What Is the Structure of DNA

Primary structure of nucleic acids The sequence of bases along the pentose-phosphodiester backbone of a DNA or RNA molecule, read from the 5 end to the 3 end. 

5 End The end of a polynucleotide at which the 5 -OH of the terminal pentose unit is free. 

In Chapter 18, we saw that the four levels of structural complexity in polypeptides and proteins are primary, secondary, tertiary, and quaternary. There are three levels of structural complexity in nucleic acids, and although these levels are somewhat comparable to those in polypeptides and proteins, they also differ in significant ways. 

Draw a structural formula for the DNA dinucleotideTG that is phosphorylated at the 5 end only. 

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So, what is the structure of DNA, what are the four letters in its alphabet, how can complex information be stored by such seemingly simple molecules (compared to proteins) and how does it reproduce itself ... 

What are the most central and fundamental problems of biology today They are questions like What is the sequence of bases in the DNA What happens when you have a mutation How is the base order in the DNA connected to the order of amino acids in the protein What is the structure of the RNA is it single-chain or double-chain, and how is it related in its order of bases to the DNA What is the organization of the microsomes How are proteins synthesized Where does the RNA go How does it sit Where do the proteins sit Where do the amino acids go in In photosynthesis, where is the chlorophyll how is it arranged where are the carotenoids involved in this thing What is the system of the conversion of light into chemical energy ... 

So we are still left with two models of the stereochemistry of DNA alkylated by a PAH diol epoxide the PAH either lies in a groove of DNA or else tries to intercalate between the bass of DNA. Since it is covalently bonded to a base it must cause considerable distortion if it tries to lie between the bases. However, the stacking observed in the crystalline state seems to argue for partial intercalation. We will need crystal structures of at least one appropriately alkylated polynucleotide before this problem can be resolved. And when this is done it will be just the beginning of the answer to the problem of alkylation of DNA by activated carcinogens. The subsequent question is, what is the lesion in DNA that is important in carcinogenesis, and then what does it cause to happen so that tumor formation is initiated ... 

The primary action of ionizing radiation is, of course, ionization. This may affect the structure of DNA directly or indirectly through the formation of free radicals from, for example, water. Much attention has been given to what are the important changes in DNA. There is an extensive literature dealing with the many possible... 

How do they recognize each other How does the structure of DNA help explain how genetic information is encoded Before answering such questions, it is important to understand what proteins and nucleic acids are. 

The structure of DNA revealed how information is stored in the base sequence along a DNA strand. But what information is stored and how is this information expressed The most fundamental role ol 1 )NA is to encode the sequences of proteins. Like DNA, proteins are linear polymers. 1 lowever. proteins differ from DNA in two important ways. First, proteins are built from 20 building blocks, called ammo acids, rather than just four, as are present in L NA. The chemical complexity provided by this variety of building blocks enables proteins to perform a wide range of functions. Second, proteins spontaneously fold up into elaborate three-dimensional structures, determined only by their amino acid sequences (Figure LI9). We have explored in depth how solutions containing two appropriate... 

Complementary use, in future, of small angle solution synchrotron X-ray scattering methods with electron microscopy could contribute to the clarification of the path of the linker DNA. Thus further experiments seeking a universal model for the chromatin fibre structure need to be undertaken although the existence of such a universal structure is questionable. More importantly future investigations need to be aimed at imderstanding what relevance the structure of the fibre has to the control of DNA transcription and replication. 

In this chapter we first summarize the basics needed to consider the interactions of metal ions and complexes with nucleic acids. What are the structures of nucleic acids What is the basic repertoire of modes of association and chemical reactions that occur between coordination complexes and polynucleotides We then consider in some detail the interaction of a simple family of coordination complexes, the tris(phenanthroline) metal complexes, with DNA and RNA to illustrate the techniques, questions, and applications of metal/nucleic-acid chemistry that are currently being explored. In this section, the focus on tris(phenanthroline) complexes serves as a springboard to compare and contrast studies of other, more intricately designed transition-metal complexes (in the next section) with nucleic acids. Last we consider how Nature uses metal ions and complexes in carrying out nucleic-acid chemistry. Here the principles, techniques, and fundamental coordination chemistry of metals with nucleic acids provide the foundation for our current understanding of how these fascinating and complex bioinorganic systems may function. 

Soon after the structure of DNA was discovered, Francis Crick hypothesized that an adaptor molecule, such as the one shown in Figure 26.2, would be required for protein synthesis. It was discovered that tRNA serves as the adaptor molecule by linking the information stored in mRNA to the primary sequence of the polypeptide. The adaptor function of tRNA is mediated by base pairing between mRNA sequences called codons, and complementary sequences on tRNA called anticodons. For every codon on the mRNA, a single amino acid is delivered by the tRNA to the growing polypeptide chain. The term genetic code refers to the specific sequences in the mRNA codon that determine which tRNA molecule is going to have the complementary anticodon and, therefore, what amino acid is required at that position in the final polypeptide chain. 

X-ray diffraction studies of DNA held at low relative humidity revealed the existence of A-DNA. Since such arid conditions presumably never occur in the cell, what is the significance of the structure of this DNA ... 

The four bases that we find in dna and that are responsible for its helical structure are adenine, cytosine, guanine, and thymine. What is the derivation of these names ... 

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