Quantification Calculators

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Quantification in molecular biology and biochemistry is the measurement of the amount or concentration of a biological molecule — DNA, RNA, or protein — in a sample. Accurate quantification is essential for setting up experiments at the correct concentration and for calculating yields during purification procedures. DNA and RNA are commonly quantified by UV absorbance at 260 nm (A₂₆₀; 50 μg/mL dsDNA gives A₂₆₀ = 1.0) using a NanoDrop or Qubit fluorometer. Proteins are quantified by Bradford assay, BCA assay, or A₂₈₀. Cell numbers are quantified by hemocytometer, flow cytometry, or OD₆₀₀.

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Nucleic Acid Quantification

Beer-Lambert law: A₂₆₀ = ε × c × l. Conversion factors: dsDNA: 50 μg/mL per A₂₆₀ unit. ssDNA/RNA: 40 μg/mL per A₂₆₀ unit. ssRNA: 40 μg/mL. A₂₆₀/A₂₈₀ purity ratio: pure DNA ≈ 1.8–2.0; pure RNA ≈ 2.0. A₂₆₀/A₂₃₀ ≈ 2.0–2.2 (< 1.8 = organic contamination). NanoDrop: 1–2 μL; fast; UV path 0.1 mm. Qubit fluorometer: uses dye specific to DNA (PicoGreen) or RNA — more accurate for low concentrations; less affected by contaminants.

Protein Quantification

  • Bradford assay (Coomassie G-250): dye binds protein → A₅₉₅ shift; BSA standard curve; fast; sensitive to detergents
  • BCA (bicinchoninic acid): Cu²⁺ reduced to Cu⁺ by protein → BCA complex → A₅₆₂; more accurate; tolerates more interfering agents than Bradford
  • A₂₈₀: aromatic residues absorb at 280 nm; fast and non-destructive; requires known extinction coefficient

Cell Number Quantification

OD₆₀₀: E. coli ≈ 8×10⁸ cells/mL at OD = 1.0. Hemocytometer: count in defined volume. Flow cytometry: cell counting + characterization simultaneously.

Glossary

A₂₆₀ Quantification
DNA/RNA concentration from absorbance at 260 nm: dsDNA = A₂₆₀ × 50 μg/mL; ssRNA = A₂₆₀ × 40 μg/mL; purity check by A₂₆₀/A₂₈₀ ≈ 1.8–2.0 for DNA; measured by NanoDrop or UV spectrophotometer.
BCA Assay
Bicinchoninic acid protein quantification: protein reduces Cu²⁺ to Cu⁺; Cu⁺ + BCA → purple complex read at 562 nm; compared to BSA standard curve; tolerates detergents; more accurate than Bradford.
Qubit Fluorometer
Fluorescent quantification using dye specific to dsDNA, ssDNA, or RNA; more accurate than NanoDrop at low concentrations; less affected by contaminants; preferred for NGS library preparation.

Frequently Asked Questions

A₂₆₀ (absorbance at 260 nm) is the standard method. Conversion factors: dsDNA: 1 A₂₆₀ unit = 50 μg/mL. ssDNA: 1 A₂₆₀ = 33 μg/mL. ssRNA: 1 A₂₆₀ = 40 μg/mL. Concentration = A₂₆₀ × conversion factor × dilution factor. Example: dsDNA sample has A₂₆₀ = 0.42 (undiluted): concentration = 0.42 × 50 = 21 μg/mL. Purity: A₂₆₀/A₂₈₀ ≈ 1.8–2.0 (pure DNA); < 1.7 = protein or phenol contamination; A₂₆₀/A₂₃₀ ≈ 2.0–2.2 (< 1.8 = organic contamination from extraction). NanoDrop (ThermoFisher): requires only 1–2 μL; fast; path length 0.1 mm.

Bradford assay: Coomassie Brilliant Blue G-250 dye binds to protein hydrophobic regions → shifts absorption from 465 nm to 595 nm → read A₅₉₅. Fast (5–10 min). Sensitive: detects 1–25 μg protein. Incompatible with: SDS > 0.1%; Tween > 0.01%; high concentrations of reducing agents. Linear range: 1–25 μg. BCA (bicinchoninic acid) assay: Cu²⁺ reduced to Cu⁺ by peptide bonds in protein; Cu⁺ + 2 BCA → purple complex; read A₅₆₂. More reproducible. Compatible with more detergents (SDS up to 5%; Triton X-100 up to 1%). Takes longer (30–60 min at 37°C). Slightly more sensitive to contaminating reductants (DTT, β-ME). Both: use BSA (bovine serum albumin) or IgG standard curves for accuracy.

The Qubit fluorometer uses fluorescent dyes that are highly specific for dsDNA (PicoGreen-based), ssDNA, or RNA — the dye only fluoresces when bound to the target nucleic acid. Advantages over NanoDrop: more accurate at low concentrations (0.2–100 ng/μL for Qubit High Sensitivity; NanoDrop becomes inaccurate below ~5 ng/μL). Less affected by contaminants: protein, phenol, guanidinium that absorb at 260 nm can inflate NanoDrop readings; Qubit dye is specific for the target nucleic acid. Requires 2 μL. When to use Qubit: library preparation for next-generation sequencing (critical to have accurate DNA amounts); any downstream application where concentration accuracy matters (restriction digestion, cloning). When NanoDrop is sufficient: routine quality check of purified DNA/RNA; when throughput speed is important.

Quantitative Western blotting measures relative protein abundance using densitometry: Expose membrane to ECL and capture image (film or CCD imager). Use software (ImageJ, Bio-Rad Image Lab) to measure integrated band density (intensity × area) for: target protein band and loading control band (GAPDH, β-actin, or total protein stain). Normalized target = target band density / loading control band density. Relative expression = normalized target in treated / normalized target in control. Critical requirements: linear signal range — bands must not be saturated; if saturated, dilute sample or reduce exposure. Housekeeping gene validation — GAPDH and β-actin must not change between experimental conditions. Total protein normalization (Ponceau S or REVERT stain) is more accurate than a single housekeeping protein. Report as fold change ± SEM with n ≥ 3 biological replicates.