密度梯度超速离心

An ultracentrifuge contains a rotor that rotates at highly controlled speeds, simulating a strong gravitational field. Even a small rotor at relatively low rotational speed can create a force thousands of times stronger than Earth's gravitational field. This magnified field increases the differences in density between particles and solvent, accelerating separation.

In step gradients, liquids of decreasing density are carefully layered on top of one another, creating distinct boundaries. In continuous gradients, liquids are mixed in varying proportions so density decreases smoothly from the base upwards. Both types keep liquids in separate layers ordered by density during centrifugation.

Isopycnic density-gradient centrifugation is the simplest and most common procedure for separating cellular organelles using a step gradient. Organelles sediment based on their density, with denser structures descending further. This straightforward approach effectively isolates mitochondria, nuclei, and other cellular structures without destructive force.

Sucrose is added in increasing amounts so each solution is more concentrated and denser than the preceding one. Solution densities must fall between the components being separated, with the final solution denser than the densest component. Exact densities depend on the specific organelles or molecules being isolated from the organism.

Because nucleic acids are denser than sucrose, isopycnic centrifugation cannot separate them from organelles nondestructively. Rate-zonal centrifugation uses a continuous gradient and separates components based on sedimentation rates, which depend on both density and conformation. This technique preserves nucleic acid integrity while achieving separation.

Cesium chloride gradients, typically ranging from 1.65 to 1.75 g/mL, are used to separate nucleic acid strands. Sucrose cannot be used because it is less dense than nucleic acids. Cesium chloride provides sufficient density and low viscosity, allowing separation of DNA strands based on guanine-cytosine versus adenine-thymine content.

After centrifugation, cellular components form discrete bands between solution layers. Fractions can be collected using a syringe or by puncturing the tube bottom with a sterilized needle and collecting outflow in sterile tubes. Isolated components can then be stored at -80 degrees Celsius for further analysis or experimentation using chromatography based biomolecule purification methods.