call Hin-d-III
On binding to A-A-G-C-T-T
The site recognition gets bent easily
Phosphodiester attacking meanwhile
Has water behaving as nucleophile
To stave off the phage for a little while
Why don’t these enzymes cut cell DNAs?
The answer’s provided by a methylase
Adding a methyl group on top of what
The sequence these enzymes would otherwise cut
So cells get protected in this simple way
From nuclease chewing of their DNA
The phage is not lucky in most every case
Unless methylases win the enzyme race
If that happens then, the cell gets erased
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