Degradation
Definition
The breakdown of DNA by enzymatic, chemical, or physical processes after biological material is deposited. Degradation produces fragmented DNA that may not amplify correctly across all STR loci, leading to allele dropout and incomplete profiles. The primary drivers are heat, moisture, UV radiation, microbial nucleases, and oxidative chemicals such as bleach.
- Causes
- Heat, moisture, UV, microbial nucleases, oxidants
- Effect on STR typing
- Allele dropout, incomplete profiles
- Pattern
- Larger loci drop out before smaller ones
- Common oxidant
- Bleach and other chemical agents
Common questions
How does degradation show up on an electropherogram?+
Larger STR loci amplify poorly or not at all while smaller loci still give clean peaks, producing a downward-sloping peak height profile across fragment size. Analysts read this ski-slope pattern as a signature of degraded, not necessarily mixed, DNA.
Can a degraded sample still be typed successfully?+
Often yes, using mini-STR kits that target shorter amplicons than standard kits, or by increasing cycle number and input DNA. Severely degraded samples may still yield only a partial profile despite these adjustments.
Related terms
- Mitochondrial DNA (mtDNA)
- DNA located in mitochondria rather than the cell nucleus. Present in hundreds to thousands of copies per cell, making it recoverable from...
- ABO Blood Group System
- Classification of red blood cells by the presence or absence of A and B antigens on the cell membrane, determined by alleles...
- Biological Evidence
- Any material of biological origin recovered in a forensic context that can yield information relevant to an investigation. Includes blood, semen, saliva,...
- Chain of Custody
- The documented chronological record of who collected, handled, transferred, and examined a piece of evidence. For digital evidence, chain of custody includes...
- Leukocyte
- White blood cell. The forensically critical component of blood because it contains a nucleus with a full complement of genomic DNA. Leukocytes...
- Nuclear DNA
- The approximately 3.2 billion base pairs of DNA contained in the nucleus of most human cells, organised across 23 pairs of chromosomes....
- Presumptive Test
- A fast, low-cost, class-specific screen used to triage a caseload before instrumental work begins. Examples are Marquis reagent for opioids and amphetamines,...
- Short Tandem Repeat (STR)
- Short repetitive DNA sequences scattered throughout the genome, with the number of repeats varying between individuals at each locus. DNA profiling compares...
- Touch DNA
- Minute quantities of epithelial cells transferred by skin contact with a surface, without leaving a visible stain. Collected by swabbing contact points...
Explained in these topics
- Blood as Biological Evidence
- The Scope of Biological Evidence in Forensic ScienceThe breakdown of biological molecules over time due to enzymatic activity, microbial growth, heat, moisture, and UV radiation. Degradation shortens DNA strands...