Density Gradient Centrifugation | Principle and Applications

This section details the Meselson-Stahl experiment that proved DNA replication is semi-conservative. They used E. coli bacteria and nitrogen isotopes: N-14 (light) and N-15 (heavy). Density gradient centrifugation separates DNA by density: light DNA (1.65 g/cm³) floats, heavy DNA (1.80 g/cm³) sinks. Bacteria grown in N-15 produce heavy DNA. When transferred to N-14 and divide once, hybrid DNA forms (1.72 g/cm³) with one heavy and one light strand. When hybrid DNA divides again, two bands appear: light DNA and hybrid DNA. This 50:50 ratio confirms semi-conservative replication where each original strand serves as a template for a new complementary strand.

This lecture covers the Meselson-Stahl experiment that proved DNA replication is semi-conservative. Three models existed: conservative (parental DNA intact), semi-conservative (each strand serves as template), and dispersive (parental material dispersed). The experiment used E. coli bacteria grown in heavy nitrogen (N15), then transferred to light nitrogen (N14). DNA was extracted and subjected to density gradient centrifugation using cesium chloride. After one generation, a single intermediate band appeared, ruling out conservative replication. After two generations, two bands appeared (intermediate and light), confirming semi-conservative replication and ruling out dispersive model. Each daughter DNA contains one parental strand and one newly synthesized strand.

The Meselson-Stahl experiment (1958) proved that DNA replication is semi-conservative. Bacteria were grown in heavy nitrogen (N-15) medium, then transferred to normal nitrogen (N-14). After one generation, all DNA was hybrid (one heavy strand, one light strand) with intermediate density. After two generations, half was hybrid and half was light. This demonstrated that each daughter DNA molecule contains one original strand and one newly synthesized strand. The experiment used density gradient centrifugation to separate DNA based on density, with heavy DNA sinking to higher density regions and light DNA rising to lower density regions.

The Meselson-Stahl experiment (1958) demonstrated that DNA replication is semi-conservative by using density gradient centrifugation to distinguish between parent and daughter DNA strands; cells grown in heavy nitrogen-15 (N15) media were transferred to normal nitrogen-14 (N14) media, and after one generation, only an intermediate-density band appeared, proving that each original DNA strand serves as a template for synthesizing a new complementary strand.

The Meselson-Stahl experiment definitively proved DNA replication is semi-conservative. Scientists used nitrogen isotopes: N-14 (light, 7 protons + 7 neutrons) and N-15 (heavy, 7 protons + 8 neutrons). Bacteria were grown in N-15 medium, making their DNA heavy. After transfer to N-14 medium, DNA was analyzed using density gradient centrifugation. After one generation, a single intermediate-density band appeared, ruling out conservative replication. After two generations, two bands appeared (intermediate and light density), confirming semi-conservative replication. After multiple generations, the intermediate band decreased while the light band increased. This pattern showed that each original heavy strand serves as a template for a new light strand, producing intermediate-density molecules. Subsequent replications produce molecules with two light strands. The experiment provided definitive proof that DNA replication is semi-conservative.

Viral particle purification involves a multi-step centrifugation workflow: initial low-force centrifugation separates soluble virus from cells, followed by cell lysis and sedimentation of debris; viral precipitation using polyethylene glycol creates pellets that are clarified through washing and resuspension; finally, density gradient ultracentrifugation (using sucrose or cesium chloride gradients) isolates active virus based on size and density, achieving purification from empty particles, incomplete proteins, and cellular debris.

Viral particle purification involves a multi-step centrifugation workflow including initial low-force separation of virus from cells, cell lysis and sedimentation of debris, precipitation using polyethylene glycol, washing with sodium benzoate, and finally density gradient ultracentrifugation to isolate active virus based on size and density (1.14-1.24 g/mL), which separates fully loaded virus from empty particles and cellular debris.

Subcellular fractionation systematically isolates organelles through sequential centrifugation at increasing speeds. The nuclear fraction (5-10 μm diameter) precipitates first at 600g for 10 minutes. Mitochondria, chloroplasts, lysosomes, and peroxisomes deposit at 15,000g for 5 minutes. The microsomal fraction (plasma membrane, ER, polyribosomes) requires ultracentrifugation at 100,000g for 60 minutes. Ribosomal subunits and enzyme complexes need even higher speeds. The cytosol remains undeposited after 300,000g for 2 hours. Fractions appear in order of decreasing density: nuclear > mitochondrial > microsomal > cytosol. Organelle purity is assessed using marker molecules (specific enzymes or biomolecules for each organelle) rather than expensive methods like electron microscopy, with DNA testing confirming nuclear isolation.

