Almost everyone has heard of DNA and knows the role it plays in heredity. Another extremely important nucleic acid in the cell is RNA. Ribonucleic acid (RNA) serves many functions in a cell and is typically a linear polymer consisting of a sugar-phosphate backbone with nitrogenous bases projecting off of the backbone, a structure very similar to that of deoxyribonucleic acid (DNA). Both of these nucleic acids carry genetic information based on the order of the bases that are present along the chain of the molecule. Yet, the presence of a single atomic change in the sugar molecule that is used in the synthesis of RNA makes the chemistry of RNA very different from that of the chemistry of DNA.
The Sugar-Phosphate Backbone of RNA
The backbone structure of the RNA polymer encodes absolutely no genetic information, but provides the linear, molecular pole off of which the different nitrogenous bases (the information coding molecules) are hung. Just as in DNA, the sugar molecules are joined together through what is known as a "phosphodiester bond". This means that the sugar molecules actually had phosphate groups already attached to them. In fact, every sugar molecule containing "nucleotide" used as a precursor in the synthesis of a nucleic acid must have a chain of three phosphates separated by oxygens (O-P-O-P-O-P-O) attached to one specific carbon atom of the sugar, at what is known chemically as the number 5 position.
Similarities Between RNA and DNA
RNA and DNA both encode specific genetic information. Both molecules use nitrogenous “bases” to convey this information. Adenine, guanine and cytosine (the letters A, G, and C) are used in both RNA and DNA. And while DNA uses the base known as thymine (T) for its fourth letter, RNA molecules use the base known as uracil (U) instead. Thus the four “letters” found in DNA are A, G, C, and T, while RNA molecules have A, G, C, and U. The bases are attached to the sugar molecule through a bond at the number 1 carbon atom.
Structurally speaking, both RNA polymers and DNA polymers are able to form a helical structure. DNA is, of course, well known for being present in cells as a double helix, where two strands of complementary DNA intertwine with each other. While most RNA inside of a cell is synthesized as a single stranded molecule, RNA is well known for forming a large amount of double stranded regions when complementary sequences of bases within a single molecule form self-complementary double helices.
This occurs because A can pair with U (or T in DNA) and G can pair with C. And the base pairing rules do not require that complementary sequences of bases be present in different strands of nucleic acid, they can occur within the same strand.
RNA and DNA Contain a Different Sugar Molecule
The single chemical difference in the backbone of an RNA versus a DNA polymer occurs in the sugar. RNA is built from the 5 carbon-containing ringed sugar known as ribose, and thus the name ribonucleic acid. In DNA, the sugar is almost identical, but instead of ribose, DNA polymers use 2-deoxyribose, and thus their chemical name is deoxyribonucleic acid. In 2-deoxyribose, the number 2 carbon does not have an attached hydroxyl (-OH) group. This one chemical change makes an enormous difference in the chemistry of the molecule.
The presence of the oxygen atom at the carbon 2 position of the sugar makes RNA a much less robust molecule chemically compared to DNA. RNA molecules can be readily broken down by exposure to alkaline pH while DNA is resistant. The extra oxygen also changes the way in which an RNA molecule absorbs ultraviolet light, being able to absorb more than DNA on a molecule per molecule basis. And while RNA also can form a helical structure, the atomic spacing that occurs in an RNA helix is different from the spacing that is seen in a DNA helix. It is pretty remarkable that a change in one atom can lead to so many differences in chemical behavior.
Need help with the language of nucleic acid chemistry; see my article on nucleic acid terminology.
For further reading see: “Genes IX”, by Benjamin Lewin, Jones & Bartlett Publishers; ISBN-10: 0763740632
Kenneth Rosen - I am a medical research doctor with more than 20 years of experience studying how the nervous system and skeletal muscle develop and ...
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