Deoxyribonucleic acid, or DNA, is often called the instruction manual for life. This biological blueprint contains the genetic information for an organism to develop, survive, and reproduce. These instructions are encoded in long, intertwined strands forming a double helix. While stable, this molecular structure is not permanent and can deteriorate with exposure to environmental pressures. This damaged and fragmented genetic material is what scientists refer to as degraded DNA.
The Process of DNA Degradation
The breakdown of DNA is a natural process accelerated by several environmental and biological factors. Exposure to the elements is a primary cause of degradation. Heat can cause the DNA molecule to unwind and break apart, while moisture can lead to hydrolysis, a chemical reaction that severs the bonds holding the genetic code together. Ultraviolet (UV) radiation from sunlight directly damages the DNA structure, creating kinks and breaks in the strands.
After an organism’s death, biological processes contribute significantly to the decay of its genetic material. Microorganisms like bacteria and fungi release enzymes called nucleases. These enzymes “digest” the DNA by breaking the chemical bonds that form the backbone of the molecule, cutting it into smaller pieces. This microbial action is a major reason why ancient remains often yield very little intact DNA.
Chemical exposure and the passage of time also play a role. Certain chemicals, such as strong acids or formaldehyde, can cause rapid degradation. Even under ideal storage conditions, DNA will naturally fragment over very long periods. The cumulative effect means that DNA recovered from historical artifacts or old crime scenes is almost always a collection of short, damaged segments.
Challenges in Reading a Damaged Blueprint
Analyzing degraded DNA presents considerable challenges for scientists. The most significant problem is fragmentation, where the long strands of the double helix are broken into numerous short, random pieces. This can be compared to shredding an instruction manual, leaving a pile of disconnected words and sentences.
Compounding the issue of fragmentation is the low quantity of usable material. The processes that break the DNA apart also reduce the total amount of recoverable genetic information. In many forensic or archaeological contexts, scientists may only have a few cells to work with, and the DNA within those cells is already severely compromised. This scarcity makes it difficult to obtain enough data for a reliable analysis.
The chemical letters of the genetic code, known as bases, can also be altered by degradation. These chemical modifications can cause one type of base to mimic another, leading to misinterpretations when scientists attempt to read the genetic sequence. Such errors can complicate efforts to identify an individual or accurately reconstruct an ancient genome.
Scientific Methods for Piecing Together Fragments
To overcome fragmentation and low quantity, scientists employ several techniques. One of the most established methods is the Polymerase Chain Reaction (PCR), which functions like a molecular photocopier. PCR can take the few remaining intact DNA fragments in a degraded sample and generate millions of identical copies, providing enough material for analysis.
A specialized application of this technique involves targeting mini-STRs (Short Tandem Repeats). STRs are specific, repeating sections of DNA that vary between individuals. Because mini-STR analysis focuses on very short segments of the DNA strand, it is more likely to find and successfully copy these regions even in highly fragmented samples.
For more comprehensive analysis, researchers often turn to Next-Generation Sequencing (NGS). This technology can process millions of tiny DNA fragments at once, reading the genetic sequence of each piece. Powerful computer programs then take this massive dataset of short sequences and, by looking for overlapping segments, assemble them back into their correct order.
When nuclear DNA is too degraded to yield results, scientists can turn to mitochondrial DNA (mtDNA). Unlike nuclear DNA, mtDNA is found in the mitochondria. Since each cell contains hundreds of mitochondria, there are far more copies of mtDNA available, increasing the chances of recovering a usable genetic sequence from a compromised sample.
Unlocking History and Solving Crimes
The ability to analyze degraded DNA has had a profound impact on multiple fields. In forensic science, these techniques are used to solve cold cases where evidence collected decades ago was previously unusable. DNA extracted from old bones, teeth, or hair can now be analyzed to identify victims of unsolved homicides or mass disasters.
This technology also plays a part in paleogenomics, the study of ancient genetics. Scientists have successfully sequenced degraded DNA from the fossilized remains of extinct species, such as Neanderthals and woolly mammoths. This has provided insights into their biology, their relationship to modern species, and the reasons for their extinction.
High-profile historical investigations have also relied on the analysis of degraded genetic material. One example is the identification of the remains of the Romanov family, the last imperial family of Russia, who were executed in 1918. By piecing together fragmented DNA from the skeletons and comparing it to living relatives, scientists were able to confirm their identities.