Secondary Structure of DNA The Double Helix

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... 

The double heUx is the predominant secondary structure of DNA. The sugar—phosphate backbones, which run in antiparallel directions on the two strands, lie on the outside of the helix. Pairs of bases, one on each strand, are held in alignment by hydrogen bonds. 

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... 

The secondary structure of DNA is shown in Figure B. This "double helix" model was first proposed in 1953 by James Watson and Francis Crick, who used the x-ray crystallographic data of Rosalind Franklin and Maurice Wilkins. Beyond that, they were intrigued by the results of analyses that showed that in DNA the ratio of adenine to thymine molecules is almost exactly 1 1, as is the ratio of cytosine to guanine ... 

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. 

In 1953, Watson and Crick proposed a three-dimensional structure of DNA which is a cornerstone in the history of biochemistry and molecular biology. The double helix they proposed for the secondary structure of DNA gained immediate acceptance, partly because it explained all known facts about DNA, and partly because it provided a beautiful model for DNA replication. 

DNA The secondary structure of DNA consists of two polynucleotide chains wrapped around one another to form a double helix. The orientation of the helix is usually right handed witii the two chains running antiparallel to one another (Fig. 4.3). 

Similarly to proteins, both DNA and RNA have a secondary and a tertiary structure. The secondary structure of DNA shows two chains running in opposite directions, coiled in a left-handed (double) helix about the same axis. All the bases are inside the helix, and the sugar phosphate backbone is on the outside (see e.g. [1]). The chains are held together by hydrogen bonds between the bases with adenine always paired with thymine and guanine paired with cytosine. The base pairing in DNA is shown below ... 

Pig. C5.10 The most common secondary structure of DNA is a double helix. Under normal physiological conditions (of temperature and ionic strength) it has about 10.4 base pairs per helical turn, with the distance between base pairs along the helix about 0.34 nm. 

What stabilizing forces hold the double helix together in the secondary structure of DNA proposed by Watson and Crick ... 

Reasoning from these data, Watson and Crick proposed a double helix as a model for the secondary structure of DNA. According to this model, two nucleic acid chains are held together by hydrogen bonds between base pairs on opposite strands. This double chain is wound into a helix with both chains sharing the same axis. The base pairs are on the inside of the helix, and the sugar-phosphate backbone is on the outside (Fig. 25.8). The pitch of the helix is such that 10 successive nucleotide pairs give rise to one complete turn in 34 A (the repeat distance). The exterior width of the spiral is about 20 A, and the internal distance between 1 positions of ribose units on opposite chains is about 11 A. 

Double helix A type of secondary structure of DNA molecules in which two antiparallel polynucleotide strands are coiled in a right-handed manner about the same axis. 

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