Ancient DNA

Ancient DNA is DNA isolated from ancient specimens. It can be also loosely described as any DNA recovered from biological samples that have not been preserved specifically for later DNA analyses. Examples include the analysis of DNA recovered from archaeological and historical skeletal material, mummified tissues, archival collections of non-frozen medical specimens, preserved plant remains, ice and permafrost cores, Holocene plankton in marine and lake sediments, and so on.



Unlike modern genetic analyses, ancient DNA studies are characterised by low quality DNA. This places limits on what analyses can achieve. Furthermore, due to degradation of the DNA molecules, a process which correlates loosely with factors such as time, temperature and presence of free water, upper limits exist beyond which no DNA is deemed likely to survive. Current estimates suggest that in optimal environments, i.e. environments which are very cold, such as permafrost or ice, an upper limit of around 1 million years exists. As such, early studies that reported recovery of much older DNA, for example from Cretaceous dinosaur remains, may have stemmed from contamination of the sample.

DNA may contain a large number of postmortem mutations, increasing with time. Some regions of polynucleotide are more susceptible to this degradation so sequence data can bypass statistical filters used to check the validity of data. Due to sequencing errors, great caution should be applied to interpretation of population size. Substitutions resulting from deamination cytosine residues are vastly overrepresented in the ancient DNA sequences. Miscoding of Cytosine to Thmine and Guanine to Adenine accounts for the majority of errors.

article source: wikipedia 

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