greater the migration distance of the sample, the better is the resolution of separation. The average size of synthetic nucleic acid polymers are frequently determined by zonal centrifugation Isopycnic or equilibrium density centrifugation separates based on particle density, not size. Particles migrate to a location where he density of the particle matches the density of the centrifuga tion medium Purification of plasmid DNA on cesium chloride is an example of isopycnic centrifugation Voet and Voet(1995)and Rickwood (1984)discuss the techniques and applications of isop- cnic separations. Pelleting exploits differences in solubility, size, or density in order to concentrate material at the bottom of a centrifuge tube (Figure 4.1). The rotors recommended for these procedures are described in Table 41 etie(a) DENSIT GRADIENT a83 Figure 4.I Types of density gradient centrifugation.(a) Rate-zonal centrifugation. The mple is loaded onto the top of a preformed density gradient (leff), and centrifugation results in a series of zones of particles sedimenting at different rates depending upon the particle sizes (right).(b) Isopycnic centrifugation using a preformed density gradient. The sample is loaded on top of the gradient (lefr), and each sample particle sediments until it reaches a density in the gradient equal to its own density (right). Therefore the final posi- tion of each type of particle in the gradient(the isopycnic position) is determined by the particle density.(c) Isopycnic centrifugation using a self-forming gradient. The sampl mixed with the gradient medium to give a mixture of uniform density (left). During sub- sequent centrifugation, the gradient medium re-distributes to form a density gradient and the sample particles then band at their isopycnic positions(right). From Centifugation: A Practical Approach(2nd Ed. ) 1984 Rickwood, D, ed. Reprinted by permission niversity Press. Troutman et al56 Troutman et al. greater the migration distance of the sample, the better is the resolution of separation.The average size of synthetic nucleic acid polymers are frequently determined by zonal centrifugation. Isopycnic or equilibrium density centrifugation separates based on particle density, not size. Particles migrate to a location where the density of the particle matches the density of the centrifuga￾tion medium. Purification of plasmid DNA on cesium chloride is an example of isopycnic centrifugation. Voet and Voet (1995) and Rickwood (1984) discuss the techniques and applications of isopy￾cnic separations. Pelleting exploits differences in solubility, size, or density in order to concentrate material at the bottom of a centrifuge tube (Figure 4.1). The rotors recommended for these procedures are described in Table 4.1. Figure 4.1 Types of density gradient centrifugation. (a) Rate-zonal centrifugation. The sample is loaded onto the top of a preformed density gradient (left), and centrifugation results in a series of zones of particles sedimenting at different rates depending upon the particle sizes (right). (b) Isopycnic centrifugation using a preformed density gradient. The sample is loaded on top of the gradient (left), and each sample particle sediments until it reaches a density in the gradient equal to its own density (right). Therefore the final posi￾tion of each type of particle in the gradient (the isopycnic position) is determined by the particle density. (c) Isopycnic centrifugation using a self-forming gradient. The sample is mixed with the gradient medium to give a mixture of uniform density (left). During sub￾sequent centrifugation, the gradient medium re-distributes to form a density gradient and the sample particles then band at their isopycnic positions (right). From Centifugation: A Practical Approach (2nd Ed.). 1984. Rickwood, D., ed. Reprinted by permission of Oxford University Press