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Question: How is DNA used to help solve crimes?
One crucial aspect of criminal investigations is determining the relationship between biological evidence and suspects. For example, if blood, hair, or other biological material are collected from a crime scene, investigators would like to know if these belonged to the victim, identified suspects, or perhaps someone else. DNA can be very useful in these determinations.
In most cells of our bodies we have about 3,000,000,000 base pairs of DNA that code for roughly 20-30,000 structural proteins and the regulatory sequences that control the timing and location of their expression that together make up our genome. Different regions of our genome have different functions and evolve at different rates. Genes that code for proteins for basic biological structures like histones, the "spools" around which our DNA are wrapped into chromosomes, are extremely conserved across all animal species. In contrast, there are other regions of DNA that consists of variable numbers of repeats of simple sequences, have no apparent function and thus, evolve very rapidly. When we compare DNA from two randomly chosen people the number of these simple repeats are often different.
If we were to compare all 3,000,000,000 base pairs of DNA from two individuals we would find many differences, especially in the quickly evolving repeat regions. Even "identical" twins have some differences in their DNA that result from mutations that occurred during their separate development from a single fertilized egg. Since it is not feasible to compare the entire genomes of individuals, forensic geneticists usually focus on a set of rapidly evolving repeat regions. The idea is that each region of DNA compared is an opportunity to observe differences between the sample collected and the suspect. If we look at enough sites we should be able to find differences if the two DNA samples came from different individuals. Thus, if we examine 30 rapidly evolving sites and the DNA from the crime scene and suspect are identical, the courts can be sure beyond a reasonable doubt that the evidence originated from the suspect.
On the other hand, a single difference at one of the examined sites is all that is needed to provide a "molecular alibi" for a suspect. Currently across the nation law enforcement agencies are re-examining biological samples from crime scenes with modern DNA techniques and in many cases are freeing innocent people who have languished on death row, in some cases, for decades for crimes they did not commit.
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