Hello Byungkyung Do

The shape and size of comet is dependent on many factors like the condition of electrophoresis namely, voltage and time and the buffers used. The appearance depends upon the concentration of the stain used, the microscope lamp intensity and the camera setting.

The lower image shows comets with the head and the tail. But the upper image gives an appearance of complete DNA fragmentation. Usually Hedgehog-like comets come from cells in early stages of apoptosis before major fragmentation takes place. Have you treated the cells under same conditions? The concentration and the time of exposure of the genotoxic agent also matters.

Assuming that you have used a genotoxic agent and not a cytotoxic compound because a cytotoxic compound could give an image which you have obtained in the upper half.

Best Wishes.

Hi!

Based on the information that some other slides from the same experiment had "expected" shape, I would guess a possible problem with electrophoresis on some slides. F.e. some part of the slides was not fully submerged in an electrophoretic buffer for the whole time, preventing comet tails to migrate in a consistent way.

Very best,

Magdalena

Hello, I would echo Magdalena's response and say there is a problem with either agarose itself, alkaline unwinding step or the phoresis itself. Since you have slides that look good I wouldn't question the experiment and the drug treatment itself. If you ran the slides in the electrophoretic chamber all at once, then I'll place my bet on the issue with the alkaline unwinding. Good luck!