How many restriction sites fragments per genome?

[john brennand]

A 6 base site will occur on average once every 4096 (4x4x4x4x4x4) bp - 
although this will vary depending on the GC/AT ratio in the DNA and in 
the site you are interested in (& whether there are repeats) - but it is 
a good place to start.

Therefore in 3x109 bp there will be 3x109/4096 individual fragments
(approximately)

John