Have calculated the marker size in base pairs and plotted on a calibration curve. I then measured relative mobility of the DNA fragments and from my graph have obtained their base pairs. However, when I compared the DNA frags to markers on my gel, the base pairs of the DNA do not match up with where the markers are, they are higher up than some of the markers and yet have a lower base pair number. Could this be because my line of best fit on my calibration curve is wrong, or could there be some other reason? Please help!