I have seen gel images in several literature. Almost in all, markers and controls are loaded in the extreme lanes (before 1st sample or after last sample) of the gel. I am just curious, is there any reason or logic to do so? Is there any problem if I load it in the middle lanes, in between my samples
No important reason. It is for esthetics, and also for ease of description in figure legends. One could argue that it might be confusing to have the markers or the control in the middle, but since the relative locations of these wells would be recorded, it does not matter where you put them. In publication, it would be easier to say, for example, "The control is in the far right lane, and the ladder is in the far left lane" Rather than saying, for example "Third from the left".