sorry, why we can only see the size of E.coli genome is more than 23 kilo base pair on agarose electrophoresi

Answer 1

The E. coli genome is actually about 46 kb long. The reason it ran shorter is probably because you didn't digest it. It ran supercoiled, which is a much more compact, rod like shape than relaxed DNA would be. This means that supercoiled DNA can run farther on a gel than relaxed DNA, and looks like it is "smaller".

Answer 2

Any agarose gel can only resolve a particular range of sizes. Anything bigger than that comes out in a single band, regardless of size. Thus, although the E. coli genomic DNA is much bigger than 23 kB (which is only about half the size of a virus, very small) it will run in the same position as DNA of that size.

FYI, if you used pulsed field electrophoresis to disentangle the DNA as it runs through the agarose gel, you could actually separate these DNA samples by size. But it's pretty expensive compared to regular electrophoresis equipment.

Answer 3

Did you digest it or run it undigested?

Isolated undigested E. coli genomic DNA is larger than 23 kb and runs as a smear. If you tried to digest it I would think your endonuclease is bad. Try some other enzymes...