Thus the two polynucleotide chains have complementary base sequences. Base pairing is specific so that adenine pairs with thymine and guanine with cytosine. The purine (adenine and guanine) and pyrimidine (thymine and cytosine) bases are turned inwards and linked by hydrogen bonds, each base on one chain being paired with a base on the other chain. Thus each phosphate links the 3′-carbon atom on one sugar to the 5′-carbon on the sugar of the next nucleotide (Fig. The only OH groups available for ester linkages on the pentose sugar are those on the 3′ and 5′ carbon atoms. The sugar units on adjacent nucleotides are linked by phosphate groups to form an outer sugar-phosphate backbone. The coiling of the two chains is such that they cannot be separated except by unwinding the coils this is called plectonemic coiling. Watson and Crick’s model consists of a double, right-handed helix in which polynucleotide chains are helically coiled about the same axis. However, the molecular arrangements of the component and the three- dimensional spatial geometry of DNA was not known. All this information was available in the 1940’s. One molecule of a phosphate joins two nucleoside units by forming an ester linkage with the hydroxyl on the 3′ carbon atom of one sugar molecule and another ester linkage with the 5′ hydroxyl of the sugar of the adjacent nucleotide. The polynucleotide chain is made up of nucleotide units held together by phosphate diester linkages. The phosphoric acid (H 3 PO 4) forms an ester linkage with one of the free hydroxyl groups in the pentose sugar which is a deoxyribose. The phosphate ester of a nucleoside is called nucleotide. The first carbon atom of the sugar is linked to the nitrogen in 9th position in a purine base, and to the first nitrogen in a pyrimidine. ![]() The combination of a base and a sugar is called a nucleoside. The basic building blocks of DNA are shown in Fig. In genetic discussion, one strand of the double helix is conveniently referred to as Watson (W), the other strand as Crick (C). In 1962 Watson, Crick and Wilkins were awarded Nobel Prize for this work. The model also suggested a mechanism by which the genetic material could be accurately replicated. ![]() In 1953 they finally postulated a precise, three-dimensional, double helical model which could account for many of the observations on the chemical and physical properties of DNA (Fig. ![]() On the basis of the evidences available and a knowledge of interatomic distances and bond angles, Watson and Crick proceeded to construct molecular models of DNA. In this article we will discuss about the double helix of DNA.
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