Specific techniques must be chosen for the isolation of DNA from some samples. Typical samples with complicated DNA isolation are:
* archaeological samples containing partially degraded DNA, see ancient DNA
* samples containing inhibitors of subsequent analysis procedures, most notably inhibitors of PCR, such as humic acid from the soil, indigo and other fabric dyes or haemoglobin in blood
* samples from microorganisms with thick cellular walls, for example, yeast
*samples containing mixed DNA from multiple sources
Extrachromosomal DNA is generally easy to isolate, especially plasmids may be easily isolated by cell lysis followed by precipitation of proteins, which traps chromosomal DNA in insoluble fraction and after centrifugation, plasmid DNA can be purified from soluble fraction.
A Hirt DNA Extraction is an isolation of all extrachromosomal DNA in a mammalian cell. The Hirt extraction process gets rid of the high molecular weight nuclear DNA, leaving only low molecular weight mitochondrial DNA and any viral episomes present in the cell.