DNA is stored in the nucleus of eukaryotic cells. This means that DNA can be harvested from any sample containing eukaryotic cells apart from a pure sample of mature red blood cells because red blood cells do not contain a nucleus.
Usually, when a DNA sample is required, a blood sample is collected, and DNA is extracted from the white blood cells. However, if blood sampling is impossible or too risky (children, animals, forensics), smaller amounts of DNA may also be extracted from a mouth swab (containing epithelial cells) or a few hair follicles.
There are several types of DNA, including genomic, mitochondrial and complementary DNA.
Genomic DNA is the full-length DNA sequence, including coding (exons) and non-coding (introns) DNA. Non-coding DNA denotes DNA sequences that do not code for any specific proteins. Humans have a high percentage of non-coding DNA in the genome; in fact, almost 98% of our genome consists of non-coding DNA! While this sequence may not code for specific proteins, research indicates that it is important for gene regulation and more functions of this non-coding DNA is being discovered all the time. All somatic cells in our body will have the same genomic DNA sequence unless there has been a somatic mutation or aberrations have occurred during the course of life.
Mitochondrial DNA is found inside the mitochondria of cells and differs from the nuclear DNA in that it comes from the mother only whereas nuclear DNA comes from both the mother and father of the organism.
Complementary DNA (cDNA) is not found inside cells but is created by reverse transcription of RNA in a test tube. RNA is first isolated from cells and then reverse transcribed into cDNA. The RNA isolated from a cell contains ribosomal RNA, transfer RNA and messenger RNA. By analysing the messenger RNA that is contained within a cell, scientists can find out which genes were active when the cell was isolated. RNA is easily degraded so it is reverse transcribed into cDNA which is much more stable. cDNA is used in the quantitative PCR method to quantify the level of gene expression. Because complementary DNA is created on the basis of an RNA template, it only contains the exons of the gene; the introns are spliced out during RNA production.