Polynucleotides double helix

FIGURE 1.5 The DNA double helix. Two complementary polynucleotide chains running in opposite directions can pair through hydrogen bonding between their nitrogenous bases. Their complementary nucleotide sequences give rise to structural complementarity. 

The DNA isolated from different cells and viruses characteristically consists of two polynucleotide strands wound together to form a long, slender, helical molecule, the DNA double helix. The strands run in opposite directions that is, they are antiparallel and are held together in the double helical structure through interchain hydrogen bonds (Eigure 11.19). These H bonds pair the bases of nucleotides in one chain to complementary bases in the other, a phenomenon called base pairing. 

H bonding also vitally influences the conformation and detailed structure of the polypeptide chains of protein molecules and the complementary intertwined polynucleotide chains which form the double helix in nucleic acids.Thus, proteins are built up from polypeptide chains of the type shown at the top of the next column. 

In 1953, James Watson and Francis Crick made their classic proposal for the secondary structure of DNA. According to the Watson-Crick model, DNA under physiological conditions consists of two polynucleotide strands, running in opposite directions and coiled around each other in a double helix like the handrails on a spiral staircase. The two strands are complementary rather than identical and are held together by hydrogen bonds between specific pairs of... 

Molecules of DNA consist of two complementary polynucleotide strands held together by hydrogen bonds between heterocyclic bases on the different strands and coiled into a double helix. Adenine and thymine form hydrogen bonds to each other, as do cytosine and guanine. 

Double helix (Section 28.2) The structure of DNA in which two polynucleotide strands coil around each other. 

The polycondensation of several nucleoside monophosphates gives oligonucleotides (up to 40-50 units). If the chain is even longer, the polymer is referred to as a polynucleotide. Initial experiments on the polycondensation of nucleotides to give longer chains were carried out about ten years after the discovery of the DNA double helix (G. Schramm, Sect. 6.3). 

The protection of a reactive intermediate complex by the DNA double helix versus a neutral oxidising agent in solution, has also been demonstrated by studying a photo-electron transfer process. In this example the intermediate complex is produced photochemically on the DNA, and is examined spectroscopically after a laser pulsed excitation [73]. Thus Ru(TAP)2(HAT) physically bound to nucleic acid is photo-reduced by hydroquinone during the laser pulse. The intermediate [Ru(TAP)2(HAT)] so-produced, detected by its absorption at 480 nm, is reoxidised by benzoquinone purposely added as oxidant to the solution. It is shown that this reoxidation of the mono-reduced complex is slower in the presence of polynucleotide than in its absence, indicating a protection of the transient mono-reduced complex in the DNA grooves. 

Secondary structure of DNA consists of two strands of polynucleotides coiled around each there in the form of double helix. The backbone of each strand is sugar-phosphate unit and the base unit of each strand are pointed into the interior of the helix and are linked through H-bonds. G and C are held by three H-bonds, A and T are held by two bonds. Unlike DNA, RNA has a single strand. 

The absorption coefficients of polynucleotides are different from those calculated from the sum of the mononucleotides in part this reflects the secondary structure. The abrupt increase in the absorption of DNA at the melting point, where the secondary structure changes from the double helix to a random coil, is well known. It is therefore... 

Melting Temperature. The double helix of polynucleotides described above becomes thermodynamically unstable at particular temperatures (with specified conditions of solute concentration, pH, etc.) and is transformed into the open random-coil arrangement. This transformation is rather sharp, and can be measured by the concurrent changes in a number of physical properties of the nucleic acid, such as the optical absorption coefficient. The midpoint of the transition region is called the melting point. 

DNA, a constituent of the cell nucleus, consists of two strands of polynucleotides that are coiled to form a double helix. The strands are held together by H-bonding between the nitrogen bases. The pyrimidines always form H-bonds with a specific purine i.e., cytosine with guanine and thymine with adenine. However, in RNA the pairing is between uracil and adenine. 

Nucleic acids are of great interest because they are the units of heredity, the genes, and because they control the manufacture of proteins and the functions of the cells of living organisms. Hydrogen bonds play an important part in the novel structure proposed for deoxyribonucleic acid by Watson and Crick.1,5 This structure involves a detailed eomplement riness of two intertwined polynucleotide chains, which form a double helix.117 The complementariness in structure of the two chains was attributed by Watson and Crick to the formation of hydrogen bonds between a pyrimidine residue in one chain and a purine residue in the other, for each pair of nucleotides in the chains. 

The bases of one strand are paired with the bases of the second strand so that an adenine is always paired with a thymine, and a cytosine is always paired with a guanine. Therefore, one polynucleotide chain of the DNA double helix is always the complement of the other. Base pairs are held together by hydrogen bonds. 

Watson and Crick proposed that DNA is a double helix of two antiparallel polynucleotide chains (Figs. 5-2 and 5-3). The structure was deduced from model building together with knowledge of the X-ray diffraction data of Maurice F. Wilkins and Rosalind Franklin93 on... 

The nucleic acids are among the most complex molecules that you will encounter in your biochemical studies. When the dynamic role that is played by DNA in the life of a cell is realized, the complexity is understandable. It is difficult to comprehend all the structural characteristics that are inherent in the DNA molecules, but most biochemistry students are familiar with the double-helix model of Watson and Crick. The discovery of the double-helical structure of DNA is one of the most significant breakthroughs in our understanding of the chemistry of life. This experiment will introduce you to the basic structural characteristics of the DNA molecule and to the forces that help establish the complementary interactions between the two polynucleotide strands. 

Very shortly, the first question was answered in principle by Watson and Crick who proposed the three-dimensional structure of DNA in 1953. Their proposal that DNA is composed of two polynucleotide chains forming a double helix was based upon studies of x-ray diffraction patterns of DNA libers. 

In A- or B-form DNA. two self-complementary polynucleotide strands associate with one another to form a right-handed double helix. The two polynucleotide chains arc antiparallcl. 

Watson-Crick model for DNA replication. The double helix unwinds at one end. New strand synthesis begins by absorption of mononucleotides to complementary bases on the old strands. These ordered nucleotides are then covalently linked into a polynucleotide chain, a process resulting ultimately in two daughter DNA duplexes. 

Crick when they proposed the duplex structure for DNA (fig. 26.1). First, the double helix unwinds next, mononucleotides are absorbed into complementary sites on each polynucleotide strand and finally these mononucleotides become linked to yield two identical daughter DNA duplexes. What could be simpler Subsequent biochemical investigations showed that in many respects this model for DNA replication was correct, but they also indicated a much greater complexity than was initially suspected. Part of the reason for the complications is that replication must be very fast to keep up with the cell division rate, and it must be very accurate to ensure faithful transfer of information from one cell generation to the next. 

Double helix. A structure in which two helically twisted polynucleotide strands are held together by hydrogen bonding and base stacking. 

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