Dr. Wahied Khawar Balwan
DNA is a molecule which contains information about the organism from which it’s extracted. Extraction of ancient DNA (aDNA) from the remains/fossils of the organisms enables to establish phylogenetic relationships (relating two organisms to a common ancestor) among extinct and modern species. The cloning of aDNA promises the reintroduction of extinct or endangered species into wild in the near future.
DNA (Deoxyribonucleic acid) is a molecule present in all the multicellular organisms. It contains information about the makeup and character of organisms. Many animals become critically endangered or extinct in the wild and once they are extinct they are lost forever. However, after recent advances in cloning and genomics it is now possible to clone extinct or endangered animals. The prime requirement for cloning is naturally the organism’s DNA. But it’s not so easy to clone DNA from fossils or preserved remains of dead animals, such DNA is also called as ancient DNA. Mostly, the ancient DNA (aDNA) obtained from organisms belongs to the Pleistocene era which is about 200,000 to 10,000 years ago.
The aDNA acts as an indirect evidence of historical processes that might have occured long time ago. The DNA normally becomes degraded by enzymes in the dead body which cleave nucleic acids such as DNA, RNA etc. commonly known as nucleases. But sometimes, under few conditions such as high salt concentrations, rapid drying and low temperatures, nucleases get destroyed or inactivated before all nucleic acids are broken down into nucleotides (which are the building blocks of DNA).
RETRIEVING ANCIENT DNA
To retrieve the DNA from dead cells, they are often lysed and methods such as filtration and centrifugation are utilized for the removal of cellular material and debris. The PCR or polymerase chain reaction, a technique which enables to generate multiple copies of DNA in a matter of few hours has made the retrieval of aDNA sequences very easy and hence many samples which have been stored in museums for over two centuries have been utilized and analyzed.
To prevent the contamination of samples by modern DNA, treatment of the laboratory equipment with alcohol (95%–100%), irradiating the entire laboratory and facilities in the laboratory with Ultra Violet (U.V.) radiation, and wearing protective clothing and face shields etc. are executed in the laboratory.
The specimens themselves might be contaminated with modern DNA, and this can be possible despite taking all the necessary measures to prevent contamination. The difficulties increase when we go for extraction of DNA from animals which actually exist even today, for example it’s very difficult to extract proper authentic human DNA from fossils as, human DNA is present in the laboratories or any other site or place of work. Hence, contamination of ancient DNA sequences by human DNA is often mistaken as ancient human DNA in the samples thus making the findings unreliable.
It’s a known fact that the mitochondria and chloroplasts (cell organelles) have multiple copies of their own DNA. Higher amount of copies of the same DNA have high probability of survival as they occur in higher numbers per cell, hence mostly DNA is attempted to be retrieved from mitochondria or chloroplasts. However it’s also to be noted that the nucleic acid sequence of the organism often give more phylogenetic information about the organism.
STUDY OF ANCIENT DNA IN BIRDS
Avian or bird DNA is small all streamlined compared with those of mammals by virtue of having fewer repetitive nucleotides and less non-coding DNA (i.e. DNA contains some regions which don’t show any character of the organism, research suggests that it is present for the stability of the molecule). This condition has been suggested to represent a key adaptation for flight in birds, by reducing the cost of producing energy for the body, associated with having large genome and cell sizes. However, the evolution of genome architecture in birds, or any other lineage, is difficult to study because genomic information is often absent for long-extinct relatives. Recent finding by Morten E. Allentof et. al. showed the identification of aDNA from extinct moa species (Pachyornis elephantopus).
STUDY OF ANCIENT DNA IN MAMMALS
Many fossils of mammals have been found frozen in ice and also in tar pits (such as Rancho la Brea in United States). DNA samples have been isolated and have been studied to find out the phylogentic relationship between them and modern or other ancient species. One such animal is the woolly mammoth (Mammuthus primigenius), which is one of the most intensively studied extinct species at the DNA level, basically because of its well preseved fossils. The comparison of the nuclear DNA of the mammoths with that of modern elephants showed that Asian elephants are the closest relatives of mammoths.
The second animal, a cave bear (Ursus spelaeus), is a relative of modern brown and black bears that lived in caves throughout Europe in the Late Pleistocene. The DNA of this species was isolated from the teeth
samples (about 40,000 years old) and cloned. The obtained fragments of DNA were compared to that of a related organism which was the dog, as its DNA was completely sequenced necessary repair was done and the cave bear DNA was obtained. The DNA sequence was then compared with the DNA sequences obtained from modern black, brown polar bears and phylogenetic tree was constructed. About 0.05% of human DNA contamination was present but it was easily distinguishable. Many ancient human DNA sequences have been retrieved and studies pertaining to the classification of humans, their geographical distribution and relation of Neanderthals (closest relative of modern man) with modern humans (Homo sapiens) have been established with the help of sequencing genomic DNA.
Many other mammalian samples belonging to the Pleistocene era such as the Patagonian Ground sloth (Mylodon darwinii) samples, the saber toothed cat (Smilodon spp.), the marsupial wolf (Thylacinus cynocephalus) and Selerikan horse (Eqqus spp.) samples were cloned and sequenced successfully. Samples from coprolities (fossilized excreta) have also been amplified and sequenced. The coprolites from extinct Shasta ground sloth (Nothrotheriops shastensis) also contained DNA sequences from various plants which it has eaten, further comparison lead to the conclusion that the Shasta ground sloth used to travel high altitudes (about 80 meters) for search of food.
Cloning has become a common process today; the utilization of single embryo in order to develop a complete animal with the same genetic makeup is an established fact. Scientists have succeeded in cloning mice from DNA of mice in frozen conditions this have become successful with modern DNA. This procedure might soon be applied in case of extinct mammals such as the wooly mammoth (Mammuthus primigenius), Saber toothed cat (Smilodon sp.) and marsupial wolf (Thylacinus cynocephalus) where the aDNA has been successfully extracted and phylogenetic classifications have been established. Many attempts have been made to clone various animals which has become either endangered or extinct in the wild such as Wild Ox (Bos gaurus) the Bucardo mountain goat (Capra pyrecenia) their tissues are being preserved at low temperatures and it is hoped that one day with the advent of better cloning procedures we can bring them back into the wild and establish the natural ecological balance.
“Any error in this document is silent testimony of the fact that it was a Human Effort.
Dr. Wahied Khawar Balwan
Assistant Professor & Head
Department of Zoology
Govt. Degree College Kilhotran, Doda.
E-mail: [email protected]
Mob. No.: 9419369557