DNA as the Genetic Material

The nucleic acids were recognized as chemical substances more than 70 years before DNA was found to be responsible for the transmission of inherited characteristics. Later it was suspected that DNA might be the genetic material because of its high concentration in chromosomes and in some viruses. The premise was complicated, however, because the concentration of protein in these structures was 

One property expected of the genetic material is a constancy of amount in every cell of the body under every environmental situation. DNA, not RNA or protein, fulfills this expectation. Its content per nucleus is the same in every cell except the germ cells, which have exactly half that found in the somatic cells. Again, this is expected if progeny obtain half their characteristics from each parent. This constancy is so dependable that the measurement of the DNA concentration in a tissue can be used to calculate die number of nuclei and thus the number of cells. This works well for diploid cells such as those of the kidney, but corrections must be made for polyploid mammalian liver or cancer cells. 

Subsequently, similar experiments were done with viral nucleic acids. The pure viral nucleic acid, when added to cells, led to the synthesis of complete virus particles the protein coat was not required. This process is called transfection. More recently, DNA has been used in cell-free extracts to program the synthesis of RNA that functions as the template for the synthesis of proteins characteristic of the DNA 

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Other experiments also pointed to the conclusion that DNA was the genetic material. DNA was found localized in the nuclei of eukaryotic cells. The absolute amount per cell was constant for a given species. Studies of bacteriophage replication pointed strongly to DNA as the genetic material.11 In 1952 Hershey and Chase showed that when a phage particle infects a cell the viral DNA enters the bacterium, but the protein "coat" remains outside.13 This was demonstrated by... 

Figure 21. Simplified conceplucil representation of the Caims-Smith clay based model for origin of life. Simple organic synthesis occurs on clay surfaces by a photolransducing system. The clays sen e as a genetic " template that is coupled to organic polymerization. Synthesis of ribose and then ribonucleic acids leads to an RNA world, a transition to DNA as the genetic material and perhaps the origin of a phototroph. Phylogenetic development is unclear (Fenchel etal., 1998).

Considering how the scientific community responded to other concepts involving DNA as the genetic material, their acceptance of the Watson-Crick structure was unusually rapid. In 1962, Watson, Crick, and Wilkins were awarded the Nobel Prize in chemistry. 

Enzyme-catalyzed synthesis of deoxyribonucleotides must have preceded the appearance of DNA as the genetic material. Chemical evidence has been summarized previously that spontaneous formation of 2-deoxyribose or its 1-N-glycosides is extremely unlikely in an aqueous medium under socalled primitive earth conditions, and that the only reasonable pathway for their generation is reduction of the more readily formed, more abundant ribonucleotides. This chapter is to explore whether the properties of ribonucleotide reductases, essential catalysts in present-day biochemistry, could be reconciled with a very early origin of reductive deoxyribonucleotide formation. 

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