I don't fully understand the principle of double-sided size selection of Illumina DNA prep bead clean-up. Can someone explain to me?
I know the first clean-up is using diluted SPB beads to remove right-side large fragments and the second clean-up is using undiluted SPB beads to remove left-side small fragments.
I also know in both clean-up steps, the larger fragments are bound to beads. In the first clean-up, we keep the supernatant which contains the smaller fragments and in the second clean-up we keep the beads which contains the larger fragments.
What I do not know is why the fragments larger than a certain size are bound to the diluted beads and the fragment larder than another size are bound to the undiluted beads,