Humic Acid
Definition
Organic polymer from soil degradation that inhibits PCR by intercalating into double-stranded DNA and disrupting primer annealing.
- Origin
- Soil degradation product
- Mechanism
- Intercalates into double-stranded DNA
- Effect
- Disrupts primer annealing during PCR
- Category
- PCR inhibitor
Common questions
How is humic acid inhibition typically detected before it wastes a PCR run?+
Internal PCR controls or quantification assays with an internal amplification control can flag inhibition by showing delayed or absent amplification of the control target relative to expected quantity, prompting a cleanup or dilution step before the main STR reaction is run.
How do forensic labs typically remove humic acid from a DNA extract?+
Common approaches include additional purification steps such as silica-based or magnetic bead cleanup, sample dilution to reduce inhibitor concentration relative to target DNA, or the use of inhibitor-tolerant PCR master mixes formulated for challenging samples.
Why is humic acid a particular concern for buried remains casework?+
Skeletal or soil-contact samples recovered from burial contexts are especially prone to humic acid co-extraction alongside degraded DNA, compounding the low-template challenge already posed by degraded biological material.
Related terms
- Allelic Drop-Out
- Failure of one allele of a heterozygous locus to amplify detectably, a stochastic effect of low-template amplification where sampling of limited template...
- ESSplex SE QS
- Qiagen kit covering the European Standard Set of 17 loci plus DYS391 and Amelogenin; optimised for EU Prüm database upload.
- GlobalFiler
- Applied Biosystems 25-plex STR kit covering 24 STR loci plus Amelogenin, compatible with both CODIS 20 and ESS17 requirements.
- Haematin
- Degradation product of haemoglobin that inhibits Taq polymerase by chelating Mg2+ ions and binding to the polymerase active site; most common inhibitor...
- Identifiler Plus
- Applied Biosystems 16-plex STR kit covering the original 13 CODIS loci plus D2S1338, D19S433, and Amelogenin. Widely used through the 2010s; insufficient...
- Investigator 24Plex QS
- Qiagen 26-plex STR kit with Quality Sensor (IPC) and covering 24 STR loci plus DYS391 and Amelogenin. GO! variant uses increased cycles...
- IPC (Internal PCR Control)
- A non-forensic DNA sequence spiked into the PCR reaction at fixed concentration; a shift in IPC Ct value relative to the expected...
- Low-Copy-Number (LCN) PCR
- PCR protocol using increased cycle numbers (typically 34 cycles) developed at the UK FSS to generate profiles from template inputs below ~100...
- PowerPlex Fusion 6C
- Promega 23-plex STR kit covering all 20 CODIS core loci, all 17 ESS loci, DYS391, and Amelogenin across 6 dye channels.
- Probabilistic Genotyping
- Software-based interpretation of complex DNA electropherograms using continuous statistical models (e.g. STRmix, TrueAllele, ArmedXpert). Generates a likelihood ratio for complex mixtures previously...