DNA EVIDENCE

DNA sequences
STRs
STRs & individuals
Electrophoresis
DNA length
STR patterns
PCR
Sources
Mitochondrial DNA
Mitochondrial patterns

 

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Getting enough DNA to see - the polymerase chain reaction

You can't actually see DNA, because it is colorless. In order to see the DNA in gel electrophoresis, fluorescent dyes of various colors are bound to it. With these dyes it is possible to see tiny amounts of DNA, less than a microgram. However, this is still much more than is present in typical DNA samples. How do we get enough DNA to see for STR analysis?

The polymerase chain reaction (PCR) uses enzymes, called DNA polymerases, which copy DNA to increase its amount by more than a million times. This amplification is very specific - a single STR at a time, out of all the genes in a cell, can be amplified by the PCR technique.

DNA is made of 2 complementary strands: A on one strand always pairs with T on the other strand, and G always pairs with C. So if you know the sequence of one strand, you know the sequence of the other strand too. There are enzymes called polymerases which read the sequence of one strand and string together DNA bases to make the second strand. After the polymerase reaction, there are two DNA molecules for each original DNA. In round two of the polymerase reaction the two molecules are increased to four. Each round doubles the number of DNA molecules.

DNA polymerases need a short stretch of DNA, called a primer, to start their copying. By choosing the right primers you can select which gene to copy.