Ultracentrifuges operate at extremely high speeds ranging from 60,000 to 100,000+ RPM, far exceeding the capabilities of standard high-speed centrifuges. These powerful instruments enable separation of very small particles including proteins, nucleic acids, subcellular organelles, viral particles, and macromolecular complexes. The extreme centrifugal forces allow resolution of molecular weight differences that cannot be achieved with conventional centrifugation, making ultracentrifuges indispensable in advanced biochemistry and molecular biology research.

Organelle isolation begins with gentle cell disruption using osmotic protectors (0.25 M sucrose) and methods like sonication or filtration. Differential centrifugation separates organelles by sedimentation rate: low-speed spins pellet nuclei; higher speeds pellet mitochondria, chloroplasts, lysosomes, and peroxisomes; ultracentrifugation pellets membranes and ER. Equilibrium density gradient centrifugation further purifies organelles by buoyant density (lysosomes ~1.12 g/cm³, mitochondria ~1.15 g/cm³, peroxisomes ~1.23 g/cm³). Antibody-based purification targets specific proteins using protein A from Staphylococcus aureus. Mass spectrometry identifies proteins through trypsin digestion, MALDI-TOF analysis, and database comparison, enabling comprehensive proteomic characterization of organelle compositions.

Analytical Ultracentrifugation (AUC) is a specialized technique using an ultracentrifuge with absorbance and interference detection optics to monitor macromolecules in solution under native conditions, capable of detecting particles at low concentrations without standards; the instrument spins samples at speeds up to 60,000 RPM while measuring optical density or refractive index along the cell radius over time, with two primary experimental approaches—sedimentation velocity (high-speed, creating pellets to calculate sedimentation coefficients, diffusion coefficients, and molecular weights) and sedimentation equilibrium (lower-speed, establishing gradients to determine molecular weight, homogeneity, association constants, and aggregation states)—enabling characterization of proteins, nanoparticles, peptides, polymers, cells, liposomes, drug conjugates, and viral payloads.

Analytical ultracentrifugation uses very high acceleration (up to 250,000 g) to sediment biological macromolecules in solution. Modern systems use optical detection (absorbance and interference) to monitor concentration gradients in real-time. It operates in two modes: sedimentation velocity (measuring sedimentation rate) and sedimentation equilibrium (measuring isopycnic positions). Applications include determining relative molecular mass, oligomeric state, and hydrodynamic properties; assessing sample purity and homogeneity; studying ligand binding interactions. Advantages include working in solution without immobilization, analyzing a wide range of particle sizes, and providing absolute measurements.

Analytical ultracentrifugation uses UV absorption optics (280 nm for proteins) and interference optics (for any macromolecule) to detect sedimenting molecules. Sedimentation velocity experiments measure how fast molecules sediment under centrifugal force, with the sedimentation coefficient (s) representing the rate per unit centrifugal field. Larger particles sediment faster, while asymmetric particles sediment more slowly due to increased friction. At low speeds, opposing forces of sedimentation and diffusion reach equilibrium, creating a stationary concentration gradient dependent only on molecular weight. This equilibrium state enables absolute molecular weight determination independent of calibration standards. The sedimentation coefficient relates to molecular weight through the power law equation s = K' * M^B, where B is the frictional coefficient dependent on conformation.

Ultracentrifugation is an advanced centrifugation technique operating at 60,000-150,000 rpm to separate biological molecules that normal centrifugation cannot handle. Refrigeration prevents heat-induced degradation of DNA and RNA. The sedimentation principle states denser particles settle faster under centrifugal force, with the sedimentation coefficient (v/ω²r) characterizing particle behavior. Biological molecules are too small for gravitational separation due to thermal motion. Analytical ultracentrifugation (AUC) analyzes particle properties during centrifugation using real-time detection systems (absorbance, interference, fluorescence). The sedimentation coefficient enables determination of molecular mass, hydrodynamic properties, and characterization of size/shape changes under different experimental conditions.

Modern AUC instruments like the Optima AUC feature 4-hole rotors (60,000 rpm) or 8-hole rotors (50,000 rpm) with sector-shaped cells containing air gaps, buffer regions, and sample regions. Detection uses interference or absorption optics to record concentration versus radial distance over time, tracking the meniscus boundary movement. Two experimental approaches exist: sedimentation equilibrium (low speed, long duration, 6-sector centerpieces) for accurate molecular weight and binding constants; sedimentation velocity (high speed, short duration, 2-sector centerpieces) for molecular shape determination. Multi-wavelength detection enables simultaneous monitoring of multiple components within single experiments. Sample loading uses narrow-stem pipette tips, with typical volumes around 450 μL requiring absorbance 0.1-1.0 at 230nm for optimal detection.