Because of their biochemical importance, the nucleic acids receive much more attention than the others. DNA was first isolated in 1869 by F. Miescher from cell nuclei. Nearly 80 years of research have been carried out to identify the major building block units and the basic structure of nucleic acids. DNA molecules from different cells and viruses vary in ratio of the four major types of nucleotide monomers, in their nucleotide sequence, and in their molecular weight. Four major bases (adenine, guanine, thiamine, and cytosine) are found in all DNA. The DNA isolated from different organisms and viruses normally has two strands in complementary double-helical arrangement, and it is the basic compound of genetic material (chromosomes). In its double-helical structure it has a 20 A (1 A=10-8 cm) diameter width and the nucleotides are repeated in every 3.4 A.
This polymeric molecule, DNA, is the chemical basis of heredity and is organized into genes, the fundamental units of genetic information. It was first demonstrated in 1944 in a series of experiments that genetic determination of the character (type) of the capsule of a specific pneumococcus could be transmitted to another of a distinctly different capsular type by introducing purified DNA from the former coccus into the latter. This agent (later shown to be DNA) was called 'transforming factor'. Subsequently ,this type of genetic manipulation has become commonplace. Similar experiments have recently been performed utilizing yeast, cultured mammalian cells, and insects and rodents as recipients, and cloned DNA as the donor of genetic information.
DNA has the ability to replicate itself. It is this property that holds the genetic material in the same type and number sequence during the cell divisions. In prokaryotic cells, which contain only a single chromosome, essentially all the DNA is present as a single double-helical, two-stranded macromolecule exceeding 2 x 10- 9 in molecular weight. In eukaryotic cells, which contain either several or many chromosomes, there ate either several or many DNA molecules.
The double-stranded structure of DNA can he melted in solution by increasing the temperature or decreasing the salt concentration. The denaturation of DNA is used to analyze its structure. Not only do the two stacks of bases pull apart but the bases themselves unstack while still connected in the polymer by phosphodiester backbone.
Careful examination of the model reveals a major groove and a minor groove winding along the molecule parallel to the phosphodiester backbone. In these grooves,proteins can interact specifically with exposed atoms of the nucleotides (usually H bonds) and thereby recognize and hind to specific nucleotide sequences without disrupting the base pairing of the double-helical DNA molecule. As easily seen, regulatory proteins can control the expression of specific genes via such interactions.
The genetic information stored in the nucleotide sequences of DNA serves two purposes. It is the source of information for the synthesis of all proteins of the cell and organism, and it provides the information inherited. Both these functions require that the DNA molecule serve as template - in the first case for the transcription of the information into RNA and in the second ease for the replication of the information into daughter DNA molecules.
DNA is likely to survive for millions of years in some conditions. Amber can provide some of the right conditions to preserve DNA. The most reliable report of DNA to date from amber is that of termite-like sequences from around 30 million year-old Dominican amber. The earlier recovery of DNA from a 20 million year-old magnolia leaf, near Moscow, was more remarkable because the leaf was preserved in a sequence of easily split soft clays and silts, interbedded with layers of volcanic ash.
Recent developments in genetics are providing very powerful new techniques for the analysis of DNA from remains of ancient organisms. The young field of ancient DNA research is less than a decade old but is growing exponentially. In spite of some serious technical difficulties, the study of ancient DNA promises to become a revolutionary research tool in archaeology; anthropology and molecular biology.The earliest report of retrieval of informative DNA sequences from extinct animals - in this case from the desiccated skin of a quagga, a member of the horse family which became extinct more than one hundred years ago - was that by A. Wilson and his colleagues in 1984. Shortly after this, DNA was isolated by S. Pbo from the skin of a pre-dynastic Egyptian mummy; it was shown by DNA hybridization that a small amount of recognizable human DNA was left in the ancient tissue. In 1988, the first report of an amplified ancient DNA sequence was made by S. Pbo and colleagues who retrieved it from a 7,000 year-old skull found in a peat bog.
It seems likely that studies of this kind will become even more popular in the near future. With the ancient DNA sequence studies, one of the most controversial problems in biology - evolution - will possibly be enlightened. No there is a drive to gather genetic information from indigenous groups to increase our understanding of human origins, history and migrations. Ancient DNA studies will play a very important role by providing a direct source of objective evidence on past populations, and perhaps the only reliable insight into the genetic characteristics of vanished peoples.