Cells Contain RNA as well as DNA

In the nineteenth century it had been discovered that cells have a second kind of nucleic acid - what we now call ribonucleic acid (RNA). Unlike DNA, which is located primarily in the nucleus, RNA is found in abundance in the cytoplasm as well as in the nucleus. Within the nucleus, RNA is concentrated in few dense granules (nucleoli) that are attached to chromosomes.

Both DNA and RNA resemble proteins in that they are constructed from many smaller building blocks linked end to end. However, nucleotides, the building blocks of nucleic acid, are more complex than any amino acid. Each nucleotide contains a phosphate group, a sugar moiety and either a purine or pyrimidine base. When nucleotides are linked together in large numbers, they are called polynucleotides.

Early on, the sugar component of RNA was known to be different from that DNA. Yet it was not until the 1920's that the work of Phoebus Levine of the Rockefeller Institute revealed that the sugar of DNA is deoxy ribose while the sugar in RNA is ribose. Two purines and two pyrimidines are found in both DNA and RNA. The two purines, adenine and guanine is used in both DNA and RNA; the pyrimidine cytosine is like wise found in DNA and RNA. However, the pyrimidine thymine is found only in DNA, while the structurally similar pyrimidine - Uracil appears in RNA.

In both DNA and RNA, the nucleotides are linked together to form very long poly nucleotide chains. The linkage consists of chemical bonds running from the phosphate group of one nucleotide to the deoxyribose (or ribose) group of the adjacent nucleotide. Each deoxyribose (ribose) residue contains several atoms to which phosphate groups might attach, and there was initially much difficulty in identifying the exact atoms that are bridged by the phosphate group.

Another question that went unanswered for a long time was how the four different ribonucleotides are ordered along a DNA (or RNA) molecule. No methods existed for even estimating the exact amounts of the four nucleotides within the DNA or RNA ; as late as the 1940s, the possibility could not be dismissed that DNA and RNA had regular repeating structures in which each base was repeated every four nucleotides along the poly nucleotide chain. More interesting, however, was the alternative possibility, that there were a very large number of different DNA and RNA molecules, each with its own specific irregular sequence of bases. If this were the way these molecules were constructed, DNA and RNA could have encoded in their varying base sequences the massive amount of information needed to specify the order of the very large number of amino acid sequences found in the proteins of the living world