The structure of DNA and RNA

We first consider the structure of the macromolecules DNA and RNA that carry genetic information. We will view these macromolecules as another type of polymer accordingly, we will refer to the subunits from which they are composed as 

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There are three interesting derivatives of the pyrimidine molecule. 

UracU (t/), in which two of the H atoms of pyrimidine which were bonded to C atoms are replaced by O atoms. There is a double bond between the C and O atoms, which eliminates two of the double bonds in the hexagonal ring. 

Cytosine (C), in which one of the H atoms of pyrimidine which was bonded to a C atom is replaced by an O atom and another one is replaced by a NH2 unit. 

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The structures of DNA and RNA are similar in that each has a sugar-phosphate backbone with one organic base bound to each sugar. However, there are four distinct differences between RNA and DNA ... 

The basic monomers of nucleic acids are nucleotides which are made up of heterocyclic nitrogen-containing compounds, purines and pyrimidines, linked to pentose sugars. There are two types of nucleic acids and these can be distinguished on the basis of the sugar moiety of the molecule, Ribonucleic acids (RNA) contain ribose, while deoxyribonucleic acid (DNA) contains deoxyribose. The bases cytosine (C) adenine (A) and guanine (G) are common in both RNA and DNA. However, RNA molecules contain a unique base, uracil (U), while the unique DNA base is thymidine (T). These differences in the base structure markedly affect the secondary structures of these polymers. The structures of DNA and RNA are outlined in Appendix 5.2. 

As we said earlier, nucleic acids are the architects and construction contractors for synthesizing proteins. There are two kinds of nucleic acids. DNA, or deoxyribonucleic acid, is the blueprint for synthesis of proteins. RNA, or ribonucleic acid, is the construction contractor. Messenger RNA reads the instructions for synthesis of a protein encoded on a strand of DNA and carries those instructions to the worksite, where transfer RNA brings the amino acids in for incorporation into the polypeptide chain. Now, let s take a closer look at the structures of DNA and RNA. 

Recognize the structures of DNA and RNA, and draw the structures of the common ribonucleotides and deoxyribonucleotides. 

One of the most important discoveries of modern science was the elucidation of the structures of DNA and RNA as the famous double helix by Watson and Crick in 1953. They realized that the basic structure of base-sugar-phosphate was ideal for a three-dimensional coil. The structure of a small part of DNA is shown opposite. 

The Structure of DNA and RNA Double Helices is Determined by Watson-Crick Base-Pair Geometry... 

Chandrasekaran R, Arnott S (1989) The structures of DNA and RNA in oriented fibers. In Saenger W (ed) Nucleic acids, Landolt-BOrnstein New Series Group VII. Biophysics, vol. 1 b. Springer, Berlin, pp 31-170... 

In order to understand the mechanisms of pathogen disinfection by UV light, it is necessary first to be familiar with the structure of DNA and RNA, as they play key roles in two important aspects of reproduction of microorganisms protein synthesis and the replication of chromosomes. 

One of the most important discoveries of modern science was the elucidation of the structures of DNA and RNA as the famous double helix by Watson and Crick in 1953. They realized that the basic structure of base-sugar-phosphate was ideal for a three-dimensional coil. The structure of a small part of DNA is shown on the right. Notice that the 2 (pronounced two prime ) position on the ribose ring is vacant. There is no hydroxyl group there that is why it is called deoxyribonucleic acid. The nucleotides link the two remaining OH groups on the ribose ring and these are called the 3 - and 5 -positions. This piece of DNA has three nucleotides (adenine, adenine, and thymine) and so would be called -AAT- for short. 

Contrast the structures of DNA and RNA and list the function of the three types of cellular RNA. 

Nucleosides and nucleotides are found in places other than as part of the structure of DNA and RNA. We have seen, for example, that adenosine units are part of the structures of two important coenzymes, NADH and coenzyme A. The 5 -triphosphate of adenosine is, of course, the important energy source, ATP (Section 22. IB). The compound called 3, 5 -cyclic adenylic acid (or cyclic AMP) (Fig. 25.6) is an important regulator of hormone activity. Cells synthesize this compound from ATP through the action of an enzyme, adenylate cyclase. In the laboratory, 3, 5 -cyclic adenylic acid can be prepared through dehydration of 5 -adenylic acid with dicyclohexylcarbodiimide. 

Considering the structural diversity of natural products, the structures of DNA and RNA are simple. All their components, called nucleotides, are polyfunctional, and it is one of the wonders of nature that evolution has eliminated all but a few specific combinations. Nucleic acids are polymers in which phosphate units link sugars, which bear various heterocyclic nitrogen bases (Figure 26-10). 

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