Degraded DNA: The Limits of Detection

Fire and Very Hot Temperatures

Fire and very high heat are among the quickest ways to ruin DNA. For example, during cremation or big fires, extreme heat can destroy it. When temperatures reach around 760°C to 982°C (1400°F to 1800°F), all living material, including DNA, simply burns up. This intense heat breaks the DNA’s inner structure, unwinds its famous double-helix shape, and turns its small parts into ash. What’s left is only the non-living parts of bones or just ash.

Even strong body parts like teeth or a tough bone in the skull (the petrous bone), which are often chosen for DNA because they protect it better, almost never have useful DNA after being fully cremated. If something is only partly burned at lower temperatures (like 200–500°C), tiny pieces of DNA might remain. However, these pieces are usually too damaged to be copied or read properly. So, trying to get DNA from burned remains typically fails. This often means police must use other clues. These can include dental records, special chemical tests, or information about the place where the remains were found.

Damage from Chemicals

Chemicals can really hurt DNA a lot. Very strong liquids, like powerful acids or bases (such as drain cleaner), or even things like bleach, hydrogen peroxide, or a chemical called formalin, can melt or break DNA into tiny pieces. When this happens, the DNA can’t be used for tests.

At crime scenes, if DNA samples get cleaning products on them, or if a body is kept in formalin after being checked, there’s often not enough good DNA left to make copies. Formalin is a common chemical used to stop body parts from rotting. But it also sticks DNA to other things called proteins. This stops a special process, called PCR, that we need to make many copies of DNA to study it.

Also, in very old historical places, DNA can be broken or changed by natural chemicals in the ground, like tannins in sour soil, or by dirt from factories. All these chemicals make the DNA lose its perfect shape, which it needs to be understood. So, these samples aren’t very useful for finding out about crimes or learning about the past.

Damage from the Environment

The place where DNA is found greatly affects how long it can survive. If DNA is exposed to water, heat, sunlight (UV rays), or tiny living things (like germs) for a long time, it breaks down faster. This is especially true in acidic dirt or in places that are hot and wet.

Water causes a process called hydrolysis. This process uses water to cut the chemical links in the DNA, breaking it into smaller pieces. This happens even more easily when certain chemicals (enzymes) or acidic conditions are present. Tiny living things like bacteria and fungi also help destroy DNA by eating the organic material it’s part of.

DNA inside bones can last for thousands of years in perfect conditions. These perfect spots are very dry, cold, and stable, like frozen ground or desert caves. However, if samples are stored poorly after they are dug up, the DNA can get much worse very quickly.

Rooms with changing temperatures and too much or too little moisture will cause more water damage and allow tiny germs to grow. This makes it much less likely that any useful DNA will be found. For example, old samples from archaeological sites might decay in just a few months if they aren’t stored in special, controlled conditions, even if they looked well-preserved when first discovered.

Broken DNA and Testing Problems

Sometimes, even if DNA lasts, it’s broken into such tiny pieces that normal tests used by police or archaeologists can’t use it. DNA fragments that are very short (often smaller than 100 base pairs) are too hard for common PCR tests. These tests need longer, complete sections of DNA to make good copies.

When this happens, scientists might only be able to look at mitochondrial DNA (mtDNA). There’s a lot more mtDNA in each cell (hundreds or thousands of copies) compared to nuclear DNA (just two copies).

However, mtDNA is less special. It only comes from the mother and doesn’t have the specific genetic differences that make each person unique, like nuclear DNA does. This makes it harder for police to identify someone exactly, or for archaeologists to study the genes of whole groups of old people.

Newer, more complex ways like next-generation sequencing (NGS) or hybridization capture can sometimes find and read these short DNA bits. But these methods cost a lot of money and are not always possible for labs that don’t have much money or advanced tools.

What This Means for Crime Solvers and Historians

When DNA is totally broken, it causes big trouble. This affects both police and people who study the past. For police, if DNA is missing, they cannot learn who a dead person is. They cannot find the person’s family. And they cannot connect clues from a crime to the bad person. So, they must use other ways. They check teeth. They look at fingerprints. Or they learn from bones.

For people who study the past, lost DNA is also bad. They cannot learn much about old groups of people. They cannot see how people moved around the world. And they cannot learn how humans changed during many, many years. This means they need to use other tools. They might test old items to find what people ate or where they went. They might look at bone shapes. Or they might read old books or writings. These other ways help a lot. But they often do not give answers that are as clear or as full as DNA can.

How to Help and Other Solutions

It’s very important to protect DNA right from the start so it doesn’t get damaged. When people dig up old items or remains, they must wear gloves. This stops their own DNA from getting onto the samples. After finding them, samples should be kept dry, cool, and clean. This slows down any harm.

It’s best not to use strong chemicals like formalin to keep samples. Instead, alcohol (like ethanol) or special drying powders (like silica) are better. These help DNA last longer.

For very old things found by archaeologists, move them quickly to a special storage place. This room should be cool (around 4°C) and not wet. This is key.

At crime scenes, clear rules must be followed. This stops samples from touching cleaning products or dirt from the outside.

If we can’t get DNA from a sample, other methods can still give us important facts:

• Looking at chemicals: Scientists can check elements like strontium, oxygen, or carbon in bones or teeth. This can tell us where a person lived, what they ate, or places they went.
• Studying bone shapes: By looking at a skeleton’s form, we can guess a person’s age, sex, or ancestry. But this is not as exact as DNA.
• Clues from around: Old papers, items found close by, or how someone was buried can give hints about who they were or their way of life.
• Checking proteins: Proteins stay good longer than DNA. Studying them can show what kind of animal or person a sample came from, if it was male or female, or if it had any sicknesses.

Conclusion

People who study crimes and old things need to know why DNA sometimes cannot be found. This often happens because of very hot temperatures, strong chemicals, bad weather, or if the DNA breaks into tiny bits. If experts understand these problems, they can try their best to keep DNA safe and collect as much as possible. When DNA cannot be used, they can then look for other clues instead. This smart way of working helps us learn important facts, even if the DNA is totally damaged. It helps us figure out who people were and understand history much better.