DNA
sequences
STRs
STRs & individuals
Electrophoresis
DNA length
STR patterns
PCR
Sources
Mitochondrial
DNA
Mitochondrial
patterns
DNA evidence has emerged as the most powerful technique to identify individuals. Why? Each person has a unique DNA sequence (except for identical twins), and only a tiny amount of DNA is needed to match a person with a forensic sample.
DNA is made of strings of units, called bases, joined together in a specific sequence. There are four different bases: A, C, G, and T.
These two short DNAs have the same number of A's, C's, G's, and T's, but their order, or sequence, is different:
1) ACGTACGGGA
2) TCCAAGAGGG
Each human cell has a 3 billion-long sequence of DNA. The sequences of two individuals are, on average, 99.9% identical - but that still leaves several million sequence differences between them. How can we distinguish between individuals? It isn't possible to look at all 3 billion bases. Instead, we can look at a few short stretches of DNA which tend to differ between individuals. These are called Short Tandem Repeats (STRs).